Table of Contents
Advertisement
9.6.7.2 Power Shutoff Circuit
9.6.7.3 Vcc Power Supply
9.6.7.4 Raw Power Supplies
This circuit is illustrated in Figure 9-25, Power Supply Circuit.
Fuse F1 protects the unit from excessive current draw. CR24 protects against
large voltage transients caused by ESD, EMI, and so on.
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 and
conducts current. If batteries are installed backward, Q15 part 2 switches off and
disables current flow. This protects the unit's 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.
This circuit is illustrated in Figure 9-25, Power Supply Circuit.
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%).
This circuit is illustrated in Figure 9-25, Power Supply Circuit.
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 SpO
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
=
–6.0 V to
–4.0 V
7.5 V
to
11.0 V
12.0 V to
15.0 V
Technical Supplement
analog section
2
9-25
Advertisement
Table of Contents
