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MAX6363HUT Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX6363HUT Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page Detailed Description The Typical Operating Circuit shows a typical connection for the MAX6361–MAX6364 family. OUT powers the stat- ic random-access memory (SRAM). OUT is internally connected to VCC if VCC is greater than the reset thresh- old, or to the greater of VCC or VBATT when VCC is less than the reset threshold. OUT can supply up to 150mA from VCC. When VCC is higher than VBATT, the BATT ON (MAX6363) output is low. When VCC is lower than VBATT, an internal MOSFET connects the backup battery to OUT. The on-resistance of the MOSFET is a function of backup-battery voltage and is shown in the Battery to Out On-Resistance vs. Temperature graph in the Typical Operating Characteristics section. Backup-Battery Switchover In a brownout or power failure, it may be necessary to preserve the contents of the RAM. With a backup bat- tery installed at BATT, the MAX6361–MAX6364 auto- matically switch the RAM to backup power when VCC falls. The MAX6363 has a BATT ON output that goes high when in battery-backup mode. These devices require two conditions before switching to battery- backup mode: 1) VCC must be below the reset threshold. 2) VCC must be below VBATT. Table 1 lists the status of the inputs and outputs in bat- tery-backup mode. The device will not power up if the only voltage source is on BATT. OUT will only power up from VCC at startup. Manual Reset Input (MAX6361 Only) Many µP-based products require manual reset capabili- ty, allowing the operator, a test technician, or external logic circuitry to initiate a reset. For the MAX6361, a logic low on MR asserts reset. Reset remains asserted while MR is low, and for a minimum of 150ms (tRP) after it returns high. MR has an internal 20k Ω pull-up resistor to VCC. This input can be driven with TTL/CMOS logic lev- els or with open-drain/collector outputs. Connect a nor- mally open momentary switch from MR to GND to create a manual reset function; external debounce circuitry is not required. If MR is driven from long cables or the device is used in a noisy environment, connect a 0.1µF capacitor from MR to GND to provide additional noise immunity. Watchdog Input (MAX6362 Only) The watchdog monitors µP activity through the input WDI. If the µP becomes inactive, the reset output is asserted in pulses. To use the watchdog function, con- nect WDI to a bus line or µP I/O line. A change of state (high to low or low to high) within the watchdog timeout period (tWD) with a 100ns minimum pulse width clears the watchdog timer. If WDI remains high or low for longer than the watchdog timeout period, the internal watchdog timer runs out and a reset pulse is triggered for the reset timeout period (tRP). The internal watchdog timer clears whenever reset asserts or the WDI sees a rising or falling edge within the watchdog timeout period. If WDI remains in a high or low state for an extended period of time, a reset pulse asserts after every watchdog timeout period (tWD) (Figure 1). Reset In (MAX6364 Only) RESET IN is compared to an internal 1.235V reference. If the voltage at RESET IN is less than 1.235V, reset is asserted. The RESET IN comparator may be used as an undervoltage detector to signal a failing power sup- ply. It can also be used as a secondary power-supply reset monitor. To program the reset threshold (VRTH) of the secondary power supply, use the following equation (see Typical Operating Circuit): where VREF = 1.235V. To simplify the resistor selection, choose a value for R2 and calculate R1: Since the input current at RESET IN is 25nA (max), large values (up to 1M Ω) can be used for R2 with no signifi- cant loss in accuracy. For example, in the Typical SOT23, Low-Power µP Supervisory Circuits with Battery Backup 6 _______________________________________________________________________________________ Table 1. Input and Output Status in Battery-Backup Mode PIN STATUS VCC Disconnected from OUT OUT Connected to BATT BATT Connected to OUT. Current drawn from the battery is less than 1µA (at VBATT = 2.8V, excluding IOUT) when VCC = 0. RESET/RESET Asserted BATT ON High state MR, RESET IN, WDI Inputs ignored VV R R RTH REF =+ ⎛ ⎝⎜ ⎞ ⎠⎟ 1 2 1 RR V V RTH REF 12 1 / = () − [] |
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