SG3526B MICROSEMI [Microsemi Corporation], SG3526B Datasheet - Page 6

• Manufacturer Part Number: SG3526B
• Description: REGULATING PULSE WIDTH MODULATOR
• Manufacturer: MICROSEMI [Microsemi Corporation]

4/90 Rev 1.1 2/94
Copyright
CHARACTERISTIC CURVES
APPLICATION INFORMATION
VOLTAGE REFERENCE
The reference regulator of the SG1526B is a “band-gap” type; that is, the precision +5 volt
output is derived from the very predictable base-emitter voltage of an NPN transistor.
Since this is a sub-surface phenomenon, the resulting output exhibits excellent stability
compared to earlier surface-breakdown zener designs.
The reference output is stabilized at input voltages as low as +8 volts, and can provide
up to 20mA of load current to external circuitry. An external PNP transistor can be used
to boost the available current to many hundreds of mA. A rugged low-frequency audio-
type transistor should be used, and lead lengths between the PWM and transistor should
be as short as possible to minimize the risk of oscillation.
UNDERVOLTAGE LOCKOUT
The undervoltage lockout circuit protects the SG1526B and the power devices it controls
from inadequate supply voltage. If +V
and holds the RESET pin LOW. This prevents spurious output pulses while the control
circuitry is stabilizing, and holds the soft-start timing capacitor in a discharged state.
The circuit consists of a merged bandgap reference and comparator circuit which is
active when the reference voltage has risen to 2V
reference voltage rises to approximately +4.4 volts, the circuit enables the output drivers
and releases the RESET pin, allowing a normal softstart. The comparator has 200mV
of hysteresis to minimize oscillation at the trip point. When +V
and the reference drops to +4.2 volts, the undervoltage circuit pulls RESET LOW again.
The soft-start capacitor is immediately discharged, and the PWM is ready for another
soft-start cycle.
The SG1526B can operate from a +5 volt supply regulated to within ±4% by connecting
the V
SOFT-START CIRCUIT
The soft-start circuit protects the power transistors and rectifier diodes from high current
surges during power supply turn-on. When supply voltage is first applied to the
SG1526B, the undervoltage lockout circuit holds RESET LOW with Q3. Q1 is turned on,
which holds the soft-start capacitor voltage at zero. The second collector of Q1 clamps
the output of the error amplifier to ground, guaranteeing zero duty cycle at the driver
outputs. When the supply voltage reaches normal operating range, RESET will go HIGH.
Q1 turns off, allowing the internal 100 A current source to charge C
amplifier output to 1.0 V
to +5 volts, the duty cycle of the PWM linearly increases to whatever value the voltage
regulation loop requires for an error null. Figure 7 gives the timing relationship between
C
S
FIGURE 19.
OSCILLATOR PERIOD VS. R
ramp time to 100% duty cycle.
1994
REF
pin to the +V
IN
T
pin.
AND C
BE
above the voltage on C
T
(continued)
IN
is too low, the circuit disables the output drivers
S
. As the soft-start voltage ramps up
BE
or 1.2 volts at 25
IN
6
to the PWM is removed
S
. Q2 clamps the error
o
C. When the
SG1526B/SG2526B/SG3526B
FIGURE 20.
EXTENDING REFERENCE OUTPUT CURRENT
FIGURE 21.
SIMPLIFIED UNDERVOLTAGE LOCKOUT
FIGURE 22.
SOFT-START CIRCUIT SCHEMATIC
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