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Equipment Overview: Theory of Operation
provides the constant current drive, and derives LED operating power from the
MAX782 (U27) 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 pair of
INA138s acting as 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 R127 and is nominally 1.8A/V for a full-scale (3.3V)
current of 6Amps. A low pass filter (R128 and C134) provides filtering to remove
switching noise from the signal.
Revision E
MAC 5500 resting ECG analysis system
2-35
2020299-020
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