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Principles of Operation - 4
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When the downprogramming stage is turned on (in either CV or CC mode), the CV/CC control circuit
causes the Control signal to go low, which in turn causes the downprogramming transistors to conduct
current away from the load and speed up downprogramming.
During operation, a PM_Inhibit signal will cause the output stage bias/shutdown circuit to turn off the gated
15 V bias voltages and shut down the output if any of the following occur:
The output is programmed off.
An overvoltage condition is detected (OV_Detect* signal is received).
The line voltage falls below 90 volts (approximately).
Current readback is provided by three separate circuits. The previously discussed high range current signal
(Imon_H) returns the high range currrent measurement. When the unit is operating in the low current
readback mode, a separate low range current shunt and amplifier provides low-current readback via the
Imon_L signal . A shunt clamp (Q302 and Q304) clamps the voltage across RmLo to approximately 1.8 V.
The third current readback circuit consists of a high bandwidth current amplifier that returns dynamic
current measurements from the output filter capacitor via the Imon_P signal. Note that the Imon_H and the
Imon_P signals are combined to return the actual output current measurement.
An overvoltage detect circuit compares the output voltage to the programmed overvoltage setting. When the
output exceeds the programmed setting, the OV_Detect* signal goes low, which informs the logic array that
an OV condition has occurred. The crowbar control circuit is enabled when the OV_SCR* signal is
received. When an overvoltage condition occurs, the SCR control circuit generates the OV signal, which
causes the following actions to occur:
1. The SCR fires, shorting the supply's output.
2. The microprocessor circuits are notified of the OV condition (OV_Detect* is low) in order to
program the ouput off, turn off the gated 15V bias supplies, and update the status of the unit.
3. The PM_Inhibit signal goes high, programming the output off and shutting down the gated 15V bias
for the output regulators.
4. When a output protection clear command is executed, the microprocessor circuits resets the OV
circuits, turns on the gated 15V biases, and programs the output to its previous level.
The fan driver control circuit provides the DC voltage to operate the cooling fan. The Fan_Prog signal
from the secondary interface circuit varies this voltage according to the ambient and heatsink temperature as
well as the output voltage and current of the supply.