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ML4632CP

Fiber Optic LED Driver

Micro Linear Corporation 

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 RTSET. 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 RPEAK

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.

ML4632

A typical application is shown in figure 3. When the

resistors RRPK and RPEAK are both set to 20Ω, a pulse will

be generated at the PEAK pin of magnitude 1.2V and

equivalent resistance 20Ω (assuming VDRV = 1.2V).

PEAK

PEAK

RPK

RPEAK

20Ω CPEAK

100pF

RPK

RTSET

20Ω

20Ω

LED

DRIVER

RTSET

Figure 3. Application of the Peaking Circuit

The peaking current is coupled through the 100pF

capacitor, CPEAK, 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 0Ω LED impedance. Peaking

currents will be slightly lower for real LED’s.

IOUT

CPEAK

100pF

ECLN

VBIAS

LED

GND

20Ω

RTSET

20Ω

RPK

PEAK

20Ω

ECLP

TTL

PTAT

DRV

VREF

IOFF

VCC

0.1µF

IOUT = 60mA

IOFF = 0.7mA

TTL IN

4.7µF

+5V

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.

TTL Driven Implementation

(No Temp. Comp)

5

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