Nellcor N-180, Manual

Page: 19 / 83

Section 3: Functional Block and Circuit Analysis
3-15
CPU PCB
The CPU PCB:
• controls display functions
• manages the RS-232 link to external devices
• has multiple support circuits
– speaker drive
– LED current check
– battery condition monitor
– battery charging and power control
CPU (Sheet 1 of 4)
Note: Power and Ground connections for the ICs on the CPU board are
shown as “power terminal” on the schematic
■
Speaker drive circuit
The speaker drive circuit is composed of transistor Q1 and FET Q2, in
series with speaker and waveform conditioning components D1 and C3.
The audio pulse tone (beep) is determined by the output frequency of
processor U9 I/O, (lead A) CXS/PA5. This signal is buffered by U7B,
which controls FET Q2. The beep frequency is proportional to the
patient’s oxygen saturation percentage, increasing in frequency with
increasing saturation percentage.
Speaker volume is determined by the DC value of VOLUME, which
controls the conduction of Q1. The DC value of VOLUME is determined
by the output of DAC U3. Processor U9 sets the default value initially,
but you can change the level by rotating the front-panel control knob.
■
Back panel switch SW1
Back panel switch SWI is a dual-in-line package (DIP) switch. Switch
settings control communications baud rate and the communications
format (conversation or computer). In addition, a switch section in this
DIP controls the default alarms for adult or neonate patients
Battery Charging and Power Control (Sheet 2 of 4)
Device U4B performs two functions: battery in use determination, and low
battery notification. The circuit is a comparator with a logic input from
processor U9 (BAT_THRESH). The comparator normally monitors VPS,
which reflects the value of the monitor DC operating voltage. When AC-
powered, the value of VPS is typically 16 VDC. When battery-operated, VPS
is typically less than 13.2 V (the actual value depends on the battery charge
level).
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AC Operation
The circuit performs as follows when the monitor is operating on AC
power. VPS is equal to 16 V. Signal BAT_THRESH is a logic high (+5 V)
most of the time, and drops to a logic low for 200
µ
s every 100 ms.
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,