GE MAC 5000, Field Service Manual

Page: 84 / 160

Revision B 5-18
CPU Theory of Operation: Theory of Operation
MAC 5000 resting ECG analysis system
2000657-002
peak currents (drawn during printing) are clipped to the current
monitor’s full scale value. The effects of this are minimal as such high
density printing occurs for short periods of time and represents only a
small portion of system energy consumption. Quantization error limits
the ability to measure the small current that flows when the system is
off. To compensate for this, Moe presumes a small constant quiescent
current flow from the battery. This flow serves to drain the gauge at a
rate estimated to mimic the self discharge and system quiescent current
draws.
Operation with Discharged Batteries
On AC power, the MAC 5000 can operate indefinitely with a discharged
battery, though it may not be able to print until sufficient charge has
been placed in the pack to support peak current demands. Normally,
only a minute or two of charge is sufficient to get through the printing of
a single page report.
Main Power Supplies
The MAC 5000 contains a mix of 3.3 and 5V logic. Both supply rails are
provided by a high efficiency synchronous buck regulator device
(Maxim MAX782). This device is the "Swiss army knife" of switching
regulators, providing the main logic rails as well as 3V-C, which powers
Moe. A medium current (1A) semi regulated 13V output is also provided
to feed a pair of low current linear regulators as well as provide power
for other loads. One of the low current linear regulators is used to
provide 12V for the analog output hardware. The semi-regulated rail is
also used to power the acquisition module.
Acquisition Module Power Switch/Regulator/Current Limiter
A discrete MOSFET based linear regulator applies crude regulation to
the MAX782 semi-regulated rail for use by the acquisition module. The
regulator may be turned on/off under software control and contains a
built in current limiter/sensor to protect against short circuits.
Main 12V Rail
An integrated switching regulator provides 12VDC for the LCD
backlight, motor and external peripheral devices. Input power to the
regulator is controlled by a MOSFET power switch under software
control.
External 12V Rail Switch/Current Limiter
12V power for external peripherals (modem or suction electrode pump)
is switched, current limited and fault detected by a MOSFET switch
under software control. The current limiter allows momentary
excursions above the nominal current limit to allow for KISS suction
pump startup transients.