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TPS542951: Is it acceptable to output 8.8V?

jon watte
Prodigy 30 points
Part Number: TPS542951
Other Parts Discussed in Thread: TPS65270

I need to generate two intermediate rails, about 8.8V and 5.1V, from a 4 cell Lithium battery (so input voltage in the 12.8V to 16.8V range.)

I need about 1.5 Amps on the 9V rail and up to 2 Amps on the 5V rail. These will be reasonably well behaved, i e step changes in load will be small.

Looking at the data sheet for the TPS542951, the switch node should be able to deal with the voltages just fine, and I can calculate an appropriate feedback resistor value.

However, the "recommended components" Table 1 on page 14 of the datasheet only goes up to 6.5V output.

Would it be acceptable to use a 6.3uH inductor, 232 kOhm resistor, 2x22 uF X5R capacitors, and 22 pF feedforward coupling capacitor to generate 8.8V? The main concern here is that the corner frequency of the LC filter will be about 9.1 kHz. (On the other hand, the "extreme" values for the 5V rail of 68 uF and 4.7 uH give an 8.9 kHz corner frequency, which is why I think it might be OK.)

  • 0
    TI__Genius 9550 points

    Hi,

    I am looking into your request and will give you feedback tomorrow.

  • 0
    TI__Genius 15135 points

    Hi, Sir

    TPS542951 max output voltage is 7V, so it cannot support your application.

  • 0 in reply to Zhao Ma
    Prodigy 30 points

    Jason,

    Thanks for the answer.

    Note that the "features and capabilities" at the top of the datasheet are only nominal, not normative. (You say so yourself in your documentation!)

    In the "absolute maximum ratings" and in the "operating characteristics ratings," there is no mention of output voltage for the chip, because this voltage is only seen by the top of the voltage divider, and by the output inductor/capacitor.

    What, in the chip control circuit, is it that doesn't work with a 8.8V output voltage?

    There is no rating for maximum output inductance, either, only a rating for maximum peak output current.

    If there is something in the chip that ends up not working in this situation, is there an alternate part you can recommend?

  • 0 in reply to jon watte
    TI__Genius 15135 points

    Hi, jon

    Some comments:

    1. The DS first page usually provides the most important information for this part, the Max output is 7V.

    2. The IC doesn't have output pin, so output voltage isn't included in "absolute maximum ratings" and in the "operating characteristics ratings".

    3. TPS542951 uses D-CAP2 architecure, when Vout > 7V, internal ripple injection circuit's common-mode voltage will be out of range for comparator, which will cause IC work abnormally.

    4. Usually, we should choose inductor's saturation current > max current limit.

    5. Suggest TPS65270 for your application, it is peak current mode and can support high output voltage.