March 1997
ML4632
Fiber Optic LED Driver
BLOCK DIAGRAM
FEATURES
s
Current Driven Output for accurate Launch Power
s
Programmable output current from 20mA to 100mA
s
Programmable temperature coefficient, 0 to 3300ppm/
C
s
High Efficiency Output Stage
s
Programmable LED pre-bias current
s
Low EMI/RFI Noise
s
ECL or TTL inputs
s
Optional Peaking circuit
APPLICATIONS
s
IEEE 802.3, 10BASE-F
s
IEEE 802.5 Fiber Optic Token Ring
s
IEEE 802.4 Fiber Optic Token Bus
s
Fiber Optic Data Communications and
Telecommunications
GENERAL DESCRIPTION
The ML4632 is a fiber optic LED driver suited for network
applications up to 20Mbps. The part is capable of driving
up to 100mA of current through a Fiber Optic LED from
an ECL or TTL level input signal. Its efficient output stage
provides a high current that can be programmed for
accurate absolute output level as well as automatic
temperature compensation. The combination of automatic
temperature compensation and a highly accurate current
driven design insures precise launch power.
The LED driver's output stage provides fast, well matched
rise and fall times through a unique class B output stage
that burns supply current only when the LED is on. A
positive temperature coefficient of up to 3300ppm/
C can
be programmed into the output current to compensate for
the negative temperature coefficient of the LED optical
output power. An optional peaking circuit may also be
employed.
The ECL and TTL inputs are ANDed so one can be used
for data and the other for an enable input. An ECL
compatible BIAS voltage is also provided for single ended
ECL applications.
ECLP
BUFFER
PEAK
DRIVER
PTAT
DRV
AMP
REF
+
14
1
2
13
8
4
9
12
10
11
5
3
6
7
ECLN
VBIAS
TTL
V
CC
GND
I
OFF
PTAT
VREF
DRV
RTSET
LED
RPK
PEAK
1
ML4632
2
PIN CONFIGURATION
NAME
FUNCTION
ECLN
Negative ECL data input. Tie to VBIAS for
single ended ECL operation or when ECLP is
used as an enable. Tie to ground during TTL
only operation.
VBIAS
BIAS voltage for single ended ECL operation.
LED
Fiber optic LED drive pin. Connect the LED
between this pin and V
CC
.
GND
Negative power supply. The pin should be
tied to the grounded side of RTSET to improve
output accuracy and avoid a ground loop.
RTSET
Output current programming pin. Connect a
resistor of value V
DRV
/I
LED
from this pin to
ground to set the high LED output current.
RPK
Peaking circuit bias pin. Connect a resistor of
value V
DRV
/I
PEAK
from this pin to ground
when using the peaking circuit. Leave open
circuited when peaking is not used.
PEAK
Peaking circuit output pin. When using
peaking, connect this pin to V
CC
through a
resistor of value RRPK. Then connect a
capacitor from this pin to the LED cathode.
When peaking is not used, open circuit RPK.
PIN DESCRIPTION
NAME
FUNCTION
V
CC
Positive power supply. +5 volts.
I
OFF
Connect a resistor from this pin to V
CC
to
increase the off current to the LED, i.e. 4.3K
for 1mA. With this pin open, the default I
OFF
current is between 0.51.0mA.
VREF
A constant 1.2V reference output used to set
up DRV.
DRV
A DC input that sets the positive swing on
RTSET and the high level output current to
the LED.
PTAT
Proportional to Absolute Temperature. A 1.0V
reference at 25
C that moves proportional to
absolute temperature, also used to set up
DRV. (See figure 1)
TTL
TTL data input. Can also be used as an enable
during ECL operation. TTL = High (enabled),
TTL = Low (disabled).
ECLP
Positive ECL data input controls signal to the
LED. Tie to VBIAS during TTL only operation
or use as an enable.
ECLP
TTL
PTAT
DRV
VREF
I
OFF
V
CC
ECLN
VBIAS
LED
GND
RTSET
RPK
PEAK
14
13
12
11
10
9
8
1
2
3
4
5
6
7
TOP VIEW
ML4632
14-Pin PDIP (P14)
ML4632
16-Pin Wide SOIC (S16W)
TOP VIEW
ECLN
VBIAS
LED
GND
RTSET
RPK
PEAK
NC
ECLP
TTL
PTAT
DRV
VREF
I
OFF
V
CC
NC
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
ML4632
3
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.
