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TPS92513: The value of VISENSE not equal to IADJ/6 as the VIN rises

David Huang
Intellectual 2285 points
Part Number: TPS92513

Hi Team

For the application with IADJ fixed and VIN rises from 24V to 29V,

IADJ suddenly dropped to about half the value after VIN rises to 27.9V and the VISENSE became not equal to IADJ/6.

Under what condition would VISENSE become not equal to IADJ/6? (What's the mechanism that trigger such IADJ drop?)

The detailed test results are shown as below.

Signal

Measured value

　

+24V(VIN)

24

27.8

27.9

29

V

IADJ

648.9

646.7

366.5

366.4

mV

VISENSE

110.2

110.1

111

127.5

mV

IADJ/6

108.2

107.8

61.1

61.1

mV

Much appreciate.

Regards

David

  • 0
    TI__Intellectual 2285 points

    Hi team

    It seems like the problem I mentioned above might have occurred because the switching cycle is lower than the minimum cycle ton(min) = 140ns according to the calculation through equation 12.

    But then for the application below, the result comes from equation 12 would be ton=130ns, which is lower than the ton(min) = 140ns and should have been malfunctioning.

    However, this worked just fine and the actual test result regarding the ton was 166ns.

    Could you check if this is normal and does the condition below work?

    Vin=24V

    Vout=1.8V

    Fsw=574kHz

    Regards

    David

  • 0 in reply to David Huang
    TI__Genius 13001 points

    Hi David,

    Please refer the below formula. It's not recommended to adopt such low convert ratio.

  • 0 in reply to Yuting Shao
    TI__Intellectual 2285 points

    Hi Yuting

    Could you confirm what's the lowest recommended convert ratio?

    Also if you could let me know why the equation didn't work under the following condition it would be helpful.

    Vin=24V

    Vout=1.8V

    Fsw=574kHz

    (The result is ton=130ns from the equation but the test result was ton=166ns)

    Regards

  • 0 in reply to David Huang
    TI__Genius 13001 points

    Hi David,

    The lowest convert ratio depends on switching frequency. For 574kHz, you can calculate it about 0.08 with above formula.

    You should be aware that the minimum off time 140ns@typ(you can find this in datasheet) is the physical limitation. In other words, for any situation, the off time should be larger than 140ns@typ.

    Robin