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ADM695AN Arkusz danych(PDF) 5 Page - Analog Devices |
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ADM695AN Arkusz danych(HTML) 5 Page - Analog Devices |
5 / 16 page ADM690–ADM695 REV. A –5– PIN CONFIGURATIONS PRODUCT SELECTION GUIDE Part Nominal Reset Nominal VCC Nominal Watchdog Battery Backup Base Drive Chip Enable Number Time Reset Threshold Timeout Period Switching Ext PNP Signals ADM690 50 ms 4.65 V 1.6 s Yes No No ADM691 50 ms or ADJ 4.65 V 100 ms, 1.6 s, ADJ Yes Yes Yes ADM692 50 ms 4.4 V 1.6 s Yes No No ADM693 50 ms or ADJ 4.4 V 100 ms, 1.6 s, ADJ Yes Yes Yes ADM694 200 ms 4.65 V 1.6 s Yes No No ADM695 200 ms or ADJ 4.65 V 100 ms, 1.6 s, ADJ Yes Yes Yes CIRCUIT INFORMATION Battery Switchover Section The battery switchover circuit compares VCC to the VBATT input, and connects VOUT to whichever is higher. Switchover occurs when VCC is 50 mV higher than VBATT as VCC falls, and when VCC is 70 mV greater than VBATT as VCC rises. This 20 mV of hysteresis prevents repeated rapid switching if VCC falls very slowly or remains nearly equal to the battery voltage. Figure 1. Battery Switchover Schematic During normal operation with VCC higher than VBATT, VCC is in- ternally switched to VOUT via an internal PMOS transistor switch. This switch has a typical on-resistance of 1.5 Ω and can supply up to 100 mA at the VOUT terminal. VOUT is normally used to drive a RAM memory bank which may require instanta- neous currents of greater than 100 mA. If this is the case then a bypass capacitor should be connected to VOUT. The capacitor will provide the peak current transients to the RAM. A capaci- tance value of 0.1 µF or greater may be used. If the continuous output current requirement at VOUT exceeds 100 mA or if a lower VCC–VOUT voltage differential is desired, an external PNP pass transistor may be connected in parallel with the internal transistor. The BATT ON output (ADM691/ ADM693/ADM695) can directly drive the base of the external transistor. A 20 Ω MOSFET switch connects the V BATT input to VOUT during battery backup. This MOSFET has very low input-to- output differential (dropout voltage) at the low current levels required for battery back up of CMOS RAM or other low power CMOS circuitry. The supply current in battery back up is typically 0.6 µA. The ADM690/ADM691/ADM694/ADM695 operates with battery voltages from 2.0 V to 4.25 V and the ADM692/ADM693 operates with battery voltages from 2.0 V to 4.0 V. High value capacitors, either standard electrolytic or the farad size double layer capacitors, can also be used for short-term memory back up. A small charging current of typically 10 nA (0.1 µA max) flows out of the VBATT terminal. This current is useful for maintaining rechargeable batteries in a fully charged condition. This extends the life of the back up battery by compensating for its self discharge current. Also note that this current poses no problem when lithium batteries are used for back up since the maximum charging current (0.1 µA) is safe for even the smallest lithium cells. If the battery-switchover section is not used, VBATT should be connected to GND and VOUT should be connected to VCC. 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 TOP VIEW (Not to Scale) ADM691 ADM693 ADM695 GND V BATT V OUT PFI PFO WDO V CC RESET BATT ON LOW LINE OSC IN OSC SEL RESET CE IN CE OUT WDI GND V BATT V OUT PFI PFO WDI RESET V CC 1 2 3 4 8 7 6 5 TOP VIEW (Not to Scale) ADM690 ADM692 ADM694 |
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