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TPS548B22: DAP loop test

Hailiang Zhang
Genius 10880 points
Part Number: TPS548B22

Tool/software:

Hi Team,

I heard some voices saying that COT control mode devices are inherently stable and do not need to test Bode plots. Customers have heard similar sounds and want to know if a loop test is required for the TPS548B22, and are the standard the same as current mode devices? Could you help with suggestions and comments on this? Thanks in advance.

Regards,
Hailiang

  • 0
    TI__Guru 56795 points

    Claims that Constant On-time control are "inherently stable" are not really true, but they have different stability conditions and unstable behavior from a fixed-frequency control loop.

    Taking a bode plot is meaningless for direct capacitor measurement Constant On-Time control, because the current sense element for the loop is the ESR of the capacitor, so it can't be separated.  Instead, we look at the stability of the switching frequency since the output voltage ripple is bound by the detection and constant on-line control.

    Constant On-time with ripple injection, such as an RCC across the inductor and injected into FB, or TI's D-CAP2, D-CAP3 and D-CAP4 technologies allow a BODE plot to be taken, but the normal guidance of small signal analysis based on loop gain, bandwidth, gain and phase margin aren't the same due to the non-linear nature of the COT loop, though a converter which shows 45-degrees of phase margin and 8 dB of gain margin will be stable, COT's variable frequency allows converters with less gain and phase margin to also be stable.

    Generally, we recommend customer look at the time-domain performance such as switching frequency jitter and load transient recovery over BODE plots.  See the attached D-CAP stability letter.

    DCAP Stability Letter.pdf