MP5515 backup power

HI MPS expert,
When power failure occurs, MP5515 can only run in buck mode, but can’t run in boost mode ,right?
That means the backup storage capacitor only can transfer the energy to the set VB, will stop transfer when Vbackup < VB?
in order to improve the storage energy utilization efficiency, should set the VB to lower volatge?

Hi Allen, welcome to the forum!

Correct, while the input is stable the MP5515 will be in a boost mode to generate a high voltage on the storage capacitor. Then if the input shuts down, the device switches to buck mode and steps the storage capacitor voltage down to transfer energy to the bus and holds the bus voltage while the system consumes the energy from the storage capacitor.

Because of this, the VB bus voltage will always be less than the value of the storage caps as they are discharged.

Capture

I’m not sure what you mean by the storage energy utilization efficiency, but you should set VB to a level that allows your circuit to operate during this last dying gasp of energy.

Hi Allen, welcome to the forum!
If the input shuts down, the device switches from boost mode to buck mode and steps the storage capacitor voltage down to transfer energy to the bus and holds the bus voltage. You would better set VB to a level that allows your circuit to operate during this last dying gasp of energy.
For more details, please send emails to Daniel.Yang@monolithicpower.com

HI Kareem,HI Daniel,
THanks for your support.
In our design: VIN=12V, we want to set the Vstorage=30V
if set the VB =12V, the buck converter will stop working when Vstorage reach 12V, do it mean there is much energy can’t be used?
If set the VB=5V, the storage energy can be used till to Vstorage=5V, the storage energy utilization efficiency will be improved, right?

Hi Allen, the image I attached in the previous reply does a good job of illustrating the flow of energy during the last gasp. Imagine the voltage on the backup capacitor as the new VIN for the system during the last gasp. This voltage is being bucked down by the converter to supply the bus voltage.

If the Vin droops down to the level of the regulation voltage set point, then the buck converter will simply track the input voltage down until it triggers the UVLO (which I believe is 2.35V).

If the VB is set lower, there will be more time before this occurs, but I don’t necessarily think it would be an issue. Regardless, as long as you set the voltage high enough to power your downstream circuitry long enough for it to run its critical operations you should be fine.

HI Kareem,
Our power tree design is as below.
the downstream DC/DC minimum input is 4.5V, if we set the VB=5V, does it means we can get more time to do the operation than setting VB=12V?

When Vin drops down to the the level of the regulation voltage set point, the buck converter should start to work to covert the Vstorage to VB, why you said: the buck converter will simply track the input voltage down until it triggers the UVLO

Hi Allen, if you want to set the Vb to 5V that should be fine. Vb will regulate at the set point for a longer amount of time as you have noted.