Data Sheet

January 2000

L7556, L7557 Low-Power SLICs
with Battery Switch

Features

Auxiliary input for second battery, and internal
switch to enable its use to save power

Low active power (typical 125 mW during on-hook
transmission)

Supports meter pulse injection

Spare op amp for meter pulse filtering

16 V to 60 V power supply operation

Distortion-free on-hook transmission

Convenient operating states:
-- Forward powerup
-- Disconnect (high impedance)
-- 2-wire wink (zero loop voltage)

Adjustable supervision functions:
-- Off-hook detector with longitudinal rejection
-- Ground key detector
-- Ring trip detector

Independent, adjustable, dc and ac parameters:
-- dc feed resistance
-- Loop current limit
-- Termination impedance

Thermal protection

Description

These electronic subscriber loop interface circuits
(SLICs) are optimized for low power consumption
while providing an extensive set of features.

The SLICs include an auxiliary battery input and a
built-in switch. In short-loop applications, they can be
used in high battery to present a high on-hook volt-
age, and then switched to low battery to reduce off-
hook power.

The SLICs also include a summing node for meter
pulse injection to 2.2 Vrms. A spare, uncommitted op
amp is included for meter pulse filtering.

The switched battery is applied to the power amplifi-
ers of the device. There are two versions. The L7556
has the battery switch completely under processor
control. The L7557 can automatically switch to lower
battery when appropriate and includes hysteresis to
avoid frequent switching. To make the switch silent,
an external capacitor can be added to slow the tran-
sition.

The L7556 is suited for applications serving only
short loops, where a high on-hook voltage is required
for compatibility with preexisting standards.

The L7557 is suited for applications where a full loop
range is needed, but low short-loop power is desired.
It is a much lower-cost solution than a switching reg-
ulator, and also occupies much less PCB area, need-
ing only a battery filter capacitor and a diode for
implementation.

The device is available in a 32-pin PLCC package. It
is built by using a 90 V complementary bipolar inte-
grated circuit (CBIC) process.

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Table of Contents

Contents Page

Features ..................................................................... 1
Description .................................................................. 1
Pin Information ............................................................ 4
Functional Description ................................................. 6
Absolute Maximum Ratings ........................................ 6
Recommended Operating Conditions ......................... 7
Electrical Characteristics ............................................. 7

Ring Trip Requirements ......................................... 11

Test Configurations .................................................. 12
Applications .............................................................. 14

Design Considerations ........................................... 16
Characteristic Curves............................................. 17
dc Applications ....................................................... 20

Battery Feed......................................................... 20
Switching the Battery............................................ 20
Overhead Voltage ............................................... 21
Adjusting Overhead Voltage ................................ 21
Adjusting dc Feed Resistance.............................. 22
Loop Range.......................................................... 22
Off-Hook Detection .............................................. 22
Ring Trip Detection.............................................. 23
Ring Ground Detection........................................ 23

ac Design ............................................................... 24

First-Generation Codecs ..................................... 24
Second-Generation Codecs ................................ 24
Third-Generation Codecs .................................... 24
Selection Criteria ................................................. 24

PCB Layout Information ............................................ 26
Outline Diagram......................................................... 27

32-Pin PLCC ........................................................... 27

Ordering Information.................................................. 28

Tables Page

Table 1. Pin Descriptions ............................................ 4
Table 2. Input State Coding ........................................ 6
Table 3. Supervision Coding ....................................... 6
Table 4. Power Supply ................................................ 7
Table 5. 2-Wire Port .................................................... 8
Table 6. Analog Pin Characteristics ............................ 9
Table 7. Uncommitted Op Amp Characteristics .......... 9
Table 8. ac Feed Characteristics .............................. 10
Table 9. Logic Inputs and Outputs ............................ 11
Table 10. Parts List for Loop Start and Ground

Start Applications ...................................... 15

Table 11. 600

Design Parameters ......................... 16

Figures Page

Figure 1. Functional Diagram ..................................... 3
Figure 2. Pin Diagram (PLCC Chip) ........................... 4
Figure 3. Ring Trip Circuits ....................................... 11
Figure 4. Basic Test Circuit ....................................... 12
Figure 5. Longitudinal Balance ................................. 12
Figure 6. Longitudinal PSRR .................................... 13
Figure 7. RFI Rejection ............................................. 13
Figure 8. Longitudinal Impedance ............................ 13
Figure 9. Metallic PSRR ........................................... 13
Figure 10. ac Gains ..................................................13
Figure 11. Basic Loop Start Application Circuit

Using T7504 Type Codec ........................14

Figure 12. Ring Ground Detection Circuit ................. 14
Figure 13. Receive Gain and Hybrid Balance vs.

Frequency ............................................... 17

Figure 14. Transmit Gain and Return Loss vs.

Frequency ............................................... 17

Figure 15. Typical V

Power Supply Rejection ....... 17

Figure 16. Typical V

BAT

Power Supply

Rejection ................................................. 17

Figure 17. Loop Closure Program Resistor

Selection ..................................................18

Figure 18. Ring Ground Detection Programming .....18
Figure 19. Loop Current vs. Loop Voltage ................ 18
Figure 20. Loop Current vs. Loop Resistance .......... 18
Figure 21. Typical SLIC Power Dissipation vs.

