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LM43603-Q1: LM43603-Q1: What is the max tolerance of switching frequency?

SY Yoon
Genius 3380 points
Part Number: LM43603-Q1

Hi,

LM43603-Q1 is designed with input voltage range is from 9V to 36V and output is 5V in my customer's board.

RT pin of LM43603-Q1  is connected with a resistor to make 990KHz switching frequency.

The resistor value is 40.2Kohm and it has 1% tolerance. 

But, measured switching frequency is 870KMHz.

This has an 12% error.

In the datasheet, Frequency adjust accuracy is 10% and I know that it is typical value.

What is the max tolerance switching frequency of LM43063-Q1?

Is 12% error normal?

  • 0
    TI__Mastermind 27045 points

    Hi SY,

    The freq equation is delivered by curve-fitting, so there maybe slight higher tolerance on some freq point, we don't have the max variation data.

    if you have more tight accuracy requirement, you can tune the RT resistor till the SW freq is target 990KHz at load current above 1.5A. then use this value in circuit design.

    B R

    Andy 

  • 0 in reply to Andy Chen
    Genius 3380 points

    Hi Andy,

    When the RT pin open circuit, waveform of switching output (SW pin) is not normal. 

    square wave of switching output is not clear.

    When RT pin with 40.2Kohm, desired switching frequency is 990KHz.

    But measured frequency is 866Hz. It is over tolerance.

  • 0 in reply to SY Yoon
    TI__Mastermind 27045 points

    Hi SY

    The circuit works in DCM mode according to SW waveform measured with RT open, and works in CCM mode while freq increased to 990KHz,  but actual freq may decrease at light load condition.

    So please measure SW freq at load 1.5A or higher, customer can bias the output with E-load

  • 0 in reply to Andy Chen
    Genius 3380 points

    Hi Andy,

    I understand that the typical tolerance of accuracy is 10% but it hasn't max tolerance.

    Most units have similar frequency and characteristics, but only one unit has an different characteristic.

    The normal unit has 45mA and over load current with 36Vin and 5Vout at 990KHz.

    And the switching freq is also close to 990KHz.

    But abnormal unit has 39mA load current and 870 KHz freq,

    Should I request failure analysis?

    What can I decide to be normal?

    Are there some points to check if it is normal or not?

  • 0 in reply to SY Yoon
    TI__Mastermind 28940 points

    Hello SY,

    At light load current, ie: <0.5A, the frequency of the converter will be modulated down from the frequency set point estabilished by RT resistor.

    At higher load current, ie: >0.5A, the frequency of the converter will be at set point estabilished by RT resistor..

    Higher Vin, may result in the frequency to be at set point estabilished by RT resistor, even with light load, as the switching frequency is modulated downards w.r.t peak inductor current.

    With that said, 10% accuracy setting is w.r.t. measured switching frequency at high load currents.

    There may be some variance with one part to the other at light loads due to variance in sampled peak inductor current, or variance in that  measurement.

    If tight frequency accuracy is required over all load range of this power converter, I would recommend a converter with constant frequency operation such as LM62440.