Battery Operation - Nellcor N-180 Service Manual

N-180 Pulse Oximeter Service Manual
When the processor brings BAT_THRESH to a logic low (0 V) the output
of comparator U4B, LOWBAT, is examined at the same time. The sum
of the logic low on BAT_THRESH and 16 V on VPS is still a voltage
greater (approximately 3.5 V) than the 2.5 reference input to U4B,
therefore LOWBAT remains a logic low. The processor responds by not
turning on the BATT IN USE light.
Battery Operation
When the monitor operates on battery, VPS drops to approximately 13
V. When BAT_THRESH is brought to logic low, the resulting voltage at
U4B, pin 6, is below the 2.5 V reference (typically 2 V) and the
comparator output (LOWBAT) rises to a logic high during the time that
BAT_THRESH is low. This change in LOWBAT only during the
BAT_THRESH low time period causes U9 to recognize battery operation
' and turns on the BATT IN USE light.
Resumption of AC power overrides the 2.5 V reference, notwithstanding
the-state of BAT_THRESH, which is strobed to a logic low. The
comparator output LOWBAT now rises to a logic high, the processor
turns off the BATT IN USE light, and the circuit returns to AC power
mode. .
Low Battery Warning
During battery power, VPS is 13.2 V or less; this condition switches the
output of U4B to a logic high during the BAT_THRESH strobe. As the
battery discharges signal VPS drops from the initial value. Eventually
the value of VPS is low enough (typically, 11.2 V) to drive U4B, pin 6,
permanently below the 2.5 V reference, regardless of the state of
BAT_THRESH. Now U4B output, LOWBAT, remains a logic high.
The processor logic detects LOWBAT high. Regardless of the level of
BAT_THRESH, the logic is conditioned by the low battery state to start
flashing the LOW BATT light.
"Low Battery Shutdown
To protect the battery from excessive discharge, the processor starts a
5-minute timer after turning on the LOW BATT light. At the end of five
minutes, if AC operation has not resumed, the processor initiates a
normal shut-down procedure through the WATCHDOG line
'ON/STDBY Switch
Monitor operation is controlled by the ON/STDBY switch on the front
panel. The switch is connected to the power control circuits on the CPU
PCB via connectors CN5/CN1 on the LED Dispiay PCB.
Power is applied to the monitor by connecting the source and drain of FET
- Q5, which connects the monitor ground (TP3) to the power ground (TP-2).
Flip-flop U14B controls the gate of Q5. In turn, U14B is controlled by gates
U13A, C, D, and associated components.
Standby condition
In standby, transistor Q5 is not conducting and U14B is RESET (logic
low at Q). The ON/STDBY switch is in the STDBY position, which
connects the STBY_COM pole to the STDBY pole of the switch (VPS).
Capacitor C13 charges to VPS (16 or 13 VDC depending on whether AC
is connected to the monitor). Capacitor C14 is charged to +5 V from
3-16