Loop Resistance ...................................... 19

Figure 22. Power Derating ........................................ 19
Figure 23. Longitudinal Balance Resistor Mismatch

Requirements .......................................... 19

Figure 24. Longitudinal Balance vs. Protection

Resistor Mismatch ................................... 19

Figure 25. Loop Current vs. Loop Voltage ................ 20
Figure 26. SLIC 2-Wire Output Stage ....................... 21
Figure 27. Equivalent Circuit for Adjusting the Over-

head Voltage ........................................... 21

Figure 28. Equivalent Circuit for Adjusting the dc

Feed Resistance ...................................... 22

Figure 29. Adjusting Both Overhead Voltage and dc

Feed Resistance .....................................22

Figure 30. Off-Hook Detection Circuit

Applications ............................................. 22

Figure 31. Ring Trip Equivalent Circuit and

Equivalent Application ............................. 23

Figure 32. ac Equivalent Circuit Not Including Spare

Op Amp ................................................... 25

Figure 33. ac Equivalent Circuit Including Spare

Op Amp ................................................... 25

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Pin Information

12-2548.q (F)

Figure 2. Pin Diagram (PLCC Chip)

Table 1. Pin Descriptions

Pin

Symbol Type

Description

BAT2

Auxiliary Battery Supply. Negative high-voltage battery, lower in magnitude than
V

BAT1

, used to reduce power dissipation on short loops.

PROG

Current-Limit Program Input. A resistor to DCOUT sets the dc current limit of the
device.

Battery Switch. See Table 2 for description.

No Connection (L7556 Only). Do not use as a tie point.

BAT

Lower Battery in Use (L7557 Only). When high, this open-collector output indicates
the device has switched to V

BAT2.

To use, connect a 100 k

resistor to V

+5 V Power Supply.

RCVP

Receive ac Signal Input (Noninverting). This high-impedance input controls the ac
differential voltage on tip and ring.

RCVN

Receive ac Signal Input (Inverting). This high-impedance input controls the ac differ-
ential voltage on tip and ring.

LCTH

Loop Closure Threshold Input. Connect a resistor to DCOUT to set off-hook thresh-
old.

DCOUT

dc Output Voltage. This output is a voltage that is directly proportional to the absolute
value of the differential tip/ring current.

BAT1

Battery Supply. Negative high-voltage power supply, higher in magnitude than V

BAT2

XMT

NLC

NRDET

RTSP

RTSN

ICM

DCR

RGDET

BGND

PRO

RCVN

RCVP

LCTH

BAT1

DCOUT

CF2

CF1

32-PIN PLCC

BS2

BS1

BAT

Agere Systems Inc.

Data Sheet
January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Pin Information

(continued)

Table 1. Pin Descriptions

(continued)

Pin

Symbol Type

Description

I/O

Protected Ring. The output of the ring driver amplifier and input to loop sensing cir-
cuitry. Connect to loop through overvoltage protection.

CF2

Filter Capacitor 2. Connect a 0.1 µF capacitor from this pin to AGND.

CF1

Filter Capacitor 1. Connect a 0.47 µF capacitor from this pin to pin CF2.

VITR

Transmit ac Output Voltage. This output is a voltage that is directly proportional to the
differential tip/ring current.

ICM

Common-Mode Current Sense. To program ring ground sense threshold, connect a
resistor to V

and connect a capacitor to AGND to filter 50/60 Hz. If unused, the pin

can be left unconnected.

RGDET

Ring Ground Detect. When high, this open-collector output indicates the presence of
a ring ground. To use, connect a 100 k

resistor to V

State Control Input. B0 and B1 determine the state of the SLIC. See Table 2.

AGND

Analog Signal Ground.

AGND

Analog Signal Ground.

DCR

dc Resistance for Low Loop Currents. Leave open for dc feed resistance of 115

or short to DCOUT for 615

. Intermediate values can be set by a simple resistor

divider from DCOUT to ground with the tap at DCR.

BGND

Battery Ground. Ground return for the battery supply.

I/O

Protected Tip. The output of the tip driver amplifier and input to loop sensing circuitry.
Connect to loop through overvoltage protection.

RTSN

Ring Trip Sense Negative. Connect this pin to the ringing generator signal through a
high-value resistor.

RTSP

Ring Trip Sense Positive. Connect this pin to the ring relay and the ringer series resis-
tor through a high-value resistor.

NRDET

Ring Trip Detector Output. When low, this logic output indicates that ringing is tripped.

NLC

Loop Detector Output. When low, this logic output indicates an off-hook condition.

I/O

State Control Input. B0 and B1 determine the state of the SLIC. See Table 2. Pin B1
has a 40 k

pull-up. It goes low in the event of thermal shutdown.

XMT

Transmit ac Output Voltage. The output of the uncommitted operational amplifier.

Summing Node. The inverting input of the uncommitted operational amplifier. A resis-
tor or network to XMT sets the gain.

No Connection. Do not use as a tie point.

BS2

Battery Switch Slowdown. A 0.1 µF capacitor from BS1 to BS2 will ramp the battery
switch transition for applications requiring quiet transition. If not needed, the pin can be
left open.

BS1

Battery Switch Slowdown. A 0.1 µF capacitor from BS1 to BS2 will ramp the battery
switch transition for applications requiring quiet transition. If not needed, the pin can be
left open.

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Functional Description

Table 2. Input State Coding

Table 3. Supervision Coding

Absolute Maximum Ratings

= 25 °C)

Stresses in excess of the Absolute Maximum Ratings can cause permanent damage to the device. These are
absolute stress ratings only. Functional operation of the device is not implied at these or any other conditions in
excess of those given in the operational sections of the data sheet. Exposure to Absolute Maximum Ratings for
extended periods can adversely affect device reliability

Note:

The IC can be damaged unless all ground connections are applied before, and removed after, all other connections. Furthermore, when
powering the device, the user must guarantee that no external potential creates a voltage on any pin of the device that exceeds the
device ratings. Some of the known examples of conditions that cause such potentials during powerup are the following:
1. An inductor connected to tip and ring can force an overvoltage on V

BAT

through the protection devices if the V

BAT

connections chatter.

2. Inductance in the V

BAT

leads could resonate with the V

BAT

filter capacitors to cause a destructive overvoltage.

State/Definition

Powerup, Forward Battery. Normal talk and battery feed state. Pin PT is positive with respect
to PR. On-hook transmission is enabled. V

BAT1

is applied to entire circuit.

