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MAX801_EPA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX801_EPA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page calculate the amount of capacitance required to allow the shutdown routine to complete before reset is asserted: CHOLD = (ILOAD x tSHDN) / (VLR(min)) where tSHDN is the time required for the system to com- plete the shutdown routine (including the VCC to low- line propagation delay), ILOAD is the current being drained from the capacitor, and VLR is the low-line to reset threshold. Output Supply Voltage The output supply (OUT) transfers power from VCC or BATT to the µP, RAM, and other external circuitry. At the maximum source current of 250mA, VOUT will typi- cally be 220mV below VCC. Decouple OUT with a 0.1µF capacitor to ground. Battery-Backup Mode Battery-backup mode preserves the contents of RAM in the event of a brownout or power failure. With a backup battery installed at BATT, the MAX801/MAX808 automati- cally switches RAM to backup power when VCC falls. Two conditions are required for switchover to battery- backup mode: 1) VCC must be below the reset threshold; 2) VCC must be below VBATT. Table 1 lists the status of inputs and outputs during battery-backup mode. BATT is designed to conduct up to 20mA to OUT dur- ing battery backup. The PMOS switch on-resistance is approximately 12 Ω. Figure 4 shows the two series pass elements (between the BATT input and OUT) that facilitate UL recognition. VBATT can exceed VCC during normal operation without causing a reset. 8-Pin µP Supervisory Circuits with ±1.5% Reset Accuracy _______________________________________________________________________________________ 9 GND VCC TO μP NMI CHOLD CHOLD > ILOAD x tSHDN VLR 4.5V to 5.5V LOWLINE MAX801 MAX808 REGULATOR Figure 3. Using LOWLINE to Provide a Power-Fail Warning to the µP MAX801 MAX808 P PP OUT VCC BATT 0.1 μF CONTROL CIRCUITRY Figure 4. VCC and BATT to OUT Switch RESET Logic low 3 3 RESET Logic high; the open-circuit voltage is equal to VCC. — VCC PIN 5 GND LOWLINE Ground—0V reference for all signals 4 4 WDI WDI is ignored and goes high impedance. — 6 BATT Supply current is 1µA max for VBATT ≤ 2.8V. 7 7 CE IN CE OUT High impedance (MAX808) 6 — Logic high. The open-circuit output voltage is equal to VBATT (MAX808). 5 — OUT OUT is connected to BATT through two internal PMOS switches in series. NAME 8 8 Battery switchover comparator monitors VCC for active switchover. 1 1 Logic low 2 2 MAX801 MAX808 STATUS Table 1. Input and Output Status in Battery-Backup Mode |
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