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LM53603: behavior when the output voltage higher then the target voltage

Vsevolod Elantsev4
Intellectual 2165 points
Part Number: LM53603
Other Parts Discussed in Thread: TPS61022

Hi team,

My customer wants to use LM53603 as a main power supply and TPS61022 as a backup power supply from a battery. There is diagram of the design:

Switching from main power supply to TPS61022 is not a problem. We just disable LM53603 and in same moment enable TPS61022.

But switching from TPS61022 to LM53603 have troubles. LM53603 have internal soft start about 3.8ms. And to avoid output voltage drop we need to keep TPS61022 enabled simultanoiusly with LM53603 during the ramping of internal soft start.

Our idea is to set the TPS61022 output voltage setting little bit higher than the LM53603 output voltage setting. And keep EN=1 for both LM53603 and TPS61022 for 10ms. Then we can disable TPS61022 because LM53603 will be already ramped up.

The questions:

1. What happens with LM53603 during soft start when it started to pre charged load? Will it stop switching or will it try to regulate the voltage down?

2.What happens with LM53603 if the output voltage is higher than the setting? Will it stop switching or will it try to regulate the voltage down?

  • 0
    TI__Mastermind 25590 points
    Vsevolod,

    LM53603 will be in diode emulation mode for the soft start period. So it will not sink current while the device is in soft start.

    As for normal operation after soft start, LM53603 will be in diode emulation mode if you operate in PFM mode (FPWM pin connected to GND) so it will not sink current.

    So if LM61022 is set slightly higher than LM53603, LM53603 will sit there (occasionally switching to charge the boot cap but essentially will do nothing until LM61022 output comes down).

    -Sam