Powerup, Forward Battery. Normal talk and battery feed state. Pin PT is positive with respect
to PR. On-hook transmission is enabled.
For the L7556 only, V

BAT2

is applied to tip/ring drive amplifiers.

For the L7557 only, the device compares the magnitude of V

BAT2

to the voltage necessary to

maintain proper loop current. Then the device automatically applies V

BAT2

to tip/ring drive am-

plifiers when possible, not affecting the desired dc template.

2-Wire Wink. Pins PT and PR are put at the same potential (near ground). V

BAT1

is applied to

entire circuit.

Disconnect. The tip and ring amplifiers are turned off, and the SLIC goes to a high-impedance
state (>100 k

).V

BAT1

is applied to entire circuit.

Pin NLC

Pin NRDET

Pin RGDET

0 = off-hook
1 = on-hook

0 = ring trip
1 = no ring trip

1 = ring ground
0 = no ring ground

Parameter

Symbol

Value

Unit

5 V Power Supply

7.0

Battery (Talking) Supply

BAT1

Auxiliary Battery Supply

BAT2

Logic Input Voltage

0.5 to +7.0

Analog Input Voltage

7.0 to +7.0

Maximum Junction Temperature

165

°C

Storage Temperature Range

stg

40 to +125

°C

Relative Humidity Range

5 to 95

Ground Potential Difference (BGND to AGND)

±3

PT or PR Fault Voltage (dc)

, V

BAT1

5) to +3

PT or PR Fault Voltage (10 x 1000 µs)

, V

BAT1

15) to +15

Current into Ring Trip Inputs

RTSP

, I

RTSN

±240

µA

Agere Systems Inc.

Data Sheet
January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Recommended Operating Conditions

Electrical Characteristics

Minimum and maximum values are testing requirements. Typical values are characteristic of the device and are
the result of engineering evaluations. Typical values are for information purposes only and are not part of the test-
ing requirements. Minimum and maximum values apply across the entire temperature range (40 °C to +85 °C)
and the entire battery range unless otherwise specified. Typical is defined as 25 °C, V

= 5.0 V, V

BAT1

= 48 V,

BAT2

= 48 V, and I

LIM

= 40 mA. Positive currents flow into the device. Test circuit is Figure 4 unless noted.

Table 4. Power Supply

1. This parameter is not tested in production. It is guaranteed by design and device characterization.

Parameter

Min

Typ

Max

Unit

Ambient Temperature

°C

Supply Voltage

4.75

5.0

5.25

BAT1

Supply Voltage

BAT2

Supply Voltage

BAT1

Loop Closure Threshold-detection Programming Range

LIM

dc Loop Current-limit Programming Range

On- and Off-hook 2-wire Signal Level

2.2

Vrms

ac Termination Impedance Programming Range

150

600

1300

Parameter

Min

Typ

Max

Unit

Power Supply--Powerup, No Loop Current:

BAT

= 48 V)

Power Dissipation (V

BAT

= 48 V)

--
--
--

2.8

2.3

125

--
--

155

mA
mA

Power Supply Rejection 500 Hz to 3 kHz (See Figures 5, 6, 15, and 16.)

BAT

35
45

--
--

dB
dB

Thermal Protection Shutdown (T

)

175

°C

Thermal Resistance, Junction to Ambient (

)

°C/W

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Electrical Characteristics

(continued)

Table 5. 2-Wire Port

1. The longitudinal current is independent of dc loop current.
2. Current-limit I

LIM

is programmed by a resistor, R

PROG

, from pin I

PROG

to DCOUT. I

LIM

is specified at the loop resistance where current limiting

begins (see Figure 19). Select R

PROG

) =1.67 x I

LIM

(mA).

IEEE

is a registered trademark of The Institute of Electrical and Electronics Engineers, Inc.

4. Longitudinal balance of circuit card will depend on loop series resistance matching (see Figure 23 and Figure 24).
5. This parameter is not tested in production. It is guaranteed by design and device characterization.

Parameter

Min

Typ

Max

Unit

Tip or Ring Drive Current:

= dc + Longitudinal + Signal Currents

Signal Current

mArms

Longitudinal Current Capability per Wire

8.5

mArms

dc Loop Current Limit

LOOP

= 100

Programmability Range
Accuracy (20 mA < I

LIM

< 40 mA)

LIM

--
--

±12

mA
mA

Powerup Open Loop Voltage Levels (includes external diode):

Differential Voltage

BAT

+ 8.4|

BAT

+ 7.9| |V

BAT

+ 7.4|

Disconnect State:

PT Resistance (V

BAT

< V

< 0 V)

PR Resistance (V

BAT

< V

< 0 V)

100
100

143
133

--
--

Ground Start State:

PT Resistance

100

143

dc Feed Resistance (for I

LOOP

below regulation level)

115

135

Loop Resistance Range (3.17 dBm overload into 600

; not

including protection):

LOOP

= 20 mA at V

BAT2

= 48 V

LOOP

= 20 mA at V

BAT2

= 24 V

1885

685

--
--

Longitudinal to Metallic Balance--

IEEE

Std. 455 (See

Figure 6.)

50 Hz to 1 kHz
1 kHz to 3 kHz

64
60

75
70

--
--

dB
dB

Metallic to Longitudinal Balance:

200 Hz to 4 kHz

RFI Rejection (See Figure 7.)

0.5 Vrms, 50

Source, 30% AM Mod 1 kHz

500 kHz to 100 MHz

dBV

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Electrical Characteristics

(continued)

Table 8. ac Feed Characteristics

1. Set by external components. Any complex impedance R1 + R2 || C between 150

and 1300

can be synthesized.

2. This parameter is not tested in production. It is guaranteed by design and device characterization.
3. Return loss and transhybrid loss are functions of device gain accuracies and the external hybrid circuit. Guaranteed performance assumes

1% tolerance external components.

