Mallinckrodt NELLCOR NPB-4000, Service Manual

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Section 15: Drawings
15-9
Neither LED is driven until CVREF is up, indicating that sufficient power is
reaching the charger.
15.9 POWER SUPPLY CONTROL LOGIC
When the battery is initially attached to the power supply, an R/C circuit
produces a single positive pulse (PWRRST) which ensures that the power and
alarm circuits are initially OFF.
15.9.1 ON/OFF Control
The NPB-4000/C system power supply is turned ON and OFF by alternate
pushes of a membrane switch on the front panel. This membrane switch is
pulled up to the battery voltage through R61, buffered, and filtered by R62/C52.
The conditioned signal toggles flip flop U8 (pin 1) with alternate switch pushes.
The second section of flip flop U8 (pin 12) produces the actual system power
supply ON/OFF control signal. The ON signal from U8 (pin 1) produces a brief
positive pulse at U8 (pin 8) that turns the system supply ON.
If the push-switch switch is pushed to turn the supply OFF, U12 (pin 1) will go
low. While the system supply is running, U12 (pin 2) will also be low. These
two low levels cause U12 (pin 3) to be low, which removes the RESET condition
from oscillator/divider U10. Oscillator U10 will begin to oscillate and will
produce a positive edge on its Q14 output after about 1 second. This edge will
clock a low into U8 (pin 11), shutting off the system supply.
15.9.2 Early Warning
The one-second delay between the request for OFF from the push-button and the
actual OFF action is called the early warning delay. This delay period allows the
processor on the Main Board to store data and make a graceful shutdown before
system power is lost. The power supply provides an early warning signal to the
Main Board via U11 (pin 6) and Q14 to alert of the pending shutdown.
The processor board can shut down the system power supply on command,
without any action from the front panel ON/OFF button. This occurs through the
PSOFF line (active low), which produces a high level on U12 (pin 5). The
resulting high on U8 (pin 10) directly shuts off system power. In general, the
processor has two reasons for shutting power down: either to truncate an early
warning cycle, or to halt operation under extreme low battery conditions.
15.9.3 SYNC Pulses
The system power supply runs at about 40 kHz without external synching, but
will move up to 50 kHz operation with an external clock signal. The external
sync signal from the processor drives Q16, which produces a 5-volt signal on its
collector that synchronizes the main supply through C45.