• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 65 subscribers
  • Views 1110 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS3808G09

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

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.

  • 0
    TI__Expert 5640 points

    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

     

  • 0
    TI__Intellectual 1330 points

    Customer came back and said:

    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.

    Stephen Taranovich said:

    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.

    TPS3808 diagram.pdf
  • 0 in reply to Stephen Taranovich
    Prodigy 10 points

    hi.. i am planning to use TPS3808 series supervisors.. my doubts regarding it are : 

    1> what if i try to supervise voltage rails above the threshold voltage For example : 3.3v rail connected directly to the sense pin of a "TPS3808g09 designed for 0.9V"

    2> what if i try and use a voltage divider circuit and monitor a 1.8v rail... Example... the voltage 1.8v divided to 0.9v at the sense point.. and fed the "TPS3808g09" instead of an adjustable Supervisor.. awaiting your valuable response..

  • 0 in reply to prashanth sujir
    TI__Mastermind 40100 points

    Hi Prashanth,

    1) In the abs max table on page 2 of the datasheet, Vsense pin is not allowed to be greater than 7V. Placing a 3.3V rail on sense will not damage the IC. When the rail reaches below the threshold voltage, RESET will trigger.

    2) Remember, there is already a voltage divider internal to the IC. You can still use an external resistor divider from 1.8V to Vsense however, in order not to ensure no offsets or loading effects of the internal resistor divider, the current throught the external resistor divider must be at least 100x the current going into SENSE (1.7uA).

    Regards,

    Darwin

  • 0 in reply to Darwin Fernandez
    Intellectual 415 points

    I have a question on this part with VDD and VSENSE. It was stated in the above post that the VSENSE pin can be driven safely with a voltage less than 7V. Does this imply that VSENSE Max is not depended on the voltage applied to the VDD pin? The data sheet does not state a dependency on VDD for Vsense.