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LM53602-Q1: Switching Modes Dynamically between FPWM and AUTO

Shinichi Yokota
Genius 9715 points
Part Number: LM53602-Q1

Hello,

I'd like to ask for urgent help. I've tried FPWM Mode so far in order to get a constant switching frequency, however, am now seriously considering AUTO Mode, too, in order to significantly reduce operating supply current at light loads.

Questions

  • Is it OK to dynamically switch between FPWM Mode and AUTO Mode while the LM53602-Q1 is operating?
    • My idea is to use FPWM Mode at heavy loads for its constant switching frequency and to use AUTO Mode at light loads to reduce supply current. The system can roughly know the load status.
  • If it's OK, how will the transient response caused by the mode change look like? Can overshoot/undershoot voltage level at the output be estimated?

Additional Questions

  • How can the Max current that flows into the BIAS pin in FPWM Mode be estimated? I saw a higher current than the data sheet specs (47 µA (Typ), 78 µA (Max)).
  • The same question as the above in AUTO Mode - how can the Max current into the BIAS pin be estimated?

Best regards,
Shinichi Yokota

  • 0
    TI__Expert 6860 points
    Yokota-san,

    Please find my comments below :

    1. Yes it is OK to switch dynamically between FPWM and auto mode .
    2. The Device would wait for few cycle and will enter (<100uS) FPWM or auto mode . The transition is smooth as mentioned in Figure 32 of the datasheet . In Auto mode you would see a little higher voltage at the output compared to FPWM mode and the same would be seen during the transition .

    3. Bias current is measured in the open loop by connecting 5V at the bias supply, connecting FPWM pin to 3.3V and the FB pin is set to 5.5 V for this test. What you are measuring would be the close loop data .

    4. Same as 3.

    ---Ambreesh
  • 0 in reply to Ambreesh Tripathi
    TI__Genius 9715 points

    Ambreesh-san,

    Thank you for quick feedback. In summary, is there no equation or formula to estimate a BIAS pin current while the device is in operation?

    Best regards,
    Shinichi Yokota

  • 0 in reply to Shinichi Yokota
    TI__Expert 6860 points
    Yakota-San,

    In FPWM mode , the Bias current will typically will be in the range of 15mA in FPWM mode and 47uA would be a typical value in the Auto mode . What you see in the datasheet is the spec with the FB connectd to 5.5V(so no switching ).

    Is the bias current requirement a concern ? The customer would be interested in Iq from the power supply, i guess, which can be estimated roughly .

    ---Ambreesh