Chapter 13 665
Display/Power Supply Section
A6 Power Supply Assembly
DC-DC Converter Control
See function block I of A6 power supply schematic diagram in the
component-level information binder.
The DC-DC converter control circuitry divides the 80 kHz squarewave
from U202A and generates two complementary 40 kHz squarewaves to
drive the FETs in the DC-DC converter. Also, U202D and its associated
circuitry monitor the voltage across sense resistor R116 in the DC-DC
converter. When the current through the FETs in the DC-DC converter
exceeds 1.8 A, the voltage across R116 will cause the output of U202D
to go high. This sets a latch in U204 which turns off U203.
Power Up
See function block M of the A6 power supply schematic diagram in the
component-level information binder.
The power up circuitry generates the PWR UP signal, which tells the
microprocessor that the supplies are up and stable. PWR UP will go
high when the +5 Vdc supply exceeds +4.99 Vdc. PWR UP will go low
when this voltage is less than +4.895 Vdc. Once PWR UP is set low, it
will stay low for at least 50 ms before going high, even if the +5 Vdc
supply exceeds +4.99 Vdc before 50 ms have elapsed.
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