I am considering the MP4570 for a new design. Input is 26V-47V (AC transformer with full bridge rectifier) and the output is 24VDC. Output current required is 2.7A.
The datasheet is unclear if this will work without an additional bias voltage (bootstrap diode) as the dutycycle can be quite high if the DC input voltage rail drops under high load.
Is this recommended or not?
What about external bootstrapping diode? required or not? if yes: does it need an additional 5V rail?
Hi damian.schneider,
Yes, you will need an external bootstrapping diode. This is because your duty cycle will be above 65% when you are below ~37V.
If you need more headroom, I would recommend using the MP8880 | 60V, 4A, High-Efficiency, Digital, Configurable, Synchronous Step-Down Converter with PMBus Interface | MPS (monolithicpower.com).
Regards,
Vinh Tran
followup question:
since the output is 24V I need a zener diode and a resistor as well. How much current should this biasing circuit provide?
Hi Damian,
The recommended diodes are IN4148 or BAT54. Similar ones will be ok.
There is no current requirement but there is a VIN-VOUT requirement of 3V. So, your minimum VIN will be 27V. Please make sure to limit UVLO using the EN pin FB divider.
Regards,
Vinh Tran
I think you misunderstood my question.
I do not have a 5V supply voltage for biasing, I have to use the 24V output for that. So If I put a 51k resistor in front of the diode and a Zener behind it to get to 5V biasing, will that work?
Hi Damian,
Yes, that could work.
Regards,
Vinh Tran
FYI: I tested this with the eval kit, no bootstrap diode, 24V output, 27V input. I get over 90% efficiency so the datasheet is not correct that the bootstrap is required in this case.