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

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

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

be generated at the PEAK pin of magnitude 1.2V and

equivalent resistance 20

PEAK

PEAK

RPK

RPEAK

20

Ω

RTSET

20

Ω

LED

RTSET

RPK

20

Ω

CPEAK

100pF

DRIVER

Figure 3. Application of the Peaking Circuit

The peaking current is coupled through the 100pF

and from the LED on each cycle of output current. The

peaking circuit shown provides approximately a 70%

overshoot current into a 0

Ω LED impedance. Peaking

currents will be slightly lower for real LED’s.

ECLN

VBIAS

LED

GND

RTSET

RPK

PEAK

20

Ω

ECLP

TTL

PTAT

DRV

VREF

0.1

µF

CPEAK

100pF

20

Ω

20

Ω

TTL IN

+5V

4.7

µF

Note:

GND plane to reduce ringing and overshoot at the LED.

TTL Driven Implementation

(No Temp. Comp)