1
LT1612
s
Operates from Input Voltage As Low As 2V
s
Internal 0.7A Synchronous Switches
s
Uses Ceramic Input and Output Capacitors
s
620mV Reference Voltage
s
800kHz Fixed Frequency Switching
s
Programmable Burst Mode Operation
The LT
1612 is an 800kHz, synchronous step-down DC/
DC converter that operates from an input voltage as low
as 2V. Internal 0.45
switches deliver output currents up
to 500mA, and the 800kHz switching frequency allows the
use of small, low value ceramic input and output capaci-
tors. Input voltage ranges from 5.5V down to 2V and
output voltage can be set as low as the 620mV reference.
The device features Burst Mode
TM
operation, keeping
efficiency high at light loads. Burst Mode operation can be
defeated by pulling the MODE pin high, enabling constant
switching throughout the load range for low noise.
No-load quiescent current is 160
A and shutdown current
is less than 1
A. The device is available in 8-lead SO and
MSOP packages.
s
Portable Devices
s
Lithium-Ion Step-Down Converters
s
5V to 3.3V Conversion
s
2-Cell Alkaline Step-Down Converters
Figure 1. 2V to 1.2V Converter
Synchronous, Step-Down
800kHz PWM
DC/DC Converter
April 2000
FEATURES
DESCRIPTIO
N
U
APPLICATIO
N
S
U
TYPICAL APPLICATIO
N
U
Final Electrical Specifications
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
V
IN
2V
0.1
F
33k
R2
232k
1%
R1
215k
1%
L1
10
H
330pF
C1
10
F
C2
68
F
3.15V
C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100
1612 F01a
V
OUT
1.2V
500mA
100pF
V
IN
SW
FB
BOOST
MODE
SHDN
V
C
GND
LT1612
LOAD CURRENT (mA)
EFFICIENCY (%)
90
80
70
60
50
40
30
100
10
500
1612 F01b
V
IN
= 2V (LINEAR)
V
IN
= 3V (LINEAR)
V
IN
= 2V
V
IN
= 3V
Efficiency for LT1612 vs Linear Regulator
V
OUT
= 1.2V
2
LT1612
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
Q
Quiescent Current
MODE = 5V
q
1
2
mA
MODE = 0V, Not Switching
q
160
220
A
SHDN = 0V
q
1
A
V
FB
FB Voltage
0.605
0.62
0.635
V
q
0.60
0.62
0.635
V
FB Line Regulation
q
0.02
0.15
%/V
FB Pin Bias Current (Note 3)
q
7
50
nA
g
m
Error Amplifier Transconductance
250
mhos
Min Input Voltage
2
V
Max Input Voltage
5.5
V
f
OSC
Oscillator Frequency
700
800
900
kHz
q
550
1100
kHz
f
OSC
Line Regulation
1
%/ V
Maximum Duty Cycle
85
90
%
q
80
%
Shutdown Threshold
Minimum Voltage for Active
q
2
V
Maximum Voltage for Shutdown
q
0.2
V
LT1612EMS8
T
JMAX
= 125
C,
JA
= 120
C/ W
ORDER PART
NUMBER
Supply Voltage (V
IN
) ............................................... 5.5V
SW Pin Voltage ....................................................... 5.5V
FB Pin Voltage ............................................... V
IN
+ 0.3V
V
C
Pin Voltage ........................................................... 2V
SHDN Pin Voltage ................................................... 5.5V
MODE Pin Voltage .................................................. 5.5V
(Note 1)
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ELECTRICAL CHARACTERISTICS
Consult factory for Military grade parts.
