# Optimising standby current of battery operated designs with momentary power activation - circuit example from digital level

My digital level flattened its 9 V battery within 2 weeks of little use, so I measured its standby current which came out at nearly 2 mA!

I took the cover off was shocked to see the battery connected directly into 5 V linear regulator (AMS1117) that is always operating.

From the datasheet I see it's got a quiescent current figure of 5 mA which to my understanding represents a standby, no-load current value.

Am I correct that this is a horrifying circuit design, and it's actually doing pretty well to come out at just under 2 mA - even though I don't deem this an anywhere near acceptable standby current for such a battery operated device?

Here's what the PCB looks like, with the battery connected in the upper-right:

I'm keen to understand how a circuit like this could be improved. To provide a starting point, I recently designed this circuit to perform momentary power latch-on as follows:

• VCC is around 14 V
• The controlled power output is in the bottom-right - this powers the circuit
• Power is activated by grounding the "EN" connection, which lets power flow to the circuit via the PNP (Q2)
• A microcontroller drives "PWR_ENABLED" high to 3.3 V while operating to keep "EN" grounded through the NPN (Q1)
• The microcontroller can ground/float this pin to turn the power off.

(note all the "extra" components are essentially protection against supply spikes and inrush current)

Are there any flaws in my approach to this problem?

One of the things I'm yet to implement is how I can allow the microcontroller to sense the state of the "EN" - ie, whether the momentary power button is pressed while the circuit is already powered. I essentially want to use the EN input to activate my circuit, but also allow the processor to use this as an input - which I can't do with my design as it stands.

• FYI your PNP is upside down. Also, with no annotations (Q? etc) it is difficult to refer to specific components Feb 26, 2023 at 10:00
• Hah I should have got you to review my circuit before manufacturing it... had to solder the transistor sideways and use a jumper to get a functional board after finding that problem the hard way. Sorry about the schematic, it's not up to date. Feb 26, 2023 at 11:47
• I've updated the schematic to the latest version... has a bunch of irrelevant components to the question now though. Feb 26, 2023 at 11:55
• when the microcontroller is OFF, PWR_EN is floating. So, better to have say 1Mohm pulldown on that pin to ground to define it. If the microcontroller supply is ON that means that EN is definitely 0. So, why do you want to connect EN to the microcontroller? Isn't it redundant?
– sai
Feb 26, 2023 at 14:05
• what is the yellow square at top of the schematic diagram? Feb 26, 2023 at 16:59

For connecting EN to the micro-controller, I tried putting together 2 schemes. Basically we need to create a replica of the EN signal. One going to this power latch and other going to the micro-controller.

This uses a DPST push button switch