How to design low battery indicator?

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

True but generally this class of switch mode regulator has very limited input voltage range where the typical maximum input voltage for this class is 5.5volts. I typically designed for a maximum input voltage 18V.
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ChickyMeal · Guest

Guyss...

Im quite confuse here..
My circuit current flow = 0.03A

The power is needed for:
1. High Voltage - for GM Detector
2. 5V for other ICs - PIC and a few other chips

Which power supply solution is highly reccomended?

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

First, I would try to get away from the 9V battery. I guessing that its just the typical 9V battery. Not much capacity and a big waste. The easiest solution would be to try and design the circuit to run on 3- 1.5V AA or AAA batteries. You circuit would probably need to work down to 3V in case the user wanted to use rechargeables which are dead at about 1V per cell. The next easiest solution would be to use 4- 1.5V cells with an LDO. You probably would not recommend using recharageables to you end user. If you want the best battery life, then go with a switcher. You can power your circuit with lower voltage cells. These can have larger capacity which means more battery life and the supply is more efficient. The switcher will cost you more however.

newguy · Joined: 24 Jun 2004 Posts: 1909

If you do end up going with a 4.5V or 3.0V (or something like that) supply, then there's a really neat low battery detection "trick" that someone posted here a long time ago. Sorry, can't remember who. This will only work if the PIC is powered directly from the battery - no regulator.

If you happen to have a spare A/D input channel (if your PIC has an A/D of course), you'll need to create a very simple zener diode voltage reference. That would be Vcc -----Resistor-----Zener diode----ground. The zener should be rated for a voltage less than your nominal supply voltage. Now you connect the resistor-zener junction to an A/D input. The A/D should be set up so that the high voltage reference is Vdd.

Now when the battery's voltage starts to fall, the A/D reading will start to increase. Very simple, and a very elegant solution. Like I said, this isn't my idea, and I'm sorry that I can't remember who posted it.

Hope someone finds this useful - I did.

Neutone · Joined: 08 Sep 2003 Posts: 839 Location: Houston

newguy · Joined: 24 Jun 2004 Posts: 1909

Neutone - what a great idea!

I'm often in awe of the ideas that float around on this forum.

MGP · Joined: 11 Sep 2003 Posts: 57

asmallri · Joined: 12 Aug 2004 Posts: 1635 Location: Perth, Australia

JimB · Posted: Fri Sep 30, 2005 8:37 pm

I have battery operated instrument project that I am going to use a dual battery supply, ±3V lithiums and plan on using a charge pump to develop the higher voltage for the LCD. Charge pumps accomplish this by charging and switching capacitors in and out to generate this voltage. Not much current capability, but simple, without any inductors required. I will then monitor the + 3 volts for battery status. The - 3 volts is less heavily loaded and should last longer than the +3.
Places to look are at Linear Technology and Maxum. You don't want to waste that 9Volt current in the drop through a linear regulator. 9 Volt batteries don't have a lot of milliamp hours of energy in them.

MikeValencia · Joined: 04 Aug 2004 Posts: 238 Location: Chicago

This design last year uses a 9V battery and a MAX666 voltage regulator. It has very small quiescent current, has adjustable output, and has a pin to indicate low battery status, based on the resistor divider values you choose. "PCMPro" and "Kenny" led me to use these chips (i had a choice of the maxim or seiko chip), and i still haven't heard a complaint yet from an angry customer of a low battery chirp. By the way, the processor used is a PIC16LF627 (not the chip shown above).

ckielstra · Joined: 18 Mar 2004 Posts: 3680 Location: The Netherlands

The MAX666 is a nice chip, but is a low drop linear regulator as several other types mentioned earlier in this thread. Any lineair regulator, low drop or not, will loose 44% of energy in converting from 9V to 5V.

Most switching regulators I've studied have a reduced efficiency when running at low output currents and higher input voltages. What we are looking for is a type number of a switching regulator with:
- A maximum input voltage of at least 10V, preferably up to 18V.
- Minimum 90% efficiency at 10mA output current
- Low quiescent current.

Eugeneo · Joined: 30 Aug 2005 Posts: 155 Location: Calgary, AB

Use a LM2575.. It works great for everything as long as you size the coil properly

MGP · Joined: 11 Sep 2003 Posts: 57

Here's a link to one IC I've used in the past. It's over 90% efficient down to around 1mA and will deliver up to 800 mA (although I only tested it to about 350mA in my design).

http://focus.ti.com/docs/prod/folders/print/tps62050.html

I can't really see the point of having much larger input voltage range than 10V for a battery application that's switched down to 5V or less. If it's an automotive or SLA battery application then having a very low quiescent current really doesn't mean much as the capacity of these cells is generally not small compared to the loads we're talking about.

At any rate, almost any modern switcher IC circuit will be better than a linear regulator for regulating a 9V battery to 5V or less, more so the higher the load current gets.

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

You can try MAX1921 http://pdfserv.maxim-ic.com/en/ds/MAX1920-MAX1921.pdf. It's in SOT-23.

Guest · Posted: Sun Oct 02, 2005 3:51 am