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TPS3600D20 Datasheet(PDF) 8 Page - Texas Instruments |
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TPS3600D20 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 26 page TPS3600D20, TPS3600D25, TPS3600D33, TPS3600D50 BATTERY BACKUP SUPERVISORS FOR LOW POWER PROCESSORS SLVS336B − DECEMBER 2000 − REVISED JANUARY 2007 8 detailed description (continued) power-fail comparator (PFI and PFO) An additional comparator is provided to monitor voltages other than the nominal supply voltage. The power-fail input (PFI) will be compared with an internal voltage reference of 1.15 V. If the input voltage falls below the power-fail threshold, V(PFI), of 1.15 V typical, the power-fail output (PFO) goes low. If it goes above V(PFI) plus about 12-mV hysteresis, the output returns to high. By connecting two external resistors, it is possible to supervise any voltages above V(PFI). The sum of both resistors should be about 1 MΩ, to minimize power consumption and also to ensure that the current in the PFI pin can be neglected compared with the current through the resistor network. The tolerance of the external resistors should be not more than 1% to ensure minimal variation of sensed voltage. If the power-fail comparator is unused, connect PFI to ground and leave PFO unconnected. BATTON Most often BATTON is used as a gate drive for an external pass transistor for high-current applications. In addition it can be also used as a logic output to indicate the battery switchover status. BATTON is high when VOUT is connected to VBAT. BATTON can be directly connected to the gate of a PMOS transistor (see Figure 3). No current-limiting resistor is required. When using a PMOS transistor, it must be connected backwards from the traditional method (see Figure 3). This method orients the body diode from VDD to VOUT and prevents the backup battery from discharging through the FET when its gate is high. TPS3600 VDD VOUT BATTON DS G GND PMOS FET Body Diode Figure 3. Driving an External MOSFET Transistor With BATTON backup-battery switchover In the event of a brownout or power failure, it may be necessary to keep a processor running. If a backup battery is installed at VBAT, the devices automatically connect the processor to backup power when VDD fails. In order to allow the backup battery (e.g., a 3.6-V lithium cell) to have a higher voltage than VDD, this family of supervisors will not connect VBAT to VOUT when VBAT is greater than VDD. VBAT only connects to VOUT (through a 2-Ω switch) when VOUT falls below V(SWN) and VBAT is greater than VDD. When VDD recovers, switchover is deferred either until VDD crosses VBAT, or when VDD rises above the threshold V(SWP). (See the timing diagram) VDD > VBAT VDD > V(SW) VOUT 1 1 VDD 1 0 VDD 0 1 VDD 0 0 VBAT www.ti.com |
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