Datasheet ML4632CS, ML4632CP Datasheet (Micro Linear Corporation)

March 1997

ML4632

Fiber Optic LED Driver

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.

FEATURES

■ Current Driven Output for accurate Launch Power

■ Programmable output current from 20mA to 100mA

■ Programmable temperature coefficient, 0 to 3300ppm/°C

■ High Efficiency Output Stage

■ Programmable LED pre-bias current

■ Low EMI/RFI Noise

■ ECL or TTL inputs

■ Optional Peaking circuit

APPLICATIONS

■ IEEE 802.3, 10BASE-F

■ IEEE 802.5 Fiber Optic Token Ring

■ IEEE 802.4 Fiber Optic Token Bus

■ Fiber Optic Data Communications and

Telecommunications

BLOCK DIAGRAM

ECLP

ECLN

VBIAS

TTL

V

CC

GND

PEAK

+

14

BUFFER
–

1

2

13

8

REF

4

9

I

OFF

PEAK

DRIVER

DRV
AMP

PTAT

7

6

RPK

LED

3

RTSET

5

11

DRV

VREF

10

PTAT

12

1

ML4632

PIN CONFIGURATION

ML4632

14-Pin PDIP (P14)

ECLN

VBIAS

LED

GND

RTSET

RPK

PEAK

1

2

3

4

5

6

7

TOP VIEW

ECLP

14

TTL

13

PTAT

12

DRV

11

VREF

10

I

9

OFF

V

8

CC

PIN DESCRIPTION

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 VCC.

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/ILED

ground to set the high LED output current.

RPK Peaking circuit bias pin. Connect a resistor of

value V

DRV/IPEAK

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 VCC 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.

from this pin to

ML4632

16-Pin Wide SOIC (S16W)

ECLN

VBIAS

LED

GND

RTSET

RPK

PEAK

NC

NAME FUNCTION

V

I

OFF

CC

Positive power supply. +5 volts.

Connect a resistor from this pin to VCC to

1

2

3

4

5

6

7

8

TOP VIEW

ECLP

16

TTL

15

PTAT

14

DRV

13

VREF

12

I

11

10

9

V

NC

OFF

CC

increase the off current to the LED, i.e. 4.3KΩ

for 1mA. With this pin open, the default I

OFF

current is between 0.5–1.0mA.

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.

2

ABSOLUTE MAXIMUM RATINGS

ML4632

Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional

PEAK DC Output Current ..................................... 120mA

Storage Temperature .............................. –65°C to +150°C

Lead Temperature (Soldering 10 sec.) ..................... 260°C

device operation is not implied.

V

............................................................................... –0.3V to 6V

CC

Input Pin Voltages ............................. –0.3V to VCC +0.3V

LED Output Current ............................................. 120mA

ELECTRICAL CHARACTERISTICS

Over the recommended operating conditions of T

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

I

CC

VREF VREF Voltage No Load 1.14 1.20 1.26 V

V

PTAT

V

I

LEDH

I

LEDL

t

t

PLH

t

PHL

t

PWD

V

V

PKTR

V

PKTF

I

ECL

I

TTL

V

I

OFF

VBIAS ECL BIAS Voltage VCC = 5V, T

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

OS

R

t

F

PK

DO

Supply Current LED off 25 35 mA

PTAT Voltage No Load, T

Driver Offset VDRV = 1.2V, RTSET = 20Ω 50 mV
LED Current Accuracy VDRV = VREF, RTSET = 20Ω

High I

OFF

Low 0.5 0.7 1.0 mA

Rise Time VDRV = VREF, RTSET = 20Ω 4.5 ns
Fall Time VDRV = VREF, RTSET = 20Ω 4.5 ns
Propagation Delay VDRV = VREF, RTSET = 20Ω

Low to High TTL and ECL 10.0 ns
High to Low 10.0 ns

Pulse Width Distortion VDRV = VREF, RTSET = 20Ω 1.0 2.0 ns

Peaking Voltage R

Peaking Rise Time R

Peaking Fall Time R

ECL Input Current 20 µA
TTL Input Current 100 µA

Dropout Voltage between
pin 5 and 3 1.5 V

Additional LED Off Current VCC = 5V, R

.

A

= 0°C to 70°C, VCC = 5V ±5%, unless otherwise specified. (Note 1)

A

= 25°C 0.9 1.0 1.1 V

A

T

= 85°C 1.08 1.2 1.32 V

A

= open 54 60 66 mA

= 20Ω, C

RPK

= 20Ω, C

RPK

= 20Ω, C

RPK

= 100pF, R

PK

= 100pF, R

PK

= 100pF, R

PK

= 4.3KΩ 0.8 1.0 1.2 mA

IOFF

= 25°C 3.8 V

A

= 20Ω 1.08 1.2 1.32 V

PEAK

= 20Ω 4.5 ns

PEAK

= 20Ω 4.5 ns

PEAK

3

ML4632

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 20Ω will be

I

(HIGH) = 1.2V/R

LED

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

pin to VCC. The additional current will be equal to

OFF

(VCC – 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.