Parameter

Min

Typ

Max

Unit

ac Termination Impedance

150

1300

Longitudinal Impedance

(See Figure 8.)

Total Harmonic Distortion--200 Hz to 4 kHz

Off-hook
On-hook

--
--

0.3
1.0

%
%

Transmit Gain, f = 1 kHz (PT/PR to VITR)
Transmit Accuracy in dB

122

0.18

125

128

0.18

V/A

Receive + Gain, f = 1 kHz (RCVP to PT/PR)
Receive Gain, f = 1 kHz (RCVN to PT/PR)
Receive Accuracy in dB

7.84

7.84
0.18

8.00

8.16

0.18

--
--

Gain vs. Frequency (transmit and receive) (600

termination; reference 1 kHz

200 Hz to 300 Hz
300 Hz to 3.4 kHz
3.4 kHz to 16 kHz
16 kHz to 266 kHz

1.00

0.3
3.0

0.0
0.0

0.1

0.05
0.05

0.3
2.0

dB
dB
dB
dB

Gain vs. Level (transmit and receive)(reference 0 dBV

50 dB to +3 dB

0.05

Return Loss

200 Hz to 500 Hz
500 Hz to 3400 Hz

20
26

24
29

--
--

dB
dB

2-wire Idle-channel Noise (600

termination):

Psophometric
C-message
3 kHz Flat

--
--
--

12
20

dBmp

dBrnC

dBrn

Transmit Idle-channel Noise:

Psophometric
C-message
3 kHz flat

--
--
--

12
20

dBmp

dBrnC

dBrn

Transhybrid Loss

200 Hz to 500 Hz
500 Hz to 3400 Hz

21
26

24
29

--
--

dB
dB

Data Sheet
January 2000

Agere Systems Inc.

with Battery Switch

L7556, L7557 Low-Power SLICs

Electrical Characteristics

(continued)

Table 9. Logic Inputs and Outputs

All outputs except RGDET and L

BAT

are open collectors with internal, 30 k

pull-up resistor. RGDET and L

BAT

are

open collectors without internal pull-up. Input pin B1 has a 40 k

pull-up; it goes low in the event of thermal shut-

down.

Parameter

Symbol

Min

Typ

Max

Unit

Input Voltages:

Low Level (permissible range)
High Level (permissible range)

0.5

2.0

0.4
2.4

0.7

V
V

Input Currents:

Low Level (V

= 5.25 V, V

= 0.4 V)

High Level (V

= 5.25 V, V

= 2.4 V)

75
40

115

200
100

µA
µA

Output Voltages (open collector with internal pull-up resistor):

Low Level (V

= 4.75 V, I

= 360 µA)

High Level (V

= 4.75 V, I

= 20 µA)

2.4

0.2

0.4

V
V

Ring Trip Requirements

Ringing signal:
-- Voltage, minimum 35 Vrms, maximum 100 Vrms.
-- Frequency, 17 Hz to 23 Hz.
-- Crest factor, 1.4 to 2.

Ringing trip:
--

100 ms (typical),

250 ms (V

BAT

= 33 V, loop

length = 530

Pretrip:
-- The circuits in Figure 3 will not cause ringing trip.

12-2572g (F)

Figure 3. Ring Trip Circuits

RING

100

10 k

TIP

200

SWITCH CLOSES <12 ms

Agere Systems Inc.

Data Sheet
January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Test Configurations

(continued)

12-2583.b (F)

Figure 6. Longitudinal PSRR

= 0.5 Vrms 30% AM 1 kHz MODULATION,

f = 500 kHz--1 MHz
DEVICE IN POWERUP MODE, 600

TERMINATION

5-6756.b (F)

is a registered trademark of Hewlett-Packard Company.

Figure 7. RFI Rejection

12-2585.a (F)

Figure 8. Longitudinal Impedance

12-2582.b (F)

Figure 9. Metallic PSRR

12-2587.e (F)

Figure 10. ac Gains

4.7

100

BAT

DISCONNECT
BYPASS CAPACITOR

56.3

BAT

OR V

TIP

RING

BASIC

TEST CIRCUIT

PSRR = 20log

67.5

BASIC TEST

CIRCUIT

TIP

RING

BAT

0.01

600

2.15

82.5

* 4935A

TIMS

6, 7

LB1201

TIP

RING

BASIC

TEST CIRCUIT

LONG

4.7

100

BAT

DISCONNECT

T/R

BAT

TIP

RING

BASIC

TEST CIRCUIT

PSRR = 20log

T/R

900

BYPASS CAPACITOR

TIP

RING

BASIC

TEST CIRCUIT

600

T/R

XMT

T/R

RCV

T/R

RCV

XMT

RCV

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

12-2573.Y(F)

Notes:
Tx = 0 dB.

Rx = 0 dB.

Termination = 600

Transhybrid = 600

Figure 11. Basic Loop Start Application Circuit Using T7504 Type Codec

12-3547(F)

Figure 12. Ring Ground Detection Circuit

PROG

66.8 k

LCTH

24.9 k

L7581

RELAY

BAT1

0.1

BAT1

BAT2

0.1

RCVN

DCOUT

LCTH

RTSP

TS1

402

RTS2

0.27

RTSN

TS2

274 k

TSN

2.0 M

RING

BAT

CF2

CF1

0.47

AGND

BGND

PROG

VITR

RCVP

54.9 k

18.7 k

RCV

84.5 k

HB1

90.9 k

GSX

PWROP

FSX

FSEP

MCLK

1/4 T7504

CODEC

CONTROL

INPUT

PCM

HIGHWAY

SYNC

AND

CLOCK

L7556/L7557

SLIC

CONTROL

INPUTS

SUPERVISION

OUTPUTS

RTS1

0.022

TSP

2.0 M

0.1

AGND

BGND L

BAT

0.47

57.6 k

330 pF

ASEL

NLC

NRDET

27
17
4

26
25

B0
BS

AGND

0.1

TIP

RING

CROWBAR

PROTECTOR

CROWBAR

PROTECTOR

0.47

BAT

0.47

ICM

GROUND START

APPLICATION CIRCUIT

GDET

ICM

GDET

ICM2

154 k

100 k

GDET

Agere Systems Inc.

