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MAX690_ESA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX690_ESA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Backup-Battery Switchover In the event of a brownout or power failure, it may be necessary to preserve the contents of RAM. With a backup battery installed at VBATT, the devices auto- matically switch RAM to backup power when VCC falls. This family of µP supervisors (designed for 3.3V and 3V systems) doesn’t always connect VBATT to VOUT when VBATT is greater than VCC. VBATT connects to VOUT (through a 140 Ω switch) when VCC is below VSW and VBATT is greater than VCC, or when VCC falls below 1.75V (typ) regardless of the VBATT voltage. This is done to allow the backup battery (e.g., a 3.6V lithium cell) to have a higher voltage than VCC. Switchover at VSW (2.40V) ensures that battery-backup mode is entered before VOUT gets too close to the 2.0V minimum required to reliably retain data in CMOS RAM. Switchover at higher VCC voltages would decrease backup-battery life. When VCC recovers, switchover is deferred until VCC rises above the reset threshold (VRST) to ensure a stable supply. VOUT is connected to VCC through a 3Ω PMOS power switch. Manual Reset A logic low on – M — R – asserts reset. Reset remains asserted while – M — R – is low, and for tWP (200ms) after – M — R – returns high. This input has an internal 70µA pull-up current, so it can be left open if it is not used. – M — R – can be driven with TTL or CMOS logic levels, or with open-drain/collector outputs. Connect a normally open momentary switch from – M — R – to GND to create a manual-reset function; external debounce circuitry is not required. __________Applications Information These µP supervisory circuits are not short-circuit protected. Shorting VOUT to ground—excluding power- up transients such as charging a decoupling capacitor—destroys the device. Decouple both VCC and VBATT pins to ground by placing 0.1µF capacitors as close to the device as possible. Using a SuperCap as a Backup Power Source SuperCaps™ are capacitors with extremely high capacitance values (e.g., order of 0.47F) for their size. Figure 3 shows two ways to use a SuperCap as a backup power source. The SuperCap may be connected through a diode to the 3V input (Figure 3a) or, if a 5V supply is also available, the SuperCap may be charged up to the 5V supply (Figure 3b) allowing a longer backup period. Since VBATT can exceed VCC while VCC is above the reset threshold, there are no special precautions when using these µP supervisors with a SuperCap. Operation without a Backup Power Source These µP supervisors were designed for battery- backed applications. If a backup battery is not used, connect both VBATT and VOUT to VCC, or use a different µP supervisor such as the MAX706T/S/R or MAX708T/S/R. Replacing the Backup Battery The backup power source can be removed while VCC remains valid, if VBATT is decoupled with a 0.1µF capacitor to ground, without danger of triggering RESET/ – R — E — S — E — T – . As long as VCC stays above VSW, battery-backup mode cannot be entered. Adding Hysteresis to the Power-Fail Comparator The power-fail comparator has a typical input hysteresis of 10mV. This is sufficient for most applica- tions where a power-supply line is being monitored through an external voltage divider (see the Monitoring an Additional Power Supply section). If additional noise margin is desired, connect a resistor between – P — F — O – and PFI as shown in Figure 4a. Select the ratio of R1 and R2 such that PFI sees 1.237V (VPFT) when VIN falls to its trip point (VTRIP). R3 adds the hysteresis and will typically be more than 10 times the value of R1 or R2. The hysteresis window extends both above (VH) and below (VL) the original trip point (VTRIP). 3.0V/3.3V Microprocessor Supervisory Circuits 8 _______________________________________________________________________________________ SuperCap is a trademark of Baknor Industries. PIN NAME STATUS VOUT Connected to VBATT through an internal 140 Ω switch VCC Disconnected from VOUT PFI The power-fail comparator is disabled when VCC < VSW – P — F — O – Logic low when VCC < VSW or PFI < VPFT WDI The watchdog timer is disabled – M — R – Disabled – R — E — S — E — T – Low logic RESET High impedance VBATT Connected to VOUT Table 1. Input and Output Status in Battery-Backup Mode |
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