In a new design, I’m using a TPS3808 to supervise the voltage of an AA battery. If the voltage is below 0.9 V then the RST pin switches to another battery. In another circuit of the same project, the RST pin simply turns off a switching regulator and allows current from another power supply to supply the circuits powered by the converter just turned off. I selected the version 0.9 because AAs don’t have much more juice left below 0.9 V. In re-reading the datasheet I was not sure about the specs. In page 2 it shows TPS3808G09 with a nominal supply voltage of 0.9 V and a threshold voltage of 0.84 V. What is 0.9 V? Should I use the version G01 and add two resistors so I can have a thresold a bit highr that 0.84? Or should I live it with the 0.9 V version? Why is the threshold voltage so low in the G01 version (0.405 V)? The supervisory chips are powered by a 3 V power supply powered by a lithium battery which cannot be turned off.
There are four voltages relating to the operation of the TPS3808 (or any of our voltage supervisors). There is the voltage applied to VDD of the IC which is used to power the internal workings of the chip. Then there is the voltage rail you wish to monitor (this can be the same as VDD or not). The voltage rail to be monitored is applied to a resistor divider which produces a voltage at the SENSE pin (Vsense). When the voltage at the SENSE pin falls below the threshold voltage (nominal 0.4V for the TPS3808) then the reset output will be driven low.
For fixed voltage versions of a supervisor, the resistor divider is internal to the IC. The TPS3808G09 is typically used to supervise a 0.9V rail. 0.9V is the nominal voltage of the rail so we want the TPS3808 to assert reset low when the voltage on the 0.9V rail drops. The internal resistor divider is programmed so that when the 0.9V rail drops below 0.84V, then the reset pin is driven low. If you need reset asserted when the voltage rail drops below 0.9V then you would need to use the TPS3808G01 with an external voltage divider.
Another important point here is the voltage on VDD. The TPS3808 requires a voltage on the VDD pin between 1.7 - 6.5V in order to power the internal workings of the IC. If you plan on powering the VDD pin from the battery, you will not have valid reset pin results below 1.7V. The reset pin is forced low for VDD voltages between 0.8V - 1.7V. The reset output versus VDD can be sumarized as follows:
VDD RANGE RESET OUTPUT
0-0.79V High impedance, will float to whatever voltage it is pulled up to
0.80 - 1.69V low impedance, forced low regardless of voltage on sense pin
1.7V - 6.5V Normal operation, output determined by voltage on sense pin
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