Agilent Technologies 6612B, Service Manual, Page: 53 / 74

Principles of Operation - 4
53
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 (Q304 and Q305) 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 dignal 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 aray
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 15V bias supply, and update the status of the unit.
3. The PM_Inhibit signal goes high, programming the output off and shutting down the +15V bias for
the output regulators.
4. When a output protection clear command is executed, the microprocessor circuits resets the OV
circuits, tutns on the +15V bias supply, 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 heaatsink temperature
as well as the output voltage and current of the supply.