BOOST Pin Voltage ....................................... V
IN
+ 5.5V
Junction Temperature ........................................... 125
C
Operating Temperature Range (Note 2) ... 40
C to 85
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
LT1612ES8
ORDER PART
NUMBER
MS8 PART MARKING
T
JMAX
= 125
C,
JA
= 120
C/ W
S8 PART MARKING
1612
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25
C, VIN = VSHDN = 3V
V
C
FB
V
IN
GND
1
2
3
4
8
7
6
5
SHDN
MODE
BOOST
SW
TOP VIEW
MS8 PACKAGE
8-LEAD PLASTIC MSOP
TOP VIEW
SHDN
MODE
BOOST
SW
V
C
FB
V
IN
GND
S8 PACKAGE
8-LEAD PLASTIC SO
1
2
3
4
8
7
6
5
LTMS
3
LT1612
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SHDN Pin Current
SHDN = 2V
q
10
15
A
SHDN = 5V
q
30
45
A
BOOST Pin Current
BOOST = V
IN
+ 2V
4
mA
Switch Current Limit (Note 4)
Duty Cycle = 0%
MODE = OV
600
710
900
mA
MODE = 5V
550
650
900
mA
Burst Mode Operation Current Limit
MODE = 0V
180
mA
Switch Voltage Drop
I
SW
= 500mA
200
280
mV
Rectifier Voltage Drop
I
RECT
= 500mA
300
400
mV
SW Pin Leakage
V
SW
= 5V
q
1
A
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The LT1612E is guaranteed to meet performance specifications
from 0
C to 70
C. Specifications over the 40
C to 85
C operating
ELECTRICAL CHARACTERISTICS
V
C
(Pin 1): Compensation Pin. This is the current sink/
source output of the error amplifier. By connecting an RC
network from this pin to ground, frequency response can
be tuned for a wide range of circuit configurations. The
voltage at this pin also sets the current limit, and if
grounded, the switch will remain in the OFF state.
FB (Pin 2): Feedback Pin. This pin is the negative input to
the error amplifier. Connect the resistor divider tap to this
point which sets V
OUT
according to:
V
OUT
= 0.62V (1 + R1/R2)
V
IN
(Pin 3): Supply Pin. Bypass capacitor C1 must be right
next to this pin.
GND (Pin 4): Ground Pin. Connect directly to local ground
plane.
PI
N
FU
N
CTIO
N
S
U
U
U
SW (Pin 5): Switch Pin. Connect inductor and boost
capacitor here. Minimize trace area at this pin to keep EMI
down.
BOOST (Pin 6): This is the supply pin for the switch driver
and must be above V
IN
by 1.5V for proper switch opera-
tion. Connect the boost capacitor to this pin.
MODE (Pin 7): Burst Mode Operation Disable Pin. For
continuous switching operation (low noise), pull this pin
above 2V. For Burst Mode operation which gives better
light load efficiency, tie to ground. Output ripple voltage in
Burst Mode operation is typically 30mV
P-P
. See applica-
tions section for more information about this function.
SHDN (Pin 8): Shutdown Pin. Pull this pin low for shut-
down mode. Tie to a voltage between 2V and 5.5V for
normal operation.
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25
C, VIN = VSHDN = 3V
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3: Bias current flows out of the FB pin.
Note 4: Duty cycle affects current limit due to slope compensation.
4
LT1612
OPERATIO
U
The LT1612 employs fixed frequency, current mode con-
trol. This type of control uses two feedback loops. The
main control loop sets output voltage and operates as
follows: A load step causes V
OUT
and the FB voltage to be
perturbed slightly. The error amplifier responds to this
change in FB by driving the V
C
pin either higher or lower.
Because switch current is proportional to the V
C
pin
voltage, this change causes the switch current to be
adjusted until V
OUT
is once again satisfied. Loop compen-
sation is taken care of by an RC network from the V
C
pin
to ground.
Inside this main loop is another that sets current limit on
a cycle-by-cycle basis. This loop utilizes current compara-
tor A2 to control peak current. The oscillator runs at
800kHz and issues a set pulse to the flip-flop at the
beginning of each cycle, turning the switch on. With the
switch now in the ON state the SW pin is effectively
connected to V
IN
. Current ramps up in the inductor linearly
at a rate of (V
IN
V
OUT
)/L. Switch current is set by the V
C
pin voltage and when the voltage across R
SENSE
trips the
current comparator, a reset pulse will be generated and the
switch will be turned off. Since the inductor is now loaded
up with current, the SW pin will fly low and trigger the
rectifier to turn on. Current will flow through the rectifier
decreasing at a rate of V
OUT
/L until the oscillator issues a
new set pulse, causing the cycle to repeat.
If the load is light and V
C
decreases below A3's trip point,
the device will enter the Burst Mode operation region (the
MODE pin must be at ground or floating). In this state the
oscillator and all other circuitry except the reference and
comparator A3 are switched on and off at low frequency.
This mode of operation increases efficiency at light loads
but introduces low frequency voltage ripple at the output.