V
CC ...............................................................................
0.3V to 6V
Input Pin Voltages ............................. 0.3V to V
CC
+0.3V
LED Output Current ............................................. 120mA
PEAK DC Output Current ..................................... 120mA
Storage Temperature .............................. 65
C to +150
C
Lead Temperature (Soldering 10 sec.) ..................... 260
C
ELECTRICAL CHARACTERISTICS
Over the recommended operating conditions of T
A
= 0
C to 70
C, V
CC
= 5V
5%, unless otherwise specified. (Note 1)
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
I
CC
Supply Current
LED off
25
35
mA
VREF
VREF Voltage
No Load
1.14
1.20
1.26
V
V
PTAT
PTAT Voltage
No Load, T
A
= 25
C
0.9
1.0
1.1
V
T
A
= 85
C
1.08
1.2
1.32
V
V
OS
Driver Offset
VDRV = 1.2V, RTSET = 20
50
mV
LED Current Accuracy
VDRV = VREF, RTSET = 20
I
LEDH
High
I
OFF
= open
54
60
66
mA
I
LEDL
Low
0.5
0.7
1.0
mA
t
R
Rise Time
VDRV = VREF, RTSET = 20
4.5
ns
t
F
Fall Time
VDRV = VREF, RTSET = 20
4.5
ns
Propagation Delay
VDRV = VREF, RTSET = 20
t
PLH
Low to High
TTL and ECL
10.0
ns
t
PHL
High to Low
10.0
ns
t
PWD
Pulse Width Distortion
VDRV = VREF, RTSET = 20
1.0
2.0
ns
V
PK
Peaking Voltage
R
RPK
= 20
, C
PK
= 100pF, R
PEAK
= 20
1.08
1.2
1.32
V
V
PKTR
Peaking Rise Time
R
RPK
= 20
, C
PK
= 100pF, R
PEAK
= 20
4.5
ns
V
PKTF
Peaking Fall Time
R
RPK
= 20
, C
PK
= 100pF, R
PEAK
= 20
4.5
ns
I
ECL
ECL Input Current
20
A
I
TTL
TTL Input Current
100
A
V
DO
Dropout Voltage between
pin 5 and 3
1.5
V
I
OFF
Additional LED Off Current
V
CC
= 5V, R
IOFF
= 4.3K
0.8
1.0
1.2
mA
VBIAS
ECL BIAS Voltage
V
CC
= 5V, T
A
= 25
C
3.8
V
Note 1: Limits are guaranteed by 100% testing, sampling or correlation with worst-case test conditions.
Note 2: Low Duty cycle pulse testing is performed at T
A
.
ML4632
4
FUNCTIONAL DESCRIPTION
The ML4632 accepts ECL and TTL input signals and
generates a high speed, high accuracy output current
which is independent of supply voltage variations. The
output current is programmable from 20mA to 100mA.
A temperature coefficient can be programmed into the
output current and a peaking circuit can be added with
a few external components.
The input of the LED driver accepts both ECL and TTL
signals. The ECL input stage is a standard NPN differential
pair with a common mode range of between 3V and 4.5V
with a +5V supply. A bias voltage VBIAS is available for
biasing either ECL input for single-ended operation. The
TTL input has a standard switching range of between 0.8V
and 2.0V. These inputs are ANDed so that the extra input
can be used as an enable.
Output current to the LED is set by connecting the
appropriate resistance from RTSET to ground. With the
VREF and DRV pins tied together, the high level output
voltage at RTSET will be 1.2V. The current through the
LED. The output current with RTSET set to 20
will be
I
LED
(HIGH) = 1.2V/R
TSET
= 1.2V/20
= 60mA.
The low level output current is set internally by a resistor
at approximately 0.7mA. This current prebiases the LED
and results in faster optical rise times. The value of this
current can be increased by connecting a resistor from the
I
OFF
pin to V
CC
. The additional current will be equal to
(V
CC
0.7V)/R
IOFF
.
