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BQ25628: TS voltage measurement signal is not a DC signal, it is a pulse train

Aaron_Rangel
Intellectual 440 points
Part Number: BQ25628

Tool/software:

Hello,

I have designed a battery charger using BQ25628 and I designed RT1 & RT2 resistors for an 103AT NTC thermistor.

Basically, I followed suggestions in the datasheet for this design.

I tried to get all the values in the TS pin (which for this PN is referenced to TS_BIAS and this internally comes from REGN) by means of a potentiometer that allows it to move from 0ºC to almost 80ºC.

So, basically I would expect to see a voltage values proportional to an equivalent resistance as per table below:

The fact is, when I measured the signal in the TS pin I got pulses that are not symmetric so it is impossible to get a constant DC voltage level.

In addition, for all the range of the operating voltage the amplitude of the signal seems to increase or decrease, too. This depends on the equivalent resistance as per potentiometer setting, indeed.

This picture was captured without connecting the potentiometer but it has more or less the same pattern for all the temperature measurement range.

That is to show the potentiometer is not adding external influence.

How is it that I get a non DC level in this pin? If so, how can I get the voltage levels to verify that this voltage matches with resistances and so the temperatures?

I would like to put values on this table.

On the other hand, the Battery charger seems to work well, I can charge and discharge when connecting a battery and so on.

I measured REGN and I got a clean DC level of 5V, which is correct.

Best Regards,

  • 0
    Intellectual 440 points

    I guess that TS_ADC register (REG0x34) could show  measurements the way I need, but is there a way to get this voltage without interacting by software (I2C)?.

    Could you please help me with these questions?

    Best Regards,

  • 0 in reply to Aaron_Rangel
    TI__Mastermind 27710 points

    Hi Aaron, 

    Please see my comments below. 

    The observed pulses of the TS voltage waveform are expected because function of TS_BIAS is to internally disconnect the TS resistor divider circuit from REGN LDO, which is pullup source, whenever a temperature measurement is not being taken. This is described in datasheet section 8.3.5.4.5 on page 28. 

    For the ease of your testing you have two options to force the TS voltage signal to be DC. One option is to simply short TS_BIAS pin to REGN pin and the other option is to enable the ADC in continuous mode. With ADC enabled the TS pin signal will be DC. To enable ADC you need to set REG0x26[7] = 1. 

    Please let me know if you have any further questions. 

    Best Regards,

    Garrett

  • +1 in reply to Garrett Kreger
    Intellectual 440 points

    Hello Garrett,

    So It's clear that the behaviour is normal.

    Just one comment on this, it is a little bit tricky to understand that this wording in the datasheet "When no temperature measurement is being taken, the switch is disabled to disconnect the thermistor and resistor-divider from the REGN LDO, ..." really means and imply that there is a non-DC level on this pin and it is somehow switching for saving bias current purposes.

    Specifically because it is not clear for me when the IC decides to read or not assuming that this only happens when the temperature is within the allowed zones for charging (COOL, PRECOOL, NORMAL, PREWARM and WARM). 

    Anyway, we can have a DC level just proceeding the way you described in your reply.

    Now we know a bit more of this IC.

    Thank you for the feedback, I really appreciate it.

    Best Regards,