Volt Regulator; Overvoltage/Undervoltage Sensor - HP PageWriter XL M1705B Service Manual

The low battery shutdown point occurs when the battery is
approximately 70% discharged. The low battery indicator begins
flashing and the message "Low battery, plug cardiograph into
AC." is displayed. After one minute, the cardiograph will go to
Standby unless it is connected to AC before the minute expires. If
the battery is not allowed to recharge, it will continue to discharge
until it reaches the low battery disconnect point.
Low battery disconnect occurs when the battery is approximately
80% to 90% discharged. When this occurs, the battery is completely
disconnected from the system, and memory and clocks are lost. The
disconnect circuit is described later in this section.
5 Volt Regulator
The 5 volt regulator provides logic power to the cardiograph. This
regulator converts VIN to +5 volts f5% with a "buck" (step-down)
converter. The UNITON signal from the DRAM gate array turns
the regulator on and off, and soft-start circuitry prevents voltage
overshoot at start-up. Maximum rated output current is 3 amps.
+5 Overvoltage/
This circuit monitors the +5 volt supply to sense an overvoltage or
Undervoltage
undervoltage condition.
Sensor
When the [On-Standby) key is pressed to turn the cardiograph On,
+5READY goes high as soon as +5 volts is within regulation.
+5READY enables the printer switch and preview display, and
disables the backup supply. After about 100 111s SYSRDY goes high.
This delay allows the VBACK supply to equalize voltage with the +5
volt supply.
When the (On-Standby) key is pressed to place the cardiograph in
Standby, the system software enters the power-off sequence described
earlier in this chapter. The DRAM gate array asserts the SHUTOFF
signal, which causes +5READY and SYSRDY to go low. When
+5READY goes low, it disconnects the printer switch and enables
the backup supply, which provides power to the DRAM. After 30 to
60 IIIS, the UNITON signal goes low and turns off the +5 regulator.
As long as the supply remains within tolerance, the f5READY and
SYSRDY signals remain high. When the +5 volt supply exceeds the
tolerance in either direction, +5READY goes low.
Theory of Operation 5-19