Rev 1.1a

Copyright

© 1994

11861 Western Avenue

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(714) 898-8121

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SG1526B/SG2526B/SG3526B

FIGURE 19.

OSCILLATOR PERIOD VS. R

T

AND C

T

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

FIGURE 21.

SIMPLIFIED UNDERVOLTAGE LOCKOUT

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

µA current source to charge C

amplifier output to 1.0 V

As the soft-start voltage ramps up

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

FIGURE 20.

EXTENDING REFERENCE OUTPUT CURRENT

FIGURE 22.

SOFT-START CIRCUIT SCHEMATIC

SOFT-START CIRCUIT