Buck regulator - , how much voltage ripple is too much.

Projects General Guidance
1

Hi, I am looking at buck regulators for my shield design. I am tossing up between the older and more expensive LM2736Y and the TPS562200. I am going to put the output of the buck onto Vin of the Arduino, so the 5V LDO will be ultimately regulating the board.

My major concern about the TPS562200 is that it is limited to a maximum inductor value of 4.7uH. The minimum current draw will be 100mA at which the voltage ripple from the buck is 40mV p-p.

Will 40mV be too much ripple for the Vin of the 5V LDO?

My other concern with the TPS562200 is that it has an absolute maximum of 7V OUT. So I would configure it to 6.7V for margin. I know that Arduino.cc recommend a minimum of 7V, but the NCP1117 data sheet says it can provide up to 800mA with a drop out of only 1.2V. So I think 6.7V allows for enough head room.

Will an input of 6.7 Volts be enough to keep the 5V LDO accurate and stable?

My other concern of course is that Vin needs to be above 6.6V to isolate the USB 5V supply. I've never understood why the USB needs to be isolated. Many applications are powered from an external 5V source through the 5V pin, and plugging in the USB doesn't seem to cause a problem under these circumstances.

Thanks for your help.

2

Will 40mV be too much ripple for the Vin of the 5V LDO?

No. Although the Arduino does not have a LDO regulator, you need at least 1.5V excess voltage.

3

A lineair regulator is AFAIK too slow to significantly reduce the ~0.5-1Mhz ripple from a DC/DC converter.
The caps on the input and output of the regulator reduce that HF ripple.
The higher frequency of newer DC/DC converters could be easier to filter out.
6.7volt sounds good (stops the 5volt supply being backfed into a connected computer).
1.7volt across the lineair regulator sounds good too.
Leo..