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TPS92515AHV-Q1: Comparator offset issue

user6066721
Prodigy 40 points
Part Number: TPS92515AHV-Q1
Other Parts Discussed in Thread: TPS92515, TPS92515HV

When designed to have a 700mA output current, it often generates a 720mA output instead. It is related to the start-up, where the input voltage ramps up quite repeatedly and noisy. When in this 720mA mode, a IADJ dim. down and up (2V to 0V to 2V) does not affect the mode, it still returns to 720mA (at max dim.). Our guess is that it relates to the high-side comparator, which maybe compensates an offset, at start-up, and is disturbed by the rapid input voltage ramp up and noise?

It should be noted that this is not at tolerance issue, IADJ is exactly the same voltage in both cases, but output current has these two levels/modes (700mA and 720mA), which changes on a power cycle. Furthermore, is it not a device tolerance, as this is observed on the same device, in the same circuit. So the conclusion is that it relates to how the IC initializes.

Moreover, it is observed that if the IC is "reset", by shorting Vcc, it comes out the the 720mA mode. Possibly because the offset compensation is not disturbed by the startup, as this is at a steady input voltage. Can you assist on what could be the problem?

A workaround could be to force a "reset" (short Vcc for 1-10ms), but is this advisable?

Regards Martin

  • 0
    TI__Mastermind 28655 points

    Hello,

    The TPS92515 has a calibration circuit during turn on.  It adjusts by 5 mV increments, if it is next to the edge of a state it can come up in one offset or the other.  There is another part, the TPS92515A that doesn't not have this calibration during turn on.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hello

    Thank you for the insight and quick response.

    The 5mV incremental calibration, correlates with the observed discrete steps in output current. We are using the TPS92515HV and not the TPS92515AHV (which I actually thought we did, hence the wrong reference to it), so the solution seems to be straight forward and I will try the A-version. As a comment, I don't think that it is obvious from the datasheet, that it has this calibration circuit.

    Regards Martin

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Hello

    As the TPS92515AHV does not have this offset calibration, does this mean that it has a much smaller offset variation/tolerance, as the calibration is not required? And how much does this offset vary or how does this affect the accuracy of the device?

    From Figure 1. in the datasheet, I can derive a +-1.8% accuracy, which I assume is only device and temperature variation.

    Regards Martin

  • 0 in reply to user6066721
    TI__Mastermind 28655 points

    Hello Martin,

    The TPS92515AHV is the same as the TPS92515HV minus the calibration circuit.  The VCST voltage can vary from 205 mV to 234 mV, 220 mV nominal when Viadj is held to 2.2V.  If Viadj is pulled to VCC it is slightly less accurate at 222 mV to 257 mV, 240 mV nominal.  This is over -40C to 150C operation.  See the top of page 5 of the TPS92515AHV datasheet.  Note that the TPS92515HV has the calibration circuit at turn-on but it cannot correct for operation over temperature once powered on.  You can compare the two parts via the datasheets.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 40 points

    Ok. Thank you.

    Regards Martin