12
Voltage Reference Circuit
A voltage reference circuit provides a constant
DC voltage to the microprocessor Analog to
Digital Converter (ADC). By reading this input
and comparing the value to a predetermined
range, the microprocessor can validate the
accuracy of the 5‑volt power supply. Variations
in the 5‑volt supply left undetected can result in
inaccuracy in the low battery alarm set points
and variations in other calculated values.
Table 12. CADD‑Prizm
®
PCS II pump low battery
conditions.
† The pump emits 3 beeps every 5 minutes, and the
message “9 Volt Battery Low” appears on the pump’s
display, indicating that the battery power is low, but the
pump is operable.
†† The pump emits a continuous, variable‑tone alarm,
and the message “9 Volt Battery Depleted” appears on
the display, the battery power is too low to operate the
pump, and pump operation has stopped.
Voltage
CADD
®
Pump Status
Trip Point*
>7.0V
No alarm
6.4–7.0V*
Transition to low battery
condition; battery low
message appears; 3 beeps
every 5 min.
†
6.0–6.6V*
Transition to depleted
battery condition; battery
depleted message appears;
continuous alarm
††
5.25–5.95V
Hardware reset occurs.
Pump continues to indicate
depleted battery condition.
* Voltage ranges are due to component
tolerances. Actual trip values are
guaranteed to be non‑overlapping.
Power Circuitry
Power for the pump is normally supplied by a
9‑volt alkaline battery, 9‑volt lithium battery,
or AC adapter. These types of batteries have
a fairly low internal resistance over their
discharge range, which will keep power supply
noise low. Other types of batteries, such as
carbon‑zinc, exhibit high internal resistance,
especially near depletion. A voltage drop across
the internal resistance occurs when current is
drawn by the motor during pump activations.
This current is demanded in short pulses when
the motor is first turned on and generates large
spikes in the battery voltage. This noise can
cause the low battery detection circuit to shut
down the pump.
The motor driver circuit power is taken directly
from the battery, but the microprocessor and its
associated circuitry requires closely regulated
and filtered 5‑volt power which is supplied
from the micropower voltage regulator. This
regulator will supply 5‑volt power until its input
voltage is approximately 5.3 volts. After that
point, the output of the regulator will follow the
input voltage down.
