TPS546D24A: Unity gain bandwidth

Hi Tsvetelin,

Just confirming that AVS refers to Adaptive Voltage Scaling, and yes, the device supports it.

For unity gain bandwidth, if you mean loop bandwidth, it should be between 15 kHz and 200 kHz, depending on the switching frequency. The device can switch from 225 kHz to 1500 kHz, with loop bandwidth recommended at 1/8 to 1/12 of that frequency. 

Thanks,
Tomoya

Hi Tomoya,

Yes, AVS refers to Adaptive Voltage Scaling. The device vendor used in their reference design TPS53355 in the following mode:

Vout set to : 0.809V defined by resistor divider

Vin: 12V

Iout: 17A typ, 21A peak

Switching frequency: 500 kHz

Forced CCM mode

Connection of AVS feedback is shown on the picture below.

The vendor states the following requirement:

"The AVS is suitable to work with external voltage regulators which have a unity gain bandwidth in the
range of 15–300 KHz."

I quickly checked the Bode plot for TPS546D24A  in WEBENCH Power Designer with the following inputs:

Vin: 10V-14V

Vout: 0.8V

Iout: 22A

Max ambient temperature:55C

With optimization that targets maximal efficiency, tool suggest switching frequency of 325kHz and I get the following Bode plot:

If I manually change the switching frequency to 1.5MHz, the bandwidth is increased (although the efficiency is decreasing to below 80%) but still is far from 200kHz that you mentioned:

Could you please advise how to achieve the loop bandwidth that you mentioned in your answer with the same design inputs?

Thank you!

Hi Tsvetelin,

To achieve the desired loop bandwidth, you need to set the VLOOP higher. In Webench, try increasing the VLOOP to 8 within the Compensation Loop Gain settings selectable via pin-strapping. With this configuration, you can expect a loop bandwidth of approximately 200kHz at a 1500kHz switching frequency.

Regarding AVS, I assumed you would be using PMBus for AVS and margining. If you're adjusting the output voltage through analog AVS, it is still possible. However, keep in mind the device's internal resistance at the VOSNS pin ranges between 85kΩ and 160kΩ, with a nominal value of 130kΩ. You'll need to consider this in your calculation.

If you're not using PMBus, this won't be a concern. However, note that the READ_VOUT command report the voltage at the VOSNS pin, so it won't reflect any output voltage adjustments made through analog AVS. 

Let me know if you have further questions. 

Thank you,
Tomoya