SG2524, SG3524
REGULATING PULSE WIDTH MODULATORS
SLVS077D – APRIL 1977 – REVISED FEBRUARY 2003
PRINCIPLES OF OPERATION
†
The SG2524 is a fixed-frequency pulse-width-modulation (PWM) voltage-regulator control circuit. The regulator
operates at a fixed frequency that is programmed by one timing resistor, R
T
, and one timing capacitor, C
T
. R
T
establishes a constant charging current for C
T
. This results in a linear voltage ramp at C
T
, which is fed to the
comparator, providing linear control of the output pulse duration (width) by the error amplifier. The SG2524 contains
an onboard 5-V regulator that serves as a reference, as well as supplying the SG2524 internal regulator control
circuitry. The internal reference voltage is divided externally by a resistor ladder network to provide a reference within
the common-mode range of the error amplifier as shown in Figure 6, or an external reference can be used. The output
is sensed by a second resistor divider network and the error signal is amplified. This voltage is then compared to the
linear voltage ramp at C
T
. The resulting modulated pulse out of the high-gain comparator then is steered to the
appropriate output pass transistor (Q1 or Q2) by the pulse-steering flip-flop, which is synchronously toggled by the
oscillator output. The oscillator output pulse also serves as a blanking pulse to ensure both outputs are never on
simultaneously during the transition times. The duration of the blanking pulse is controlled by the value of C
T
. The
outputs may be applied in a push-pull configuration in which their frequency is one-half that of the base oscillator, or
paralleled for single-ended applications in which the frequency is equal to that of the oscillator. The output of the error
amplifier shares a common input to the comparator with the current-limiting and shut-down circuitry and can be
overridden by signals from either of these inputs. This common point is pinned out externally via the COMP pin, which
can be employed to either control the gain of the error amplifier or to compensate it. In addition, the COMP pin can
be used to provide additional control to the regulator.
APPLICATION INFORMATION
†
oscillator
The oscillator controls the frequency of the SG2524 and is programmed by R
T
and C
T
as shown in Figure 4.
f
[
1.30
R C
T T
where: R
T
is in kΩ
C
T
is in
µF
f is in kHz
Practical values of C
T
fall between 0.001
µF
and 0.1
µF.
Practical values of R
T
fall between 1.8 kΩ and 100 kΩ.
This results in a frequency range typically from 130 Hz to 722 kHz.
blanking
The output pulse of the oscillator is used as a blanking pulse at the output. This pulse duration is controlled by
the value of C
T
as shown in Figure 5. If small values of C
T
are required, the oscillator output pulse duration can
be maintained by applying a shunt capacitance from OSC OUT to ground.
synchronous operation
When an external clock is desired, a clock pulse of approximately 3 V can be applied directly to the oscillator
output terminal. The impedance to ground at this point is approximately 2 kΩ. In this configuration, R
T
C
T
must
be selected for a clock period slightly greater than that of the external clock.
† Throughout these discussions, references to the SG2524 apply also to the SG3524.
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