For continuous switching and no low frequency output
voltage ripple, pull the MODE pin high. This will disable
comparator A3 which forces the oscillator to run
continuously.
BLOCK DIAGRA
W
+
R
SENSE
0.08
+
A1
V/I
SLOPE
COMPENSATION
SWITCH
SW
GND
1612 BD
BOOST
A3
ENABLE
+
A2
0.62V
0.7V
BOOST DIODE
SWITCH
DRIVER
V
IN
V
C
FB
RECTIFIER
RECTIFIER
DRIVE
3
1
2
5
4
6
MODE
7
8
SHDN
SHUTDOWN
OSCILLATOR
R
Q
S
FLIP-FLOP
5
LT1612
Figure 2. Recommended Component Placement. Traces
Carrying High Current are Direct. Trace Area at FB Pin and V
C
Pin Is Kept Low. Lead Length to Battery Should Be Kept Short
Layout Hints
The LT1612 switches current at high speed, mandating
careful attention to layout for proper performance.
You
will not get advertised performance with careless layouts.
Figure 2 shows recommended component placement for
a buck (step-down) converter. Follow this closely in your
PC layout. Note the direct path of the switching loops.
Input capacitor C1 must be placed close (< 5mm) to the IC
package. As little as 10mm of wire or PC trace from C
IN
to
V
IN
will cause problems such as inability to regulate or
oscillation.
The ground terminal of input capacitor C1 should tie close
to Pin 4 of the LT1612. Doing this reduces dI/dt in the
ground copper which keeps high frequency spikes to a
minimum. The DC/DC converter ground should tie to the
PC board ground plane at one place only, to avoid intro-
ducing dI/dt in the ground plane.
OPERATIO
U
Figure 3. Output Voltage Ripple is 20mV
P-P
for
the Circuit of Figure 1
Burst Mode Operation Defeat
To maintain high efficiency at light loads, the LT1612 will
automatically shift into Burst Mode operation (MODE = 0V
or floating). In this mode of operation the oscillator and
switch drive circuitry is alternately turned on and off,
reducing quiescent current to 160
A. This reduces power
consumption but also adds low frequency voltage ripple to
the output. Figure 3 shows switching waveforms for a 5V
to 3.3V converter running in Burst Mode operation. Output
voltage ripple is approximately 20mV
P-P
. If the MODE pin
is pulled high, Burst Mode operation will be inhibited and
the oscillator runs continuously with no low frequency
ripple at the output. See Figures 4 and 5.
Figure 4. Transient Response for the Circuit of Figure 1
with the MODE Pin Tied to Ground or Floating
Figure 5. With the MODE Pin Tied High, Low
Frequency Output Voltage Ripple Is No Longer Present
1612 F03
V
OUT
20mV/DIV
AC COUPLED
IL
200mA/DIV
5
s/DIV
1612 F04
V
OUT
200mV/DIV
AC COUPLED
I
LOAD
10mA TO 310mA
0.1ms/DIV
I
L
200mA/DIV
1612 F05
V
OUT
200mV/DIV
AC COUPLED
I
LOAD
10mA TO 300mA
0.1ms/DIV
I
L
200mA/DIV
1
2
8
7
3
4
6
5
L1
C2
R1
C3
LT1612
V
OUT
V
IN
GND
SHDN
1612 F02
MODE
MULTIPLE
VIAs
C1
R2
R
C
C
C
6
LT1612
V
IN
V
IN
2.7V TO 4.2V
SW
FB
BOOST
MODE
BURSTMODE
SHDN
V
C
0.1
F
30k
453k
1%
1M
1%
L1
10
H
680pF
10
F
CERAMIC
GND
LT1612
22
F
CERAMIC
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1612 TA02
V
OUT
2V
500mA
20pF
LOAD CURRENT (mA)
1
EFFICIENCY (%)
85
80
75
70
65
60
55
50
10
100
1000
1612 TA04
V
IN
= 2.8V
V
IN
= 4.2V
V
IN
= 3.5V