The voltage input at the DRV pin appears across the
RTSET pin when the LED is turned on. The current in
RTSET is directed through the LED. Therefore the voltage
set at DRV along with the RTSET resistor sets current
through the LED.
A temperature coefficient of between 0ppm/
C and
3300ppm/
C can be programmed into the high level
output current to compensate for the drop in LED optical
output power at high temperatures. This is accomplished
by driving the DRV pin from a resistor divider between the
VREF and PTAT pins.
When DRV is tied directly to PTAT, the peak voltage at
RTSET will be 1.0V at 25
C and have a 3300ppm/
C
temperature coefficient. At 85
C, PTAT is 1.2V and equal
to VREF. An arbitrary temperature coefficient less than
3300 ppm/
C can be set by using a resistor divider
between PTAT and VREF to set the voltage at DRV, as
shown in figure 1.
REF
PTAT
DRV
AMP
TO
DRIVER
DRV
(11)
VREF
(10)
PTAT
(12)
R1
R2
Figure 1. Current for Programming
Output Temperature Coefficient
In this configuration the temperature coefficient is
TC
ppm C
R
R
R
and
I
V
V
R
R
R
RTSET
ILED
LED HIGH
=
+
=
+
+
(
/
)
,
.
(
)
3300
1
1
2
1
0 2
2
1
2
The output current will be a linear function of temperature.
A plot of I
LED
versus temperature for several values of the
programming resistance, R1 and R2, in figure 2.
60
55
50
45
0
25
50
75 85
T(C)
I
LED
(mA)
R1 = 1
R2 = 3R1
R1 = R2
R1 = 3R2
R2 = 0
NOTE: R
1
+ R
2
10k
Figure 2. I
LED
vs T, R
TSET
= 20
ML4632
5
The ML4632 output stage conducts full load current only
when the LED is on, and even then power dissipation in
the part is low because most of the +5V supply voltage is
dropped across the LED and external resistor R
TSET
. Even
with a low power design, the LED driver junction
temperature will rise above ambient due to quiescent
power dissipation and won't exactly match the LED
junction temperature since it is also self-heating.
Therefore, the effectiveness of a temperature compensated
design will be related to component power dissipations,
thermal conductance of the PC board and packaging, and
the proximity of the LED driver to the LED.
The ML4632 also provides for peaking of the LED output
current. Peaking is used to counteract the effects of the
LED junction capacitance. By creating a controlled
overshoot and undershoot in the output current waveform,
charge is transferred to and from the LED capacitance on
the rising and falling edges of the output, speeding up rise
and fall times.
To provide peaking current, a second output stage is
biased up with a resistor from RPK to ground and another
from PEAK to V
CC
. When these bias resistors are set equal
to each other, a pulse will be generated across the R
PEAK
resistor with a magnitude equal to the voltage on the DVR
pin. A coupling capacitor transfers the rising and falling
edges of the output current waveform.
A typical application is shown in figure 3. When the
resistors R
RPK
and R
PEAK
are both set to 20
, a pulse will
be generated at the PEAK pin of magnitude 1.2V and
equivalent resistance 20
(assuming V
DRV
= 1.2V).
PEAK
PEAK
RPK
RPEAK
20
RTSET
20
LED
RTSET
RPK
20
CPEAK
100pF
DRIVER
Figure 3. Application of the Peaking Circuit
The peaking current is coupled through the 100pF
capacitor, C
PEAK
, which will transfer 120pC of charge to
and from the LED on each cycle of output current. The
peaking circuit shown provides approximately a 70%
overshoot current into a 0
LED impedance. Peaking
currents will be slightly lower for real LED's.
ECLN
VBIAS
LED
GND
RTSET
RPK
PEAK
20
ECLP
TTL
PTAT
DRV
VREF
I
OFF
V
CC
0.1
F
CPEAK
100pF
20
20
I
OUT
I
OUT
= 60mA
I
OFF
= 0.7mA
TTL IN
+5V
4.7
F
Note:
The LED, PEAK and V
CC
traces should be very short and shielded with a
GND plane to reduce ringing and overshoot at the LED.
TTL Driven Implementation
(No Temp. Comp)
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