0
\$\begingroup\$

Say I'm using an LDO to power an MCU such as the ATmega328p with 3.3V from 3 NiMH AA batteries.

What it isn't clear to me how to deal with brown-out situations.

I can think of 2 possible risks (i.e. when using an LDO such as the MCP1700):

  1. deep discharge of the batteries when their voltage drops to MCU brown-out levels (say 2.7 V) and the MCU continues to draw some current in reset mode
  2. damages to the LDO itself when V_in drops below V_out

Is it a good idea to look for LDO with on/off pin (such as the S-1132) and connect it to a voltage detector such as the MCP112 or TC54?

In that way one would work-around the brown-out detection of the MCU, and basically implement an external brown-out detection that just switches the MCU off (instead of keeping it in reset).

Or are there LDOs available that already include a voltage detector and just turn off V_out if V_in drops below a threshold?


Regarding LDO damages, I stumbled over a paragraph in the S-1132 datasheet:

In the S-1132 Series, a low on-resistance P-channel MOS FET is used as the output transistor. Be sure that VOUT does not exceed VIN + 0.3 V to prevent the voltage regulator from being damaged due to reverse current flowing from the VOUT pin through a parasitic diode to the VIN pin, when the potential of VOUT became higher than VIN.

(Section Operation, page 13)

How would V_out exceed V_in? I mean, I thought if V_in drops to V_out + V_dropout or less, then the LDO works in its dropout zone where it isn't regulating anymore, i.e. V_out has some linear relation to V_in. See for example the graph for the S-1132VB50 (V_out=5V) variant (page 16), where V_Out/V_in pretty much equals 1 for V_in=2.5..5:

enter image description here

So is the paragraph warning against (accidentally) connecting another power-source to V_out that exceeds V_In+0.3V?

Or is it possible to damage an LDO somehow merely by letting V_in drop under a minimum threshold for an extended time?


Another thing I don't understand in this datasheet are the output voltage/output current graphs on page 16, e.g. for S-1132B30:

enter image description here

I mean, e.g. for I_out=400 the curves have two V_out values?

\$\endgroup\$

1 Answer 1

0
\$\begingroup\$

The LDO won't be damaged if VIN falls -- VOUT will follow VIN down. There is an extreme case where if you had a huge (say 1000 uF) capacitor on VOUT, and VIN was suddenly shorted to ground, then the 1000 uF would discharge through the LDO and potentially damage it. With the normal values of VOUT capacitor, this won't be an issue.

If you are concerned about over discharging the NiMH cells, then you will need a lot voltage detector/protector.

\$\endgroup\$
1
  • \$\begingroup\$ Ok, what do mean with 'then you will need a lot voltage detector/protector'? \$\endgroup\$ Mar 12, 2022 at 21:43

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.