DER-94
LCD Monitor Internal Supply
September 12, 2005
frequency. Resistor R8 provides a zero to cancel the phase shift of C5. Resistor R11
sets the gain of the direct signal path from the supply output through U2 and U3.
Components C15 and R14 reduce the high frequency gain of U3, while C14 and R12
provide gain and phase boost near the control loop 0dB crossover frequency.
4.5 Active Preload/Output Protection
The components shown in Figure 3 provide 12 V active loading, 5 V overload protection,
and overvoltage protection for the power supply. The active preload function improves
cross regulation, especially when there is light or no load on the +12 V output and
maximum load on the +5 V output. The improvement in cross regulation makes the 12 V
output suitable for use with low cost LCD panels and removes the need for a 12 V linear
post regulator. The circuit provides overload protection for the 5 V output during LPS
testing of the +5 V output, eliminating the need for a 5 V fuse. Finally, the circuit provides
output overvoltage protection with auto-restart.
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