12V to 5V DC-DC converter reference circuit question

I am building a power supply for my MCU project so I want to turn 12V car battery into 5V. I am looking at Diode’s AP1509 step-down converter. It needs minimal external components and has a current rating of 2A at 5V. The reference design has a Schottky diode on the output side. I wonder why. My logic is that in case the converter fails, the full input voltage will appear on the output side, damaging the output. If I have the Schottky diode, say pick a 5.25V or something, then the output will be protected. But there is no serial resistor (wouldn’t make sense for normal operation to have one) to drop the rest of the voltage though. Any idea whether what I’m thinking is correct?

https://www.digikey.com/product-detail/en/diodes-incorporated/AP1509-50SG-13/AP1509-50SGDICT-ND/1301653

Here is the diagram:

ap1509 diode.PNG

No, it is a buck converter, that diode is a key component, when the internal switch is off, current flows through that. Look up how a buck converter works (Wikipedia article explains it)

I see. Thanks Doc. The spec sheet didn't say how I should pick the diode. I guess something that can have sustained current over 2A and a small voltage drop. Anything you can suggest?

I like the MP2467.

https://www.digikey.com/product-detail/en/monolithic-power-systems-inc/MP2467DN-LF/1589-1368-ND/5299125

It runs at 500kHz which means the inductor can be a low value ( 10uH is shown on the datasheet).

If you are not going to do many boards then it may be better to use a module like the OKI-78SR-5 (which has that MP2467 chip, an inductor, and the freewheeling diode all in a nicely tested part).

https://www.digikey.com/product-detail/en/murata-power-solutions-inc/OKI-78SR-5-1.5-W36H-C/811-2692-ND/3438675

The MP2467 is rated for 2.5A operation, but Murata derated it to 1.5A. It is unlikely the Diodes AP1509 is able to sustain operation above about 1.3A for the same reasons that the Monolithic Power part was derated.

liuzengqiang: I see. Thanks Doc. The spec sheet didn't say how I should pick the diode. I guess something that can have sustained current over 2A and a small voltage drop. Anything you can suggest?

TI has a "Application Report" that gives advice on what to consider in a diode as well as the inductor and capacitor etc.

Great! Thanks guys. I'll make a decision soon. My other goal is to power a raspberry pi with it besides MCU so there needs to be just a single power source. I was hoping to get something that supplies 2.xA at 5.1V. Maybe the murata module will do just fine.

After looking at the MP2467 datasheet I don't see anything that Murata could have done to change the current limit, so there OKI-78SR-5 should do at least 2.9A until it heats up and goes into thermal protection. Note the inductor will also saturate at high current flow so the switch (in the MP2467) may turn off from current limit after pushing the inductor into saturation, and that will make a lot of heat and trip the thermal protection.

I run a Pi Zero with an OKI-78SR-5 and it has run flawlessly for months now.

The price is out of my range but Murata also has a 78SR-5/2-C

https://www.digikey.com/product-detail/en/murata-power-solutions-inc/78SR-5-2-C/811-1119-ND/1926162

They also show a 78SR-5.2/2-C in the datasheet, but it is not at Digikey yet.

liuzengqiang:
Great! Thanks guys. I’ll make a decision soon. My other goal is to power a raspberry pi with it besides MCU so there needs to be just a single power source. I was hoping to get something that supplies 2.xA at 5.1V. Maybe the murata module will do just fine.

Perhaps OKR-T/3-W12-C from Murata fits your purpose… input voltage is 4.5-14 VDC. Output is up to 3 Amps with great efficiency at 2 Amps. One external component, a resistor, programs the output voltage. Fits nicely in .1" spaced holes. And, the price isn’t too bad for what you want.