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AS998/AS998A
PWM Controller
IC Block Diagram Description
Power Management
This block contains reference generators and comparators to determine the under-voltage shutdown point, the
power-on point, the primary regulation operating point, and the overvoltage shutdown point.
Internal Power / Reference
This block includes a course regulator for on-chip power and cascade voltages for HV circuitry, a bandgap for
comparator references, a bias current generator, PTAT, BiasP and BiasN.
Temperature Compensated Current Source
This block generates a constant current as a function of voltage and temperature with no off-chip components.
Oscillator
Mirrored currents from the reference block are used to charge a capacitor to the threshold voltage at which point
the direction is switched to an opposing 3X mirror to drive the capacitor to a lower threshold value. This gener-
ates a 75% duty cycle digital clock to the output latch, and a 100kHz ramp voltage to be used in the feedback
control.
Feedback Control
This block senses one of the various feedback methods to control the output duty cycle. It includes a current
sense, a low pass filter, current amplifier, summing amp, and comparators. This block sums the analog ramp
from the OSC, sense, and compensation node voltages into a comparator which triggers the falling edge of the
PWM clock signal. This provides supply voltage compensation and load regulation to the power converter sys-
tem.
Out_Drv
A high speed, high current bipolar output stage capable of providing approximately 500 mA sink and 250 mA
source current to charge and discharge the gate of a large power FET. It is understood that the total delay from
the comp pin to the output pin should be about 100 ns.
Absolute Maximum Ratings
Rating
Symbol
Rating
Unit
Supply Voltage (Low Impedance Source)
VDD
13
V
Supply Current (High Impedance Source)
IDD
15
mA
Output Peak Current
IOUT
600
mA
Regulator Current
IREG
10
mA
Continuous Power at 25°C
8L SOIC
PD
750
mW
8L PDIP
1000
mW
Junction Temperature
TJ
150
°C
Storage Temperature
TSTG
– 65 to 150
°C
Lead Temperature, Soldering 10 Seconds
TL
300
°C
Stresses greater than those listed under ABSOLUTE MAXIMUM RATINGS may cause permanent damage to the
device. This is a stress rating only and functional operation of the device at these or any other conditions above
those indicated in the operational sections of this specifications is not implied. Exposure to absolute maximum
rating conditions for extended periods may affect reliability.
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