Regulators, Diodes, and Power monitoring tips

Using Arduino General Electronics
#1

Hi everyone,

I am designing a PCB around the SAM D21 (with Arduino bootloader) and had some general optimization questions. The first question is the most important to answer in my application.

  1. I currently have a 3.3V buck DC-DC switching regulator (Max1921) in preceeding an LDO in order to regulate my USB and battery inputs down to 3.3V. I am using the buck regulator as it reduces power consumption and the LDO as it results in a low noise output. The LDO is used to regulate the output of the buck regulator. Is this overkill?

2 I am using a MBR120 schotky diode in order to be able to switch between USB and battery power. However, I have noticed that this diode has a forward voltage of almost 0.4V resulting in a significant battery voltage drop into the regulator and less battery time. Is there a better alternative to using the diode with the same functionality?

  1. Finally, I am using a voltage divider configuration (2 100k resistors) in order to monitor my battery and USB input (I can determine whether battery or usb is plugged in). However, these resistors slightly waste some current. Is there a power efficient method of measuring my battery voltage?

Thanks for all your help!

#2

ddesousa: 3. Finally, I am using a voltage divider configuration (2 100k resistors) in order to monitor my battery and USB input (I can determine whether battery or usb is plugged in).

What voltage reference is being used in the A-D converter being used to monitor your input supply?

#3

Hi,

Can you post a circuit diagram of your power circuit so we can see what you have?

What LDO are you using? Even though it is a low drop out regulator, you will still need more than 3.3V input for it to work. The 3.3V from the buck converter will not be high enough.

Tom.. :)

#4

Hi Stowhite,

The reference is 3.3V. Is is tied to the input of the mcu.

Hi TomGeorge,

I am using the AP2112k-3.3 . I agree. I do not think the output of the buck regulator will be enough. Do you think I can just use the buck regulator then?

#5

ddesousa: Hi Stowhite,

The reference is 3.3V. Is is tied to the input of the mcu.

Hi TomGeorge,

I am using the AP2112k-3.3 . I agree. I do not think the output of the buck regulator will be enough. Do you think I can just use the buck regulator then?

It would be worth a try with just the buck, you must make sure in your design you have ample bypassing, and layout to keep gnd currents away from any lowcurrent/voltage input circuitry.

Make provision for the LDO and it associated components on your PCB.

Have you breadboard prototyped your project, or at least the various features?

Tom... :)

#6

3) Maybe use two 1M resistors, and a 100nF capacitor across the lower resistor (to prevent noise pickup and lower the impedance for the analog pins.

#7

I'm not sure that a buck convertor will be much more efficient than a simple LDO regulator between 5 and 3.3v.

And at higher cost and with possible noise problems.

Allan