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LM53603: LM53603 PWM Off Time and PWM Frequency

Akihisa Tamazaki
Genius 12460 points
Part Number: LM53603

Hello team,

 

I have a similar question as below.

https://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/p/587543/2159864#2159864

 

The customer try to use LM53603EVM as the following condition.
Vin = 5.8V
Vout = 5.0V
Iout: variable

The customer found that PWM off time was changed when the Iout was changed. The Iout become larger, PWM off time is longer. He cannot understand this behavior.
I believe that LM53603 change the off time if Iout is changed, then PWM frequency is changed, when Vin is close to Vout. Is that correct?
Please advise me about the LM53603 behavior, and correct me if I took mistake.

 

 

Best Regards,

Akihisa Tamazaki

  • 0
    TI__Expert 6860 points

    Tamazai-san,

    What your customer see is intentional . Refer to Figure 28 of the LM53603 datasheet .

    The graph above depicts the frequency foldback in the dropout condition at various load . The reason for the frequency foldback is the min off time limitation which arises from the fact that the Low side FET is needed to turn on to allow for the Bootstrap cap  charge to buildup . So when Vin and vout is close and the switching frequency is high , the on time of Low side FET wont be enough for bootstrap cap chage up time . So the solution is to increase the on time of Low side FET which is achieved by decreasing the frequency .

    ---Ambreesh