Invivo Millennia 3155A, Руководство по обслуживанию

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1-5
to continue in its active state. If the pulses are absent, U1A will quickly time out and the power
supply will stop, as U14 and U11 will go into shutdown (having lost their enables).
The test switch SW1, if in its leftward position (ON), will hold the cathode of diode D1 low, and
maintain mono U1A in its active state indefinitely. In this case, the power supply will stop only if
power is removed by turning off the back-panel switch and removing the batteries, or by moving the
TEST switch back to its rightward position (OFF).
The keep-alive pulses are received from pin 4 of connector P9. The transistor Q29 conditions the
pulses for the larger logic swing required by gate U2 pin 13 (the U2 supply can go as high as 15
volts). Also, the input to transistor Q29 is AC-coupled, preventing the resting level of the keep-alive
pulses from affecting the state of transistor Q29. Capacitor C88 provides the AC-coupling, resistor
R5 is a protection resistor, capacitor C3 is a noise filter, diode D10 re-establishes the unidirectional
swing at the base of transistor Q29, and resistor R35 discharges capacitor C3 to ensure that transistor
Q29 is turned off when no pulses are received. Resistor R113 is the collector resistor for transistor
Q29, and gate U2 pin 13 receives this string of pulses. As long as pin 12 is high, the string of pulses
is passed through into mono U1A, keeping it active.
When the front-panel switch is turned OFF, the comparator U14 pin 4 (right side of schematic sheet
3) is pulled LOW through diode D24, taking the /UP_SHUTDOWN line LOW. This is a signal to
the processor to immediately perform a shutdown routine and stop the keep-alive pulses. To
over-ride the processor if it is in a loop, a shutdown signal also comes through transistors Q6 and
Q5 (schematic sheet 3), turning off Q5, and causing gate U2 pin 12 to fall LOW after a delay
established by resistor R34 and capacitor C97 (schematic sheet 1), and so blocking the keep-alive
pulses from mono U1A.
In the event that the front-panel switch is turned OFF during mono U1B's 40-second start-up time,
resistor R43 and capacitor C29 will pull U2 pin 13 low after a short delay, resetting U1B and, in
conjunction with pin 12 of gate U2 (as described before), the mono U1A will quickly time out and
shut down the power-control Ics.
Because the voltage on SWPWR may exceed the voltage on STDY_15, diode D15 clamps the
voltage on U1B pin 13 to no more than the STDY_15 value. Resistor R44 limits the maximum D15
current. Diode D14 prevents a too-fast discharge of C29 when the front-panel switch is turned OFF.
The diode D16 between SWPWR and U2 pin 6 squelches the power-fail beep during shutdown. So
that the processor may control the actual shutdown, the voltage at VSTY is maintained through diode
D4
from the +12V supply, keeping the keep-alive circuit able to run even after the front-panel switch
is turned OFF.
If any event causes the power supply to shut down with the front-panel switch still ON, the gate U2C
pin 8 raises its pin 10, turns on the local oscillator U2B and U2A, and produces a loud sound from
the speaker under the batteries.
Low battery voltage will also shut down the power supply, if the warning /EARLY_WARN is
ignored. If RAWPWR goes too low, the comparator U14 pin 8 goes low to effect this operation
through transistor Q6 turning transistor Q5 OFF, as described above.
The transistor Q8 and its associated circuitry will be described in the section dealing with the battery
indicator.
1.2.4 +5 and +3.3 Vdc Power Supplies. The +5V and +3.3V supplies are provided from U14 and
the associated circuitry (on schematic sheet 3). U14 is a MAX786 chip, which is a
synchronous-rectifier switcher. This design yields the greatest efficiency, as the rectifier for the
energy discharge in the power inductor is now a switched FET with much lower voltage drop.