Unit Power - Spacelabs 90217 Service Manual

Ambulatory Blood Pressure Monitor 90217 Service Manual

Unit Power

All power for the 90217 ABP Monitor comes from the main batteries (3 AA cells) and the
rechargeable 3-volt lithium battery.
Unregulated Power Supplies
+VSW
The switched battery voltage (+VSW) is provided by the main batteries via the unit power switch.
With this switch ON, +VSW is applied to the power converter, the air pump, and to the various
circuits, which must remain active during the processor idle down mode.
+VBB
The backup battery supply (+VBB) is always a diode drop (0.3 volts) down from the main battery
voltage (+VSW), the +5, or the lithium battery voltage, whichever is the highest.
• If the power switch is OFF and there are charged main batteries in the unit, the main batteries
supply the voltage through a diode (+VBB will be approximately 3.3 to 4.5 V).
• If the power switch is ON and the +5 V supply is higher than +VSW, the power comes from the
+5 V supply through a diode (+VBB is approximately 4.6 to 4.8 V).
• If the main batteries are removed, the voltage is developed from the lithium backup battery
through a diode (+VBB is approximately 3 V).
Power Converter
The power converter is a MAXIM "MAX655" step-up converter. It develops the +5 V supply from
the main battery voltage, which ranges from 2.5 to 4.8 VDC.
Low and High Current Modes
When the power converter is in its low current mode, it draws 40 µamps of quiescent current.
The power converter goes into a high current mode upon receiving an interrupt from any of its
three wake-up sources: cable connect, clock, or START/STOP button. A 5 msec delay following a
wake-up signal keeps the power converter in high current mode while the processor wakes up and
asserts the PWR_HI line.
The power converter develops the +5 V supply, the +5 VA supply, and the converter reference
voltage, ONNV_VREF.
+5 V Supply
The +5 V supply is developed directly from the +VSW (main batteries and is regulated to +4.75 to
+5.25 volts). The maximum current available is 60 ma (typical draw is approximately 20 ma).
3-6