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Technical Supplement
and conducts. If batteries are installed backward, Q15 part 2 switches off and disables current flow.
This protects the units power supply circuitry from an accidental reversal of battery potential.
Q15 part 1 controls the power supplies. When a logic high is placed on the gate (pin G1) signal,
PWR_ON battery ground is connected to the circuit and switched to ground via Q15 parts 1 and 2.
When the power control circuitry pulls PWR_ON low, switched ground switches to a high impedance
state. This action switches off the power supply and, therefore the unit, except for the power control
circuit.
9.10.15.3 Vcc Power Supply
Refer to Figure 9-24, "Power Supply Circuit," at the end of this section.
The Vcc power supply is a switched inductor voltage regulator operating in boost mode (U22). The
power input is provided by the batteries (VBAT). NFET (Q17) operates as a linear post regulator.
The 1 M resistor (R77) operates as a static bleed device across the switched regulator when the
regulator is switched down. The regulated output is Vcc (5 V ± 5%).
9.10.15.4 Raw Power Supplies
Refer to Figure 9-24, "Power Supply Circuit," at the end of this section.
The input to the raw power supplies is Vcc, which is a switched-capacitor voltage converter operating
in separate multiply and invert modes in conjunction with supporting circuitry. U23 inverts Vcc and
outputs raw –5 V. Raw 10 V is derived by voltage doubling Vcc with CR14, CR19, CR20, and
CR78. Raw 12 V is derived by voltage tripling Vcc with CR15, Q8, Q9, C96, C81, R119, and R120.
The raw power supplies are used as bias supplies for the SpO2 analog section and are not tightly
regulated. The normal operating range of the raw power supplies are:
raw –5.0 V
raw 10.0 V
raw 12.0 V
9.10.15.5 High Voltage Supply
Refer to Figure 9-24, "Power Supply Circuit," at the end of this section.
The input power for the high voltage supply is provided by the batteries (VBAT). 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's output to a safe level.
9.10.16 Analog Reference Voltage
The analog reference voltage circuit is illustrated in Figure 9-26.
9-24
=
–6.0 V to
=
7.5 V
to
=
12.0 V to
–4.0 V
11.0 V
15.0 V
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