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[FAQ] DRV2700: Flyback Voltage Expected Output

Intellectual 1250 points

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Part Number: DRV2700

When using the DRV2700 in it’s flyback configuration there are multiples things to consider when trying to approximate what your output will be. Here is the flyback configuration circuit found in the DRV2700 Datasheet:

First of all is the use of this equation:

The only part that is essential is the left part, as the right term is usually negligible in most applications. This will allow you properly to set our resistor values to get to the voltage you desire, and subsequently the correct capacitor values to match the ratio to the resistor values based on this:

The output will drive at max voltage if this pin on the schematic is at ground, so a popular method to control it is using PWM, where modifying the duty cycle will allow you to drive at any range between 0V and your set max voltage. Just make the sure the filter input is adequate to avoid excess ripple

It is suggested to use PWM above the 20kHz, a selection of frequency either much higher or lower than that may vary the max output. Ripple may get through to the output but can be fixed by adding more capacitance to the output, but will it cause it to take more time for the output to reach max voltage on startup.

Reccomendation about PWM are as follows:

You also must make sure to take in consideration the transformer selection and current saturation and limits talked about in these two sections. If your transformer saturation current is lower than what you set with the Rext resistor, this may cause lower voltage than expected at the output.