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Technical Supplement
9.6.7.1 Power Control Circuitry
9-24
•
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.
•
Power supply circuits consist of the following power supplies:
Regulated power supply: Power supplied by the batteries is regulated
at 5 VDC. This supply is used by all of the digital circuitry and some of
the SpO
analog circuitry.
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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
High voltage power supply: A voltage regulator/doubler converts
battery power to 70 VDC; this increase in power is needed by the
display drivers as well as the display backlight.
The power supply circuit is illustrated in Figure 9-25, at the end of Section 9.
Figure 9-4 shows the power supply block diagram.
The power control circuit is illustrated in Figure 9-26, at the end of Section 9.
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 backward. In the reverse battery error condition, excessive current can
flow from the inputs of U21 through the input protection diodes or substrate
inside the CPUs integrated circuit or through both. These resistors limit that
current path to safe levels.
analog circuits.
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