Other Parts Discussed in Thread: TPS56920
Hello,
We are looking into the TPS56920 or other member of that family for a new product. We have the development board with the TPS56C20 and doing some tests but running into an issue. Here are our high level requirements:
- Input Voltage 15V
- Output Voltage selectable between 2.4 and 10V
- Voltage in external mode: roughly 10V
- Voltage in internal mode: 2.46 to 7.66V
- Current at 10V max 4A
We want to use the internal DAC to control the voltage over a certain range. If we use the default setting of the demo board we achieve an output voltage from 0.6V to 1.87V. However we need a higher voltage, so we thought we put a resistor divider on the output back to the VOUT pin. As the datasheet is showing for the DAC the feedback in internal mode is through the VOUT pin.
Unfortunately we are not seeing the correct output voltage using the formula in the datasheet. When we use a 22k and 66.5k resistor we expect to see roughly 2.4V at the output when we set the DAC to 0x00. But we are seeing 2.85V (No load). VFB is still connected to the same point of the resistor divider. On VOUT we are measuring 0.6V, so that seems correct. Ideally we would like to achieve 3.6Vout.
Could you help with some questions:
1) Are we using the correct converter? Can we achieve Vout = 10V with Vin = 15V?
2) If not what, would be the maximum voltage output voltage possible?
3) Do we possibly have too much ripple? Should we try changing the inductor?
4) Why is the formula not working when we apply a voltage divider from the output to VOUT and use the internal DAC?
a. Is it possible there is a 100k pull-down?
b. If so how accurate is that resistor value?
5) Can we use both internal and external mode and switch between those?
6) Is it possible the VFB input is loading the line? Or is there a different equation for VOUT?
7) Should we look for a completely different part?
Thanks very much for your help. Keith