Data Sheet
January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

Table 10. Parts List for Loop Start and Ground Start Applications

* Contact your Agere Systems Inc. account representative for protector recommendations. Choice of this (and all) component(s) sh ould be

evaluated and confirmed by the customer prior to use in any field or laboratory system. Agere does not recommend use of this part in the
field without performance verification by the customer. This device is suggested by Agere for customer evaluation. The decision to use a
component should be based solely on customer evaluation.

Name

Value

Function

Integrated Circuits

SLIC

L7556/7557

Subscriber loop interface circuit (SLIC).

Protector

Crowbar protector*

Secondary protection.

Ringing Relay

L7581

Switches ringing signals.

Codec

T7504

First-generation codec.

Overvoltage Protection

, PTC or Fusible

Protection resistor.

, PTC or Fusible

Protection resistor.

Power Supply

BAT1

0.1 µF, 20%, 100 V

BAT1

filter capacitor.

BAT2

0.1 µF, 20%, 100 V

BAT2

filter capacitor.

0.1 µF, 20%, 10 V

filter.

0.47 µF, 20%, 100 V

With C

, improves idle channel noise.

0.1 µF, 20%, 100 V

With C

, improves idle channel noise.

BAT

100 V, 150 mA

Transient protection diode.

dc Profile

PROG

66.8 k

, 1%, 1/16 W

Sets dc loop current limit.

ac Characteristics

0.47 µF, 20%, 10 V

ac/dc separation capacitor.

0.47 µF, 20%, 10 V

ac/dc separation capacitor.

54.9 k

, 1%, 1/16 W

With R

and R

RCV

, sets ac termination impedance.

RCV

84.5 k

, 1%, 1/16 W

With R

and R

, sets receive gain.

57.6 k

, 1%, 1/16 W

With R

and R

RCV

, sets ac termination impedance

and receive gain.

330 pF, 10 V, 20%

Loop stability.

18.7 k

, 1%, 1/16 W

With R

, sets transmit gain in codec.

90.9 k

, 1%, 1/16 W

With R

, sets transmit gain in codec.

HB1

90.9 k

, 1%, 1/16 W

Sets hybrid balance.

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

Table 10. Parts List for Loop Start and Ground Start Applications (continued)

Design Considerations

Table 11 shows the design parameters of the application circuit shown in Figure 11. Components that are adjusted
to program these values are also shown.

Table 11. 600

Design Parameters

Name

Value

Function

Supervision

LCTH

24.9 k

, 1%, 1/16 W

Sets loop closure (off-hook) threshold.

TS1

402

, 5%, 2 W

Ringing source series resistor.

TS2

274 k

, 1%, 1/16 W

With C

RTS2

, forms first pole of a double pole,

2 Hz ring trip sense filter.

RTS1

0.022 µF, 20%, 5 V

With R

TSN

, R

TSP

, forms second 2 Hz filter pole.

RTS2

0.27 µF, 20%, 100 V

With R

TS2

, forms first 2 Hz filter pole.

TSN

2 M

, 1%, 1/16 W

With C

RTS1

, R

TSP

, forms second 2 Hz filter pole.

TSP

2 M

, 1%, 1/16 W

With C

RTS1

, R

TSN

, forms second 2 Hz filter pole.

Ground Start

ICM

0.47 µF, 20%, 10 V

Provides 60 Hz filtering for ring ground detection.

GDET

100 k

, 20%, 1/16 W

Digital output pull-up resistor.

ICM2

82.5 k

, 1%, 1/16 W

Sets ring ground detection threshold.

Design Parameter

Parameter Value

Components Adjusted

Loop Closure Threshold

10 mA

LCTH

dc Loop Current Limit

40 mA

PROG

dc Feed Resistance

180

, R

2-wire Signal Overload Level

3.14 dBm

ac Termination Impedance

600

, R

RCV

Hybrid Balance Line Impedance

600

HB1

Transmit Gain

0 dB

, R

Receive Gain

0 dB

RCV

, R

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

Characteristic Curves

(continued)

12-3015 (F)

Note: V

BAT

= 48 V.

Figure 17. Loop Closure Program Resistor

Selection

12-3016a (F)

Notes:
Tip lead is open.

BAT

= 48 V.

Figure 18. Ring Ground Detection Programming

12-3050.a(F)

Notes:
V

BAT1

= 48 V.

BAT2

= 28 V.

LIM

= 22 mA.

dc1

= 115

Figure 19. Loop Current vs. Loop Voltage

12-3051.a(F)

Notes:
V

BAT1

= 48 V.

BAT2

= 28 V.

LIM

= 22 mA.

dc1

= 115

Figure 20. Loop Current vs. Loop Resistance

LOOP CLOSURE THRESHOLD RESISTOR, R

LCTH

)

OFF

-H

THRE

SHOLD

LOO

RRENT

RING GROUND CURRENT

DETECTION RESISTOR, R

ICM

)

THRES

OLD RING GROUND

CURRE

100

150

200

250

LOOP VOLTAGE (V)

CUR

RENT (

)

10 k

LIM

Rdc

L7556

BS = 1,

L7557 BS = 0

BS = 0

LOOP RESISTANCE, R

LOOP

(W)

500

1000

2000

1500

OOP CUR

RENT (m

L7556

BS = 0

BS = 1,

L7557 BS = 0

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

dc Applications

Battery Feed

The dc feed characteristic can be described by:

where:
I

= dc loop current.

T/R

= dc loop voltage.