= 1.2V/20Ω = 60mA.

TSET

VREF

(10)

R1

DRV

REF

PTAT

PTAT

(12)

R2

(11)

DRV
AMP

TO
DRIVER

Figure 1. Current for Programming

Output Temperature Coefficient

In this configuration the temperature coefficient is

R

TC ppm C

I

LED HIGH

(/),

=°

3300

ILED

.

VV

+

102

=

()

RTSET

1

RR

12



R





RR

12

and

+



2





+

The output current will be a linear function of temperature.
A plot of I

versus temperature for several values of the

LED

programming resistance, R1 and R2, in figure 2.

60

55

(mA)

LED

I

50

45

R1 = 1Ω

R2 = 3R1

R1 = R2

R1 = 3R2

R2 = 0Ω

NOTE: R

025507585

T(°C)

Figure 2. I

LED

vs T, R

+ R

1

TSET

≥ 10kΩ

2

= 20Ω

4

ML4632

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 VCC. 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

are both set to 20Ω, a pulse will

PEAK

be generated at the PEAK pin of magnitude 1.2V and

equivalent resistance 20Ω (assuming V

PEAK

PEAK

RPK

RPEAK
20Ω

RPK
20Ω

CPEAK

100pF

RTSET

20Ω

DRV

= 1.2V).

LED

DRIVER

RTSET

Figure 3. Application of the Peaking Circuit

The peaking current is coupled through the 100pF
capacitor, C

, which will transfer 120pC of charge to

PEAK

and from the LED on each cycle of output current. The
peaking circuit shown provides approximately a 70%

overshoot current into a 0Ω LED impedance. Peaking

currents will be slightly lower for real LED’s.

I

OUT

20Ω
20Ω

CPEAK
100pF

0.1µF

Note: The LED, PEAK and VCC traces should be very short and shielded with a

GND plane to reduce ringing and overshoot at the LED.

20Ω

TTL Driven Implementation

(No Temp. Comp)

ECLN

VBIAS

LED

GND

RTSET

RPK

PEAK

ECLP

TTL

PTAT

DRV

VREF

I

OFF

V

CC

I

OUT

I

OFF

4.7µF

= 60mA

= 0.7mA

TTL IN

+5V

5

ML4632

6

ML4632

Package: P14

14-Pin PDIP

0.740 - 0.760

(18.79 - 19.31)

14

0.070 MIN
(1.77 MIN)
(4 PLACES)

0.170 MAX
(4.32 MAX)

0.125 MIN
(3.18 MIN)

PIN 1 ID

1

0.050 - 0.065
(1.27 - 1.65)

0.016 - 0.022
(0.40 - 0.56)

0.100 BSC
(2.54 BSC)

SEATING PLANE

0.240 - 0.260
(6.09 - 6.61)

0.015 MIN
(0.38 MIN)

0.295 - 0.325
(7.49 - 8.25)

0º - 15º

0.008 - 0.012
(0.20 - 0.31)

7

ML4632

Package: S16W

16-Pin Wide SOIC

0.400 - 0.414

16

(10.16 - 10.52)

0.024 - 0.034
(0.61 - 0.86)

(4 PLACES)

0.090 - 0.094
(2.28 - 2.39)

1

PIN 1 ID

0.050 BSC
(1.27 BSC)

0.012 - 0.020
(0.30 - 0.51)

0.291 - 0.301
(7.39 - 7.65)

0.095 - 0.107
(2.41 - 2.72)

SEATING PLANE

0.398 - 0.412

(10.11 - 10.47)

0.005 - 0.013
(0.13 - 0.33)

0º - 8º

0.022 - 0.042
(0.56 - 1.07)

0.009 - 0.013
(0.22 - 0.33)

ORDERING INFORMATION

PART NUMBER TEMPERATURE RANGE PACKAGE

ML4632CP 0°C to 70°C 14-Pin PDIP (P14)
ML4632CS 0°C to 70°C 16-Pin Wide SOIC (S16W)

© Micro Linear 1997 is a registered trademark of Micro Linear Corporation
Products described in this document may be covered by one or more of the following patents, U.S.: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940;
5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; Japan: 2598946. Other patents are pending.

Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design.
Micro Linear does not assume any liability arising out of the application or use of any product described herein,
neither does it convey any license under its patent right nor the rights of others. The circuits contained in this
data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to
whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility
or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel
before deciding on a particular application.

8

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DS4632-01