Passive Component Values for MP9989

AC-DC Power Converters
1

Hi All,

We are considering using an MP9989 in a PoE PD solution to squeeze every last ounce of power out of the 13W budget. To that end we are putting the MP9989 on the secondary side of the transformer replacing the Schottky diode. This is for a 5V output solution. We have followed the datasheet and reference design but are a bit stumped as to what to choose for the passive components which go alongside the main silicon. The datasheet says that Rsense should be between 100 ohms and 300 ohms but does not specify a specific value, or a function as to how to calculate it. There is no information whatsoever in the datasheet as to what to choose for the capacitor between VDD and VS.

Can anyone shed some light on what we should be choosing for these two passive components?

Thanks

Chris

2

Hello Chris,

MPS has an evaluation board for the device and I believe it will be helpful in your design process:
EV9989-S-00A | High Efficiency Synchronous Rectification EV Board for SOIC package type. | MPS (monolithicpower.com)

The cap between VDD and VS is a decoupling capacitor. This is used to short high frequency noise from the chip. Typically between 0.01uF and 0.1uF. However, our eval board uses a value of 470nF which is still fine. Please adjust the cap value to get the best performance.

The Rsense resistor value is recommended to be between 100 - 300 and is dependent on the slew rate of the power supply. Our eval board uses a value of 300, but please test to see which value works best for you.

Thank you,
Vinh Tran