WM9715L
Production Data
BATTERY ALARM AND ANALOGUE COMPARATORS
The battery alarm function differs from battery measurement in that it does not actually measure the
battery voltage. Battery alarm only indicates “OK”, “Low” or “Dead”. The advantage of the battery
alarm function is that it does not require a clock and can therefore be used in low-power sleep or
standby modes.
Figure 13 Battery Alarm Example Schematic
The typical schematic for a dual threshold battery alarm is shown above. This alarm has two
thresholds, “dead battery” (COMP1) and “low battery” (COMP2). R1, R2 and R3 set the threshold
voltages. Their values can be up to about 1MΩ in order to keep the battery current [IALARM = VBATT
/
(R1+R2+R3)] to a minimum (higher resistor values may affect the accuracy of the system as leakage
currents into the input pins become significant).
•
Dead battery alarm: COMP1 triggers when VBATT < VREF × (R1+R2+R3) / (R2+R3)
A dead battery alarm is the highest priority of interrupt in the system. It should immediately save all
unsaved data and shut down the system. The GP15, GS15 and GW15 bits must be set to generate
this interrupt.
•
Low battery alarm: COMP2 triggers when VBATT < VREF × (R1+R2+R3) / R3
A low battery alarm has a lower priority than a dead battery alarm. Since the threshold voltage is
higher than for a dead battery alarm, there is enough power left in the battery to give the user a
warning and/or shut down “gracefully”. When VBATT gets close to the low battery threshold, spurious
alarms are filtered out by the COMP2 delay function.
The purpose of the capacitor C is to remove from the comparator inputs any high frequency noise or
glitches that may be present on the battery (for example, noise generated by a charge pump). It
forms a low pass filter with R1, R2 and R3.
•
Low pass cutoff fc [Hz] = 1/ (2π C × (R1 || (R2+R3)))
Provided that the cutoff frequency is several orders of magnitude lower than the noise frequency fn,
this simple circuit can achieve excellent noise rejection.
•
Noise rejection [dB] = 20 log (fn / fc)
The circuit shown above also allows for measuring the battery voltage VBATT. This is achieved simply
by setting the touchpanel ADC input to be either COMP1 (ADCSEL = 100) or COMP2 (ADCSEL =
101) (see also Auxiliary ADC Inputs).
PD Rev 4.0 December 2007
48
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
