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Batteries—Four 1.5-V alkaline "C" size batteries provide 4-6 VDC power.
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Power control circuitry—Power control circuitry is connected to the batteries. It senses any
press of the Measure button and switches on the power supplies. Reverse current limiting protects
the N-20/N-20P from damage if batteries are inserted incorrectly.
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Power shutoff circuit—This circuit controls power to all circuits except the power control circuit.
In addition, a fuse protects the power supply from excessive current draw. The power supply is
also protected against electrostatic discharge and electromagnetic interference.
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Power supply circuits consists of the following power supplies:
Regulated power supply: Power supplied by the batteries is regulated at 5 VDC. All of the
digital circuitry and some of the SpO2 analog circuitry use this supply.
Unregulated power supplies: 5 VDC is converted by a switched capacitor network into
unregulated power supplies of –5 VDC, 10 VDC, and 12 VDC, all of which are used in the
SpO 2 analog circuits.
High voltage power supply: A voltage regulator/doubler converts battery power to 70 VDC;
the display drivers as well as the display backlight need this increase in power.
The power supply circuit is illustrated in Figure 9-24, at the end of this section.
9.10.15.1 Power Control Circuitry
The power control circuit is illustrated in Figure 9-25, at the end of this section.
The power control circuit consists of U21 and its associated components. U21 is a D flip/flop with
asynchronous preset and clear; only the preset and clear are used.
Power is applied to U21 via CR11 whenever batteries are installed. CR11 provides protection for U21
if the batteries are installed with reverse polarity. This error condition will reverse bias CR11, thereby
disabling current flow to U21.
The much larger RC time constant of R110, C67 compared to R78, C66 guarantees that the unit will
not be accidentally powered on when batteries are first installed.
Whenever the Measure button is pressed, a low on GO_SW sets the output signal PWR_ON high.
This condition connects switched and battery grounds, enables the power supplies, and switches on
the unit. Whenever the CPU determines that the power should be switched off, it forces PWR_DOWN
low. This action clears output PWR_ON to a logic low, disconnecting ground, and switching off the
power supplies (see also Power Supply).
R79 and R81 provide current limit protection to U21 inputs. They also limit the current that will flow
through U21 inputs to the CPU when the batteries are installed backwards. In the reverse battery error
condition, massive current can flow from the inputs of U21 through the input protection diodes and/or
substrate inside the CPUs integrated circuit. These resistors limit that current path to safe levels.
9.10.15.2 Power Shutoff Circuit
Refer to Figure 9-24, "Power Supply Circuit" (at the end of this section).
Fuse F1 protects the unit from excessive current draw. CR24 protects against large voltage transients
caused by ESD, EMI, etc.
Q15 is a dual-channel FET; the drain of Q15 part 2 (pin D2) is connected to battery ground; the gate
(G2) is connected to battery plus; and R155 applies a bias to the source (S2) so it will switch on when
a positive voltage is applied to G2. When batteries are correctly installed, Q15 part 2 is switched on
Technical Supplement
9-23
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