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Applications of the LM3524
Pulse-Width-Modulator
The LM3524 Regulating Pulse-Width-Modulator is com-
monly used as the control element in switching regulator
power supplies. This is in keeping with its intended purpose.
Engineers closely associate this part with switching power
supplies. Nevertheless, the flexible combination of elements
(see box) within the LM3524 also allows it to be used in a
number of other applications outside the power supply area.
Because the device is inexpensive and operates off a
single-sided supply, it can considerably reduce component
count and circuit complexity in almost any application. The
constant light intensity servo of Figure 1 furnishes a good
example.
Constant Light Intensity Servo
The circuit of Figure 1 uses a photodiode’s output to control
the intensity of a small light bulb. The constant intensity
output of the light bulb is useful in a number of areas,
including opto-electronic component evaluation and quality
control of photographic film during manufacture. In this cir-
cuit, the photodiode pulls a current out of the LF356 sum-
ming junction, which is directly related to the amount of light
that falls on the photodiode’s surface. The LF356 output
swings positive to maintain the summing junction at zero and
represents the photodiode current in amplified voltage form.
This potential is compared at the LM3524 to the voltage
coming from the 2.5k “intensity” potentiometer wiper. A
stable voltage for the “intensity” control is taken from the
LM3524’s internal five-volt regulator. The difference between
the LF356 output and the “intensity” potentiometer output is
amplified at a gain of about 70 dB, which is set by the 1 MΩ
value at pin 9. The LM3524 output transistors are paralleled
and provide drive to the 2N2219 switch transistor. The 5.6k
and .01 µF values set the switching frequency at about 30
kHz. Because the LM3524 forms a switched mode feedback
loop around the light bulb and photodiode, the average
power delivered to the light bulb will be controlled by the
photodiode output, which is directly proportional to the
