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ML4632CP

Fiber Optic LED Driver

Micro Linear Corporation 

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

ILED (HIGH) = 1.2V/RTSET = 1.2V/20Ω = 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

IOFF pin to VCC. The additional current will be equal to

(VCC – 0.7V)/RIOFF.

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.

VREF

(10)

R1

DRV DRV

TO

REF

PTAT

R2 (11) AMP

DRIVER

PTAT

(12)

Figure 1. Current for Programming

Output Temperature Coefficient

In this configuration the temperature coefficient is

TCILED

=

(3300ppm /

°C)

R1

R1+ R2

,

and

ILED

(HIGH)

=

1V

+

0.2V R1R+2R2

RTSET

The output current will be a linear function of temperature.

A plot of ILED versus temperature for several values of the

programming resistance, R1 and R2, in figure 2.

R1 = 1Ω

60

R2 = 3R1

55

R1 = R2

R1 = 3R2

50

R2 = 0Ω

45

0

25

NOTE: R1 + R2 ≥ 10kΩ

50

T(°C)

75 85

Figure 2. ILED vs T, RTSET = 20Ω

4

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