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TPS54622: RT pin connection

Given Ding
Expert 7325 points
Part Number: TPS54622
Other Parts Discussed in Thread: TPS54618

Hi Team,

Here are two questions on RT pin

1. from the block diagram in datasheet, RT pin is connected to Vsense through " OSC with PLL" , not sure on the reason why the it needs to be connected to Vsense.

To my understanding, it make sense to connect to slope compensation and PWM generator, but not to Vsense.

2. customer connect a 6.8nf cap in parallel with RT resistor and the part cannot work. So just want to know how RT resistor functions. Is it a part of internal RC oscillator or the part source a current on RT resistor to read the Fsw?

Thanks.

BRs

Given

  • 0
    TI__Mastermind 35115 points

    Hi Given,

    I believe the connection between the OSC with PLL and the VSENSE pin may have been a re-use mistake. I bet this block diagram came from another part which had frequency foldback. In these parts the Vsense voltage will influence the fsw so the connection makes sense. For example the TPS54618.

    We do not recommend connecting a cap on the RT/CLK pin, especially that big. This can affect the stability and startup time of the circuit which regulates the RT/CLK pin to 0.5V. The RT/CLK pin is regulated to 0.5V and the current out of the RT/CLK pin is what sets the frequency of the internal oscillator. The one exception to this is when synchronizing but in that case the cap should be kept very small, around 47-100 pF.

    Do you know why the customer added a cap on the RT/CLK pin?

    Best Regards,
    Anthony