BAT

| = battery voltage magnitude applied to the power

amplifier stage (V

BAT1

or V

BAT2

= overhead voltage. This is the difference between

the battery voltage and the open loop tip/ring
voltage.

= loop resistance, not including protection resistors.

= protection resistor value.

= SLIC internal dc feed resistance.

The design begins by drawing the desired dc template.
An example is shown in Figure 25.

12-3050.f (F)

Notes:
V

BAT1

= 48 V.

BAT2

= 28 V.

LIM

= 22 mA.

dc1

= 115

Figure 25. Loop Current vs. Loop Voltage

Starting from the on-hook condition and going through
to a short circuit, the curve passes through two regions:

Region 1; On-hook and low loop currents. The slope
corresponds to the dc resistance of the SLIC, R

DC1

(default is 115

typical). The open circuit voltage is the

battery voltage less the overhead voltage of the device,
V

(default is 7.9 V typical). These values are suitable

for most applications, but can be adjusted if needed.
For more information, see the sections entitled Adjust-
ing dc Feed Resistance or Adjusting Overhead Volt-
age.

Region 2; Current limit. The dc current is limited to a
value determined by external resistor R

PROG

. This

region of the dc template has a high resistance
(10 k

Calculate the external resistor as follows:

PROG

) = 1.67 I

LIM

(mA)

Switching the Battery

The L7556 and L7557 SLICs provide an input for an
auxiliary battery. Called V

BAT2

, this power supply

should be lower in magnitude than the primary battery,
V

BAT1

. Under an acceptable loop condition, V

BAT2

can

be switched to provide the loop power through the out-
put amplifiers of the SLIC. The dc template, described
in the last section, is determined by the battery that is
activated--either V

BAT1

or V

BAT2

Which device will be best for you? That mainly
depends on your loop range requirements. If you have
only short loops and no on-hook voltage requirements,
you don't need a battery switch at all. Use the L7551
instead. If you have only to guarantee a short loop
range, e.g., 22 mA into 530

, consider the L7556. The

minimum V

BAT2

can be determined by the standard dc

equations.

In these applications, the off-hook detector can be
used to indicate when to switch the battery. Just make
sure the off-hook detector will also function as required
with V

BAT2

as well as V

BAT1

Consider an off-hook threshold of 10 mA. This could
represent a 1000

loop with a 48 V V

BAT1

active or a

2000

loop with a 28 V V

BAT2

active. In this case, if

the loop is below 1000

or above 2000

, off-hook

detection will be accurate. Between 1000

and

2000

, the detector is battery-dependent. This condi-

tion must be avoided. In our example, since the maxi-
mum loop is 530

, the 10 mA detector is perfectly

acceptable.

If the PTT would like a short loop system that can also
serve long loops, the off-hook detector is not the best
indicator, and better loop intelligence is needed. In this
case, the L7557 can be used. It has an internal com-
parator that senses when there is enough potential at
V

BAT2

to switch without affecting the loop current. In

this case, the loop range is determined by V

BAT1

, and

BAT2

is only switched in when the loop is short enough

to use it. This switching is automatic and includes hys-
teresis to avoid oscillation when the loop length is close
to the V

BAT2

switch threshold.

B AT

O H

d c

----------------------------------

T R

BA T

O H

(

)

d c

--------------------------------------------

LOOP VOLTAGE (V)

LOOP CURRE

10 k

LIM

Rdc

Agere Systems Inc.

Data Sheet
January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

dc Applications

(continued)

Overhead Voltage

In order to drive an on-hook ac signal, the SLIC must
set up the tip and ring voltage to a value less than the
battery voltage. The amount that the open loop voltage
is decreased relative to the battery is referred to as the
overhead voltage. Expressed as an equation:

BAT

)

Without this buffer voltage, amplifier saturation will
occur and the signal will be clipped. The device is auto-
matically set at the factory to allow undistorted on-hook
transmission of a 3.17 dBm signal into a 900

loop

impedance. For applications where higher signal levels
are needed, e.g., periodic pulse metering, the 2-wire
port of the SLIC can be programmed with pin DCR.

The drive amplifiers are capable of 4 Vrms minimum
(VAMP). Referring to Figure 26, the internal resistance
has a worst-case value of 46

. So, the maximum sig-

nal the device can guarantee is:

Thus, R

allows 2.2 Vrms metering signals. The

next step is to determine the amount of overhead volt-
age needed. The peak voltage at output of tip and ring
amplifiers is related to the peak signal voltage by:

12-2560.e (F)

Figure 26

SLIC 2-Wire Output Stage

In addition to the required peak signal level, the SLIC
needs about 2 V from each power supply to bias the
amplifier circuitry. It can be thought of as an internal
saturation voltage. Combining the saturation voltage
and the peak signal level, the required overhead can
be expressed as:

where V

SAT

is the combined internal saturation voltage

between the tip/ring amplifiers and V

BAT

(5.4 V typ.).

(

) is the protection resistor value, and 40

is the

output series resistance of each internal amplifier.
Z

T/R

(

) is the ac loop impedance.

Example 1, on-hook transmission of a meter pulse:

Signal level: 2.2 Vrms into 200

protection resistors

LOOP

= 0 (on-hook transmission of the metering signal)

= 5.4 +

(2.2) = 10.8 V

Accounting for V

SAT

tolerance of 0.5 V, a nominal

overhead of 11.3 V would ensure transmission of an
undistorted 2.2 V metering signal.

Adjusting Overhead Voltage

To adjust the open loop 2-wire voltage, pin DCR is
programmed at the midpoint of a resistive divider from
ground to either 5 V or V

BAT

. In the case of 5 V, the

overhead voltage will be independent of the battery
voltage. Figure 27 shows the equivalent input circuit to
adjust the overhead voltage.

