I've posted similar threads about this problem, but I feel like I am close to the solution.

I have a Pololu 9V Step Up Regulator. It's specced at 1.4A maximum input current. I am supplying it with a 5.2VDC 8A power supply. The boost converter is in parallel with servo motors (2A), an Arduino Mega (70mA) and a custom circuit (40mA). The 9VDC the Pololu circuit produces us going to a 5V regulator powering an SSC32 (30mA)

My question is, do I need to be concerned with the 1.4 maximum input current? Is so, how do I limit it?

 

You limit the input current by limiting the output current. In other words don't ask a boost converter to supply more power than it can handle.

 

The maximum allowed input to the smps is 1.4A. At 5.2V input, the max allowed input power is P =IE =1.4*5.2 = 7.28W.

The smps is likely about 85% efficient, so the max power it can deliver to a 9V load is 0.85*7.28 = 6.2W.

The max current it can deliver to a load and stay within the allowed limit is I=P/E = 6.2/9 = 0.69A.

Since your load only requires 30mA at 9V, the smps will be loafing, so the output power is 0.03*9 = 270mW. The input to the smps will be 270/0.85 = 320mW (due to losses), so it will draw only 320mW/5.2V = 62mA from the 5.2V supply.

I would have used the adjustable regulator set to ~5.5V rather than the fixed 9V one. That would waste much less power than using the 9V step-up. I've given you all you need to figure the power saving in both the step-up and the on-card 5V regulator if the intermediate voltage is only 5.5V.

 

@Papabravo, that was my intuitive answer. Glad to see it confirmed.

And @MikeML , thank you for the calculations. Helped move this problem from intuition and theory to understanding.

Regarding the adjustable regulator, I had been running the SSC32 directly from 5.2VDC, but was recommended by the manufacturer to run it at 9 VDC and through the onboard regulator because I was having problems. Not convinced this will solve the problem, (I was previously supplying the source from a regulated supply and bypassing the regulator; worked for several years) But I'm first trying to follow their recommendations.

 

@Papabravo, that was my intuitive answer. Glad to see it confirmed.

And @MikeML , thank you for the calculations. Helped move this problem from intuition and theory to understanding.

Regarding the adjustable regulator, I had been running the SSC32 directly from 5.2VDC, but was recommended by the manufacturer to run it at 9 VDC and through the onboard regulator because I was having problems. Not convinced this will solve the problem, (I was previously supplying the source from a regulated supply and bypassing the regulator; worked for several years) But I'm first trying to follow their recommendations.

If the 9V were unregulated, then operating the logic supply input of the SSC32 with higher headroom would be a good idea, but because the output of the smps is regulated, then it can be set to the min drop-out of the SSC32, which is 5.5V. You waste less power stepping up all the way to 9V, and then wasting power by dropping 9V to 5V in the on-card regulator...