Revision B MAC™ 5000 resting ECG analysis system 2-37
2024917-010
Equipment Overview: Theory of Operation
The charge LED is contained in the power supply compartment and is
disconnected from the CPU board when the cover is open. When the
cover is closed electrical connections are re-established through the self-
aligning connector. As the connections are made in random order, there
is a possibility that the VPS and XChargeLED drive lines can connect
before the power supply ground. This places a high potential across the
LED drive circuit as the power supply attempts to return its output
current through the LED. To prevent damage to the LED and driver, it
is implemented as a constant current source with a large compliance
voltage. Q108 provides the constant current drive, and derives LED
operating power from the MAX782 (U24) VL output rather than from
+3V-C. Q109 level shifts Moe’s output to the level required to turn off
Q108 during off periods.
Software update status
Moe also uses the charge LED for indicating the software update
progress indicator. After system power ON, if Moe does not find a Bbus
status request within six seconds, it flashes charge LED at 1Hz rate with
50% duty cycle for two minutes.
Battery Gauge
Current flow into and out of the battery pack is monitored by Moe via a
MAX472 Battery Current Monitor. By integrating the current flow, Moe
is able to maintain a reasonable estimate of the battery pack’s state of
charge. Moe’s A/D converter hasn't sufficient dynamic range to cover the
full range of system currents at high resolution so some compromises
must be made. The current monitor's full-scale range is set to a value
that is likely to encompass normal operating currents. Peaks above this
level (6Amps) are clipped. The effects of this clipping 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.
Current monitor gain is set by R128 and is nominally 1.8A/V for a
full-scale (3.3V) current of 6Amps. A low pass filter (R129 and C134)
provides filtering to remove switching noise from the signal.
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