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TPS61087 - Loop Response at no load is very low

Brigitte
Guru 65280 points
Other Parts Discussed in Thread: TPS61086, TPS61087, TPS61085

At 0 load in PWM the feedback to output frequency response bandwidth drops to 500Hz or so. Does this mean that the converter reacts very slow?

  • 0
    TI__Guru 65280 points

    Let me try to describe the operation of TPS61085, TPS61086 and TPS61087 when no load is attached and they work in PWM mode:

    Keep in mind that the converter is non-synchronous (using a diode not a second switch). The minimum on-time of the internal switch is in the range of 100ns or so, therefore whenever the switch turns on, some energy is given to the output. When the converter is in steady-state operation, some of this energy is dissipated through the resistive divider and the rest is charging the output capacitor. After some pulses, the voltage will increase above the nominal voltage given by the resistive divider on the FB pin. If this happens, the converter has to stop switching immediately to not increase the output voltage even more. Now the converter does not switch until the output voltage falls below the nominal value. This is called skipping pulses, not PFM. You can almost observe no ripple in this mode.

    So you can see that there are times where the converter is not switching and times where the converter switches with minimum on time. This means that you are not getting a small signal answer if you are using a small signal stimulus. Therefore the measurement you get with the Venable is not really meaningful.