2
\$\begingroup\$

I'm pretty new at hardware side of things and trying to teach myself circuit and PCB design just as a hobby, so. bear with me if I'm annoyingly noob for you.

As a relatively simple first task, I decided to design a USB powered Li-Po charging - 3V3 LDO circuit. It looks like a pretty common hot-plug type design. basically when the USB cable is connected it cuts the Li-Po from the load and charges the LI-PO while powering the circuit from USB power. When USB isn't connected it uses LI-PO power. Here's how it looks:

enter image description here

The problem(?) is with the Schottky diode (D5) which is supposed to block the current from Li-Po to USB (and the MOSFET gate) when the USB cable isn't connected.

I used the following diode for my prototype: https://datasheet.lcsc.com/lcsc/2303141100_MDD-Microdiode-Electronics--SS54_C22452.pdf

Anyway, after getting 5 prototype boards I noticed: When I unplug my USB and plug in a LI-PO Battery (when the Li-Po is at around 4 V) I see 2.55 V at the Schottky's anode terminal. Now I know you're supposed to have reverse leakage with a Schottky diode and it can be high. The unit functions correctly, but that 2.55 V is giving me an itch. Is this normal for this diode? At first I thought the diode may be faulty but all 5 prototypes consistently have the same voltage at that terminal with that 4 V Li-Po.

If this is normal, is this a problem in general? (let's say I will power a MCU board with this)

I'm not sure if it's relevant, but the input impedance of the multi-meter I use is 1 MΩ.

\$\endgroup\$
1
  • \$\begingroup\$ Are you sure it's only the Schottky diode leaking? You have other components such as the charger IC between 5V node and battery voltage too. \$\endgroup\$
    – Justme
    Dec 1, 2023 at 9:51

1 Answer 1

1
\$\begingroup\$

Look at Fig.2 in the Diode's Datasheet (Typical reverse Characteristics). 25µA reverse leakage are exactly in spec for the Diode.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.