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LM53603: LM53603 switching frequency when Vin is close to Vout

Vincent Chen64988
Expert 4480 points
Part Number: LM53603
Other Parts Discussed in Thread: LM43603, LM46002

Hi Team,

My customer got a LM53603 EVM and have a question related to switching frequency.

Customer changed the Vo voltage to 8V on the EVM, the load current is around 2A. When he tried to lower the Vin to around 8.5V ~ 9V. The switching frequency supposed should be maintaining at 2.1MHz(FPWM pin is connected to high to set device at forced PWM mode). But he found the Fsw is changed to 1MHz.

Please could team help to provide your comment on this phenomenon.

Thanks very much.

Vincent Chen

  • 0
    TI__Expert 4365 points
    Yes, This is intentional behavior. There is a maximum duty cycle limit on all buck regulators as Vin gets lowered to approach Vout. Once you reach maximum duty factor the stage regulation will go into dropout and Vout will start dropping as it tracks Vin downward. This occurs because of the limit in the Off-time of the power stages.

    An alternative behavior as Vin is lowered is called On-Time Extension. This keeps the Off-time essentially at it's limit but increases On-time to maintain regulation. The byproduct is that the frequency decreases because it takes a secondary place in the control loop in this mode. LM43603 and LM46002 were the first devices to behave this way and the data sheets include plots of operating frequency as a function of Vin and load current so the effect is easier to visualize. These graphs are provided for the common output voltages.
  • 0 in reply to Alan Martin
    TI__Expert 4480 points

    Hi Martin,

    Thanks a lot for the description. I got your point and will reply customer for your comment.

    Vincent Chen