Single Li-Ion to 2V Converter
Burst Mode Operation
Li-Ion to 2V Converter Efficiency
Inrush Current at Start-Up
V
OUT
2V/DIV
V
SHDN
5V/DIV
INRUSH
CURRENT
200mA/DIV
0.2ms/DIV
1612 TA06
TYPICAL APPLICATIO
N
S
U
100
s/DIV
Transient Response
1612 TA03
V
IN
= 4V
V
OUT
= 2V
MODE = HIGH
LOAD STEP
125mA TO 300mA
I
L
200mA/DIV
V
OUT
50mV/DIV
V
OUT
20mV/DIV
I
L
100mA/DIV
MODE = LOW
1612 TA05
5
s/DIV
7
LT1612
Dimension in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTIO
N
U
MS8 Package
8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
V
IN
V
IN
5V
SW
FB
BOOST
MODE
BURSTMODE
SHDN
V
C
0.1
F
30k
332k
1M
L1
10
H
680pF
C1
10
F
CERAMIC
GND
LT1612
C2
22
F
CERAMIC
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1612 TA07
V
OUT
2.5V
500mA
20pF
LOAD CURRENT (mA)
1
EFFICIENCY (%)
85
80
75
70
65
60
55
50
10
100
1000
1612 TA08
5V to 2.5V Converter
5V to 2.5V Converter Efficiency
TYPICAL APPLICATIO
N
S
U
MSOP (MS8) 1197
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,
PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
0.021
0.006
(0.53
0.015)
0
6
TYP
0.007
(0.18)
1
2
3
4
0.192
0.004
(4.88
0.10)
8
7 6
5
0.118
0.004*
(3.00
0.102)
0.118
0.004**
(3.00
0.102)
SEATING
PLANE
0.040
0.006
(1.02
0.15)
0.012
(0.30)
REF
0.006
0.004
(0.15
0.102)
0.034
0.004
(0.86
0.102)
0.0256
(0.65)
TYP
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
SO8 0996
0.016 0.050
(0.406 1.270)
0.010 0.020
(0.254 0.508)
45
0
8
TYP
0.008 0.010
(0.203 0.254)
0.053 0.069
(1.346 1.752)
0.014 0.019
(0.355 0.483)
0.004 0.010
(0.101 0.254)
0.050
(1.270)
TYP
1
2
3
4
0.150 0.157**
(3.810 3.988)
8
7
6
5
0.189 0.197*
(4.801 5.004)
0.228 0.244
(5.791 6.197)
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
*
**
8
LT1612
1612i LT/TP 0400 4K PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1999
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX: (408) 434-0507
q
www.linear-tech.com
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC
1474
Low I
Q
Step-Down Switching Regulator
10
A I
Q
, V
IN
from 3V to 18V, MSOP Package up to 300mA
LTC1701
SOT-23 Step-Down Switching Regulator
500mA in SOT-23 Package, 1MHz Switching Frequency
LTC1707
Monolithic Synchronous Step-Down Switching Regulator
500mA, V
IN
from 2.65V to 8.5V
V
IN
V
IN
2V
SW
FB
BOOST
MODE
SHDN
V
C
0.1
F
33k
R2
232k
R1
105k
L1
10
H
330pF
C1
10
F
GND
LT1612
C2
68
F
3.15V
C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100
1612 TA09
V
OUT
0.9V
500mA
100pF
LOAD CURRENT (mA)
1
EFFICIENCY (%)
80
70
60
50
40
30
20
10
10
100
1000
1612 TA10
V
IN
= 2V
V
IN
= 2V (LINEAR)
V
IN
= 3V
(LINEAR)
V
IN
= 3V
2V to 0.9V Converter
Efficiency for LT1612 vs Linear Regulator.
V
OUT
= 0.9V.
TYPICAL APPLICATIO
N
S
U
V
IN
5V
C3
0.1
F
R3
33k
R2
232k
1%
R1
1M
1%
L1
10
H
C4
680pF
C1
10
F
C2
22
F
C1: TAIYO-YUDEN LMK325BJ106MN
C2: TAIYO-YUDEN LMK325BJ226MN
L1: SUMIDA CD43-100
1612 TA01a
V
OUT
3.3V
500mA
20pF
V
IN
SW
FB
BOOST
MODE
SHDN
V
C
GND
LT1612
LOAD CURRENT (mA)
1
EFFICIENCY (%)
85
80
75
70
65
60
55
50
10
100
1000
1612 TA01b
V
IN
= 5V
V
OUT
= 3.3V
Efficiency
5V to 3.3V Converter
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