Mindray Passport 5-Lead, Service Manual

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Passport 5-Lead, 5L, LT, XG Service Manual 0070-00-0420 2 - 51
Theory of Operation Block Diagrams
Low Voltage Shut Down Comparator / 20 Second Delay Circuit
When the battery drops to 11 volts (10.5 V after the USD945 diode drop and fuse), the low
current comparator will trip. Hysteresis on the comparator circuit will prevent the unit from
turning back ON until the supply reaches 15 V. The purpose of hysteresis is to prevent the
power from toggling ON and OFF when the battery is low and monitor functions are
enabled and disabled.
This circuit employs a low power OP-AMP U1(LM4250) as a voltage comparator. This OP-
AMP allows the designer to operate the amplifier at a unique operating point established by
R6. In this application, R6 biases the comparator at 1Ua. This equates to approximately
10Ua of supply current drain at 10V. Another low power device, the LT1004-1.2 (CR1) is a
zener diode used to develop a precision voltage reference at the negative terminal of the
comparator. This device is specified to “zener” at 1.2V with only 10Ua of current. The
10Ua zener current is provided by R3. A precision resistor divider consisting of R1, R2, and
R4 apply the appropriate voltage to the positive input terminal of the comparator. If the
supply voltage is greater than 10.5 V, the divider voltage will be greater than the reference,
and the output will jump towards the positive rail. When this occurs, CR2 will forward
conduct causing the combination of U2C and U2A to turn Q1 (2N7000) “ON” In turn, Q2
(2N7000) will be forced OFF to eliminate R4 from altering the divider circuit. This state is
the normal state for proper operation. However, if the supply dips below 10.5 V , the
following events happen.
The positive input to the comparator will be less than the reference. This will cause the
comparator output to jump towards the negative rail. When this occurs, CR2 will reverse
bias and the rail voltage on C7 will discharge through R12. If the supply does not rise above
10.5 V in a period of time greater than approximately 15 seconds (exponential decay of C7
and R12 to the trip point of U2C) Q1 2N7000 will turn OFF. Q2 the 2N7000 transistor will
now turn ON causing R4 to modify the resistor divider and force the divider input voltage to
fall further below the reference. The circuit will remain in this state until the power switch is
toggled or the supply voltage exceeds 15 V.
Turn On:
Capacitor C1 and R8 forces Q1 to turn “ON” and Q2 to turn “OFF” during power-up. This
is to insure that the hysteresis resistor R4 will not be connected during power up, to establish
the proper state for normal operation.