Mallinckrodt Nellcor N-20PA Service Manual page 74

Technical Supplement
9.6.7.5 High Voltage Supply
9.6.8 Analog Reference Voltage
9-26
This circuit is illustrated in Figure 9-25, Power Supply Circuit.
The batteries (VBAT) provide the input power for the high-voltage supply. The
high-voltage supply is a switched-inductor voltage regulator (U26) that operates
in conjunction with a capacitive voltage doubler to output 72 VDC ± 5%. To
protect against a runaway voltage condition, CR25 clamps U26 output to a safe
level.
The analog reference voltage circuit is illustrated in Figure 9-27.
U32 provides an accurate, regulated voltage that is used as the reference voltage
for the A/D converter inside the CPU. C6, C12, and R124 provide filtering. The
voltage output VREF is 5 V.
R123
RAW10V
182
R121
RAW12V
221
R122
RAW-5V
221
Figure 9-27: Analog Reference Voltage Circuit
U32
2
VIN VOUT
4
GND TRIM
C95
22UF LT1021
R161
Q23
2N3904
10K
Q24
2N3904
VREF
+12V
TP61
C91
C80
22UF
0.1UF
-5V
TP62
C93
C94
22UF
0.1UF
VREF
TP60
R124
6
5
1
C12
C6 22UF
0.1UF
TP70
GND
+5.7