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WM9711 Datasheet(PDF) 39 Page - Wolfson Microelectronics plc |
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WM9711 Datasheet(HTML) 39 Page - Wolfson Microelectronics plc |
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39 / 66 page  Production Data WM9711L w PD Rev 4.1 April 2004 39 COMP1 COMP2 + - - + V REF GPIO2/ IRQ GPIO PINS DEAD BAT LOW BAT GPIO / INTERRUPT LOGIC C R2 R3 R1 V BATT WM9711L I ALARM VOLTAGE REGULATOR AVDD, DCVDD, ... Figure 6 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 1M Ω 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/ (2 π 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) |
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