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TPS3808

Stephen Taranovich
Intellectual 1330 points
Other Parts Discussed in Thread: TPS3808

Attached is part of the circuit that includes the supervisor and the DC-DC switcher. As you can see I'm going to monitor the battery voltage. When it's 0.9 V - the battery is practically dead - I want the output of the TPS3808 U21 shuts down the switching power supply U23. Vcc is 3 V and generated by U23. When the RST pin of the 3808 is pulled down, it disables U23 and enables another power supply to guarantee VCC always be at 3 V.

 

I wouldn't mind to have a 0.84 threshold so that I could squeeze a bit more juice out of the battery. The thing is that, even though the circuits powered by Vcc always draw very little current (less than 2 mA), once in a while they may need a bit more juice (about 20 mA). Therefore, at voltages below 0.9 volts there probably is little juice or no juice at all... and the RST would toggle up and down often when that short pulse of current is requested from the battery. It's not exactly a problem because the other power supply backs it up but I would prefer disabling U23 to have a clean switch from one power supply to the other.

 

The TPS3808G09 has a Vit of 0.84 and the next is the G12. Is using two resistors the only way to get 0.9 V as the trip voltage? You said the G09 is typically used to supervise the 0.9 V rail. Do you advise me to use it as 0.9 V is its nominal supply voltage? Frankly, I would prefer a fixed value than having to use two resistors.

 

I bypassed the sense pin with a 10 nF cap as the manual suggests and used a 390 nF for a reset delay time of approximately 2 seconds. I still have the option to pull Ct up with the 100 K resistor if I need a shorter (300 ms) delay.  Therefore, I can use one or the other.

 

As that 20 mA pulse of current will be less than 1 second, would the G09 be the correct choice? Looking at the timing diagram, it would switch back to the power supply U23 "long" after (1 second later) the voltage went above the 0.9 V, right?

 

Any other ideas or suggestions would be welcome.

TPS3808 diagram.pdf
  • 0
    TI__Expert 5640 points

    First, you need to decide what your threshold voltage will be for this circuit.  The G09 version is meant to monitor a 0.9V power rail (typically the core voltage of a microprocessor) so the threshold is set below 0.9V.  In your case you probably want to have the threshold set to 0.9V as an end of charge indication of a battery.  Operating an alkaline battery below 0.9V is not recommended because it leads to battery leakage with your board covered in caustic brown goo.

    There are two ways to get a 0.9V threshold voltage.  The first is to request a custom device from the product line and make the buisness case.  The other is to use the adjustable with a resistor divider which is what I would recommend.

    As far as the input voltage fluxuating with battery current, you can add a resistor from the /reset output back to the SENSE pin to increase the hysteresis of the sensing circuit.  This would allow you to enter reset when the voltage reaches 0.9 (or what ever you set it too) and not exit reset until the battery voltage is much higher (say 1.0V).  In other words, once it reaches its absolute minimum voltage, it stays in reset until the battery is replaced.  You can look at application note SLVA360 to see how to do this.