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 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 20Ω, a pulse will
be generated at the PEAK pin of magnitude 1.2V and
equivalent resistance 20Ω (assuming V
DRV
= 1.2V).
PEAK
PEAK
RPK
RPEAK
20Ω
CPEAK
100pF
LED
DRIVER
RTSET
RPK
20Ω
RTSET
20Ω
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 0Ω LED impedance. Peaking
currents will be slightly lower for real LED’s.
I
OUT
= 60mA
I
OFF
= 0.7mA
ECLN
VBIAS
I
OUT
LED
GND
20Ω
20Ω
RTSET
RPK
PEAK
20Ω
+5V
0.1µF
ECLP
TTL
PTAT
DRV
VREF
I
OFF
V
CC
4.7µF
TTL IN
CPEAK
100pF
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)
5
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