12-2562 (F)

Figure 27. Equivalent Circuit for Adjusting the

Overhead Voltage

The overhead voltage is programmed by using the fol-
lowing equation:

= 7.9 4 V

DCR

T R

4 V

T/R

2 R

(

)

-----------------------------------------

Vamp = V

T/R

2 R

(

)

T R

------------------------------

AMP

T/R

]

T/R

O H

S AT

2 R

(

)

T R

------------------------------

T R

2 35

(

)

200

------------------------------

DCR

25 k

± 30%

5 V

7.9

25 k

--------------------------------------

7.9

25 k

--------------------------------------

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

dc Applications

(continued)

Adjusting dc Feed Resistance

The dc feed resistance may be adjusted with the help
of Figure 28.

12-2560 (F)

Figure 28. Equivalent Circuit for Adjusting the dc

Feed Resistance

The above paragraphs describe the independent set-
ting of the overhead voltage and the dc feed resis-
tance. If both need to be set to customized values,
combine the two circuits as shown in Figure 29.

12-2561 (C)

Figure 29. Adjusting Both Overhead Voltage and dc

Feed Resistance

This is an equivalent circuit for adjusting both the dc
feed resistance and overhead voltage together.

The adjustments can be made by a simple superposi-
tion of the overhead and dc feed equations:

When selecting external components, select R1 on the
order of 5 k

to minimize the programming inaccuracy

caused by the internal 25 k

resistor. Lower values can

be used; the only disadvantage is the power consump-
tion of the external resistors.

Loop Range

The equation below can be rearranged to provide the
loop range for a required loop current:

Off-Hook Detection

The loop closure comparator has built-in longitudinal
rejection, eliminating the need for an external 60 Hz
filter. The loop closure detection threshold is set by
resistor R

LCTH

. Referring to Figure 30, NLC is high in

an on-hook condition (I

= 0, V

DCOUT

= 0) and

LCTH

= 0.05 mA x

LCTH

. The off-hook comparator

goes low when V

LCTH

crosses zero and then goes neg-

ative:

12-2553g(F)

Figure 30. Off-Hook Detection Circuit Applications

DCR

DCOUT

25 k

± 30%

d c

115

500

D C R

D C O UT

--------------------

115

500

25 k

----------------------------------

DCR

25 k

± 30%

DCOUT

5 V

LCTH

= 0.05 mA x R

LCTH

DCOUT

= 0.05 mA x R

LCTH

0.125 V/mA x I

LTCH

) = 2.5 x I

(mA)

O H

7.9

25 k

----------------------------------------------

D C

115

500

25 k

--------------------------------------

BA T

O H

----------------------------

d c

ITR

RING

DCOUT

LCTH

LCTH
NLC

TIP

0.125 V/mA

0.05 mA

Agere Systems Inc.

Data Sheet
January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

dc Applications

(continued)

Ring Trip Detection

The ring trip circuit is a comparator that has a special
input section optimized for this application. The equiva-
lent circuit is shown in Figure 31, along with its use in
an application using unbalanced, battery-backed ring-
ing.

12-3014.f (F)

Figure 31. Ring Trip Equivalent Circuit and

Equivalent Application

The comparator input voltage compliance is V

BAT

, and the maximum current is 240 µA in either

direction. Its application is straightforward. A resistance
(R

TSN

+ R

TS2

) in series with the R

TSN

input establishes a

current which is repeated in the R

TSP

input. A slightly

lower resistance (R

TSP

) is placed in series with the R

TSP

input. When ringing is being injected, no dc current
flows through R

TS1

, and so the R

TSP

input is at a lower

potential than R

TSN

. When enough dc loop current

flows, the R

TSP

input voltage increases to trip the com-

parator. In Figure 31, a low-pass filter with a double
pole at 2 Hz was implemented to prevent false ring trip.

The following example illustrates how the detection cir-
cuit of Figure 31 will trip at 12.5 mA dc loop current
using a 48 V battery.

= 17.9 µA

The current I

is repeated as I

in the positive compar-

ator input. The voltage at comparator input R

TSP

is:

Using this equation and the values in the example, the
voltage at input R

TSP

is 12 V during ringing injection

LOOP(dc)

= 0). Input R

TSP

is therefore at a level of 5 V

below R

TSN

. When enough dc loop current flows

through R

TS1

to raise its dc drop to 5 V, the comparator

will trip. In this example,

LOOP(dc)

= 12.5 mA

Ring Ground Detection

Pin ICM sinks a current proportional to the longitudinal
loop current. It is also connected to an internal compar-
ator whose output is pin RGDET. In a ground start
application where tip is open, the ring ground current is
half differential and half common mode. In this case, to
set the ring ground current threshold, connect a resis-
tor R

ICM

from pin ICM to V

. Select the resistor

according to the following relation:

The above equation is shown graphically in Figure 18.
It applies for the case of tip open. The more general
equation can be used in ground key application to
detect a common-mode current I

LOOP

15 k

7 V

= I

RTSN

RTS2

2 M

RTS1

RTS2

274 k

PHONE

HOOK SWITCH

RC PHONE

RING

BAT

NRDET

TS1

TSP

RTSN

0.022

0.27

402

TSP

(

)

2.289 k

-----------------------------

R T SP

BAT

L O O P dc

( )

T S 1

T S P

5 V

402

------------------

I CM

( )

228

(

)

----------------------

I C M

( )

114

(

)

----------------------

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

ac Design

There are four key ac design parameters. Termination
impedance is the impedance looking into the 2-wire
port of the line card. It is set to match the impedance of
the telephone loop in order to minimize echo return to
the telephone set. Transmit gain is measured from the
2-wire port to the PCM highway, while receive gain is
done from the PCM highway to the transmit port.
Finally, the hybrid balance network cancels the
unwanted amount of the receive signal that appears at
the transmit port.

At this point in the design, the codec needs to be
selected. The discrete network between the SLIC and
the codec can then be designed. Here is a brief codec
feature and selection summary.

