Nellcor N-180, Manual

Page: 21 / 83

Section 3: Functional Block and Circuit Analysis
3-17
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Standby to on
When the ON/STD BY switch is placed in the ON position, the
STBY_COM and ON poles of the switch are connected. Capacitor C13
discharges through R22, producing a positive spike into gate U13D.
U13D inverts the spike that sets FF U14B and discharges C14 through
D7. The ON input then remains at 0 V.
When U14B is SET the Q output (logic high) causes Q5 to conduct,
connecting the PGND and system grounds. The processor starts
operating and places a continuous stream of 0’s and 1’s on the
WATCHDOG line. These transitions are differentiated by C16 and R20.
The positive spikes are inverted by U13C and keep C14 discharged
through D6.
The inputs to U13A (pin 1 0 V, pin 2 +5 V) set the output, pin 3, to a
logic high. Line STANDBY, which reflects the charge on capacitor C13,
will now be 0 V. The processor is now monitoring STANDBY.
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On to standby
When you place the monitor in standby the STBY_COM and STDBY
poles of the switch connect. This action quickly charges C13 to VPS, so
that the same value (logic high) is present on the STANDBY line.
Processor U9 logic detects the status change and removes the pulses
from the WATCHDOG input to the control circuit. The cessation of
negative pulses through D6 allows C14 to quickly charge to +5 V. This
condition places two logic highs at the input of U13A. Accordingly, the
output of U13A goes low, resetting U14B. The logic low on the Q output
of U14B then stops conduction in Q5, removing power from the monitor.
The circuit conditions are now as they were in the standby condition.
Processor U9 also uses this sequence to turn off the monitor in the event
of low battery. In this case the trigger is not the STANDBY line, but
information from the battery condition circuits.
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Battery charger circuit
The battery charger circuit consisting of Q8, Q9, Q10, Q11, Q12, and
associated components operates as a charging voltage source having a
maximum current of 160 mA. The circuit keeps the battery charged to a
maximum of 13.8 V.
When the battery voltage is below 13.8 V the circuit charges the battery
until the voltage reaches 13.8 V. Then, the charger switches to constant
voltage operation to maintain the battery charge at 13.8.
Resistors R52, R53, and VR1 drop the battery charge value (less the
forward voltage drop of D4). The common point of R52 and R53 is the
battery charge sense point. Regulator Q10 monitors the voltage at this
point and controls transistor Q8 to maintain this point at a voltage that
represents a battery voltage of 13.8 V. Variable resistor VR1 adjusts the
voltage at this point.
The circuit composed of transistor Q11 and associated resistors
constitutes a current-limiting circuit that has foldback characteristics.
Battery charge current flows through R54. The voltage drop across R54
is proportional to charge current. Transistor Q11 is normally turned off
by reverse bias generated by the R49/R50 divider. If the charge current
increases so that the drop across R54 exceeds the drop across R49 Q11
turns on. This allows additional current to flow through VR1/R53
raising the voltage at the sense point. Transistor Q10 reacts by reducing