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ASM690AMJA Datasheet(PDF) 5 Page - Alliance Semiconductor Corporation |
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ASM690AMJA Datasheet(HTML) 5 Page - Alliance Semiconductor Corporation |
5 / 13 page 5 of 13 Notice: The information in this document is subject to change without notice ASM690A / 692A ASM802L / 802M ASM805L µP Power Supply Supervisor With Battery Backup Switch rev 1.0 October 2003 Backup Power Sources - Batteries Battery voltage selection is important to insure that the battery does not discharge through the parasitic device diode D1 (see Figure 3) when VCC is less than VBATT and VCC > VRT. Table 2: Maximum Battery Voltages Although most batteries that meet the requirements of Table 2 are acceptable, lithium batteries are very effective backup source due to their high-energy density and very low self- discharge rates. Battery replacement while Powered Batteries can be replaced even when the device is in a powered state as long as VCC remains above the reset threshold voltage VRT. In the ASM devices, a floating VBATT pin will not cause a powersupply switchover as can occur in some other supervisory circuits. If VBATT is not used, the pin should be grounded. Backup Power Sources - SuperCap™ Capacitor storage, with very high values of capacitance, can be used as a back-up power source instead of batteries. SuperCap™ are capacitors with capacities in the fractional farad range. A 0.1 farad SuperCap™ would provide a useful backup power source. Like the battery supply, it is important that the capacitor voltage remain below the maximum voltages shown in Table 2. Although the circuit of Figure 4 shows the most simple way to connect the SuperCap™, this circuit cannot insure that an over voltage condition will not occur since the capacitor will ultimately charge up to VCC. To insure that an over voltage condition does not occur, the circuit of Figure 5 is preferred. In this circuit configuration, the diode-resistor pair clamps the capacitor voltage at one diode drop below VCC. VCC itself should be regulated within ±5% of 5V for the ASM692A/802M or within ±10% of 5V for the ASM690A/802L/805L to insure that the storage capacitor does not achieve an over voltage state. Note: SuperCapTM is a trademark of Baknor Industries Part Number MAXIMUM Battery Voltage ASM690A 4.80 ASM802L 4.80 ASM805L 4.80 ASM692A 4.55 ASM802M 4.55 100K ASM692A ASM802M VCC VOUT VBATT RESET (RESET) GND To µP + 0.1F Figure 5: Capacitor as a backup power source Voltage clamped to 0.5V below VCC VCC VOUT VBATT RESET (RESET) GND To µP +5V 0.1F + Figure 4: Capacitor as a backup power source D2 D1 To SRAM D1 ASM692A ASM802M To SRAM +5V |
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