Clarification on MP8715's 100% duty cycle operation

DC-DC Power Converters
1

Hello!

I have a problem, where my electronics needs 5V regulated power supply from an input, which varies between 5V and 20V. This input source is a regulated 5V most of the times, but occasionally it is increased to a higher regulated voltage like 9V/15V/20V for several hours.

Your MP8715 IC seems to be a perfect solution to me, as if I understand the datasheet correctly, it can act as a “pass-through” device with the 100% duty cycle feature when the input is the regulated 5V, and in case of the higher input voltages, it’ll regulate the output to 5V.

However just to be sure, I would like to ask for some clarification.

Regarding the bootstrap capacitor and its charging circuitry the datasheet has two statements:
In the electrical characteristics (for 12V input voltage) it says that the BST Under Voltage Lockout Threshold (BSTUV_Vth) is typically 2.8V.
In section “Floting Driver and Bootstrap Charging” it says that: “The floating power MOSFET driver is powered by an external bootstrap capacitor. This floating driver has its own UVLO protection. This UVLO’s rising threshold is 2.5V with a hysteresis of 150mV.”

I did not find anywhere that for what should we reference this UVLO. I assume that the VIN should be higher than the mentioned values, however I want to be sure that not the (VIN-SW) voltage should be higher than the mentioned values.
Therefore I would like to ask that am I right, that the voltage of the VIN pin should be higher than this 2.5V/2.8V in order to the bootstrap capacitor charging to work properly?

Additionally, I would like to ask, that is there any minimum output current requirement for the MP8715 to operate properly in 100% duty cycle mode?

Thank you,
Ferenc

2

Hello Ferenc,

For the MP8715 we recommend using an input voltage in the range of 4.5-21V, and we recommend using a bst cap of 0.1uF = 100nF.

On page 6 we illustrate that there is a trade-off between duty cycle and maximum current - so while in maximum duty cycle the peak current that can be provided is limited. There is no minimum output current for maximum duty cycle operation.

Further testing can be done using one of our evaluation boards.

Regards,
Jonathan

4

Hello Jonathan,

thank you for the the clarification about the minimum output current!

My question is still open about the UVLO voltage. Which voltage should be higher than the BST Under Voltage Lockout Threshold (BSTUV_Vth) mentioned in the Electrical Characteristics on page 3, and which voltage should be higher than the UVLO voltage mentioned in the “Floating Driver and Bootstrap Charging” section on page 11? The VIN voltage should be higher than these 2.8V / 2.5V?

Thank you,
Ferenc