First-Generation Codecs

These perform the basic filtering, A/D (transmit), D/A
(receive), and µ-law/A-law companding. They all have
an op amp in front of the A/D converter for transmit gain
setting and hybrid balance (cancellation at the summing
node). Depending on the type, some have differential
analog input stages, differential analog output stages,
and µ-law/A-law selectability. This generation of codecs
has the lowest cost. They are most suitable for applica-
tions with fixed gains, termination impedance, and hy-
brid balance.

Second-Generation Codecs

This class of devices includes a microprocessor inter-
face for software control of the gains and hybrid bal-
ance. The hybrid balance is included in the device. ac
programmability adds application flexibility and saves
several passive components and also adds several I/O
latches that are needed in the application. However, it
does not have the transmit op amp, since the transmit
gain and hybrid balance are set internally.

Third-Generation Codecs

This class of devices includes the gains, termination
impedance, and hybrid balance--all under micropro-
cessor control. Depending on the device, it may or may
not include latches.

Selection Criteria

In the following examples, use of a first-generation
codec is shown. The equations for second- and third-
generation codecs are simply subsets of these. There
are two examples. The first shows the simplest circuit,
which uses a minimum number of discrete components
to synthesize a real termination impedance. The sec-
ond example shows the use of the uncommitted op
amp to synthesize a complex termination. The design
has been automated in a DOS based program, avail-
able on request.

In the codec selection, increasing software control and
flexibility are traded for device cost. To help decide, it
may be useful to consider the following. Will the appli-
cation require only one value for each gain and imped-
ance? Will the board be used in different countries with
different requirements? Will several versions of the
board be built? If so, will one version of the board be
most of the production volume? Does the application
need only real termination impedance? Does the
hybrid balance need to be adjusted in the field?

Agere Systems Inc.

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Applications

(continued)

ac Design

(continued)

Selection Criteria (continued)

Example 1, Real Termination:

The following design equations refer to the circuit in
Figure 32. Use these to synthesize real termination
impedance.

Termination Impedance:

Receive Gain:

rcv

Transmit Gain:

Hybrid Balance:

bal

= 20log

To optimize the hybrid balance, the sum of the currents
at the VFX input of the codec op amp should be set to
0. The following expressions assume the test network
is the same as the termination impedance.

bal

= 20log

Example 2, Complex Termination:

For complex termination, the spare op amp is used
(see Figure 33).

The hybrid balance equation is the same as in Exam-
ple 1.

PCB Layout Information

Make the leads to BGND and V

BAT

as wide as possible

for thermal and electrical reasons. Also, maximize the
amount of PCB copper in the area of--and specifically
on--the leads connected to this device for the lowest
operating temperature.

When powering the device, ensure that no external
potential creates a voltage on any pin of the device that
exceeds the device ratings. In this application, some of
the conditions that cause such potentials during pow-
erup are the following: 1) an inductor connected to PT
and PR (this can force an overvoltage on V

BAT

through

the protection devices if the V

BAT

connection chatters),

and 2) inductance in the V

BAT

lead (this could resonate

with the V

BAT

filter capacitor to cause a destructive

overvoltage).

This device is normally used on a circuit card that is
subjected to hot plug-in, meaning the card is plugged
into a biased backplane connector. In order to prevent
damage to the IC, all ground connections must be
applied before, and removed after, all other connec-
tions.

T R

--------------

1000

T 1

G P

---------

T 1

R C V

------------

-----------------------------------

T R

--------------

RCV

---------------

RCV

---------------

T/R

-------------

-------------------------------------------------------------------------------------

GSX

T R

---------------

-----------

125

T R

-------------

GSX

---------------

------------

rcv

-------------------------

1000

T 3

G N

---------

T 3

R C V

------------

-----------------------------------

T 5

---------

(

)

k Z

(

)

r cv

RCV

--------------

RCV

G N

--------------

T/R

----------

-----------------------------------------------------------------------------

t x

T 6

-----------

125

T/R

----------

T 5

---------

Data Sheet

January 2000

with Battery Switch

L7556, L7557 Low-Power SLICs

Agere Systems Inc. reserves the right to make changes to the product(s) or information contained herein without notice. No liability is assumed as a result of their use or application.

January 2000
DS00-060ALC (Replaces DS97-172ALC)

For additional information, contact your Agere Systems Account Manager or the following:
INTERNET:

http://www.agere.com

E-MAIL:

docmaster@agere.com

N. AMERICA:

Agere Systems Inc., 555 Union Boulevard, Room 30L-15P-BA, Allentown, PA 18109-3286
1-800-372-2447, FAX 610-712-4106 (In CANADA: 1-800-553-2448, FAX 610-712-4106)

ASIA:

Agere Systems Hong Kong Ltd., Suites 3201 & 3210-12, 32/F, Tower 2, The Gateway, Harbour City, Kowloon
Tel. (852) 3129-2000, FAX (852) 3129-2020
CHINA: (86) 21-5047-1212 (Shanghai), (86) 10-6522-5566 (Beijing), (86) 755-695-7224 (Shenzhen)
JAPAN: (81) 3-5421-1600 (Tokyo), KOREA: (82) 2-767-1850 (Seoul), SINGAPORE: (65) 6778-8833, TAIWAN: (886) 2-2725-5858 (Taipei)

EUROPE:

Tel. (44) 7000 624624, FAX (44) 1344 488 045

Ordering Information

Device Part No.

Description

Package

Comcode

ATTL7556AAU

Low-Power SLIC with Battery Switch

32-Pin PLCC

107385668

ATTL7556AAU-TR

Low-Power SLIC with Battery Switch

32-Pin PLCC (Tape and Reel)

107749509

ATTL7557AAU

Low-Power SLIC with Battery Switch

32-Pin PLCC

107385841

ATTL7557AAU-TR

Low-Power SLIC with Battery Switch

32-Pin PLCC (Tape and Reel)

107749517

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Document Outline

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