# 3.0V CMOS Coin battery booster to 5V, 300mA

#### Exjay

Joined Nov 19, 2015
94
Hello Experts,
I am trying to build a scrolling led badge to be powered from a 3V CMOS Coin battery. I don't know which IC is the best for this application.

#### peterdeco

Joined Oct 8, 2019
397
You are NOT going to get 5V 300mA from a 3V coin battery unless it's the size of a pizza.

#### Audioguru again

Joined Oct 21, 2019
3,675
To get a boosted output of 5V at 300mA, the little 3V battery must produce 575mA which would be a lot of current for two much larger AA alkaline cells. As the battery voltage drops then the booster must use more battery current.
Two AA cells might last for 1 hour.

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#### Exjay

Joined Nov 19, 2015
94
S
To get a boosted output of 5V at 300mA, the little 3V battery must produce 575mA which would be a lot of current for two much larger AA alkaline cells. As the battery voltage drops then the booster must use more battery current.
Two AA cells might last for 1 hour.
What do you suggest?
However, I would be powering PIC microcontroller (uC) from the booster while the LEDs from the uC.

#### Exjay

Joined Nov 19, 2015
94
You are NOT going to get 5V 300mA from a 3V coin battery unless it's the size of a pizza.
I'm not getting the 5V and 300mA from the coin but from a boost converter

#### peterdeco

Joined Oct 8, 2019
397
I understand that but as Audioguru mentioned, if 2 AA alkaline batteries (which are about 2400mAH) are only going to last an hour, a coin cell will not work.

#### dl324

Joined Mar 30, 2015
13,328
I'm not getting the 5V and 300mA from the coin but from a boost converter
Power in = power out

At 100% efficiency, the coin cell would need to provide:
$$I_{IN} = \frac{I_{OUT}*V_{OUT}}{V_{IN}} = I_{OUT}*\frac{V_{OUT}}{V_{IN}} = 0.3A * 1.67 = 0.5A$$

You're not going to get half an amp from a coin cell.

#### Irving

Joined Jan 30, 2016
2,197
However, I would be powering PIC microcontroller (uC) from the booster while the LEDs from the uC.
But all the energy ultimately comes from your primary power source...

I'm not getting the 5V and 300mA from the coin but from a boost converter
Yes, but you can't create energy from thin air as Dennis and others have said. You simply don't have enough energy storage in a coin cell, booster or no booster, to power a scrolling LED display.

Having said that, there are other types of display that might work... so lets go back to the drawing board...

Describe exactly what you want to build with some salient info, like, for example the number of characters to display at one time, how big you want them to be and so on....

#### Exjay

Joined Nov 19, 2015
94
I want to build a charlieplexed LED badge with 90 LEDS. Each LED is 2mA, a low power LED.
The LEDs to be run from PIC uC. I want the overall package to be so small that it can be attached to small pocket. I intend to use 18×5 for the LED combination. All LEDs are red.

#### ericgibbs

Joined Jan 29, 2010
13,881
hi Exjay,
Options..
E

#### Exjay

Joined Nov 19, 2015
94
hi Exjay,
Options..
E
Dear Eric,
Thanks for your proposed solution. These modules are expensive. I want to produce it in large scale. I prefer the LEDs. What is the supply voltage of the setup in the video? Is it replicable to badge usage?

#### Irving

Joined Jan 30, 2016
2,197
I want to build a charlieplexed LED badge with 90 LEDS. Each LED is 2mA, a low power LED.
The LEDs to be run from PIC uC. I want the overall package to be so small that it can be attached to small pocket. I intend to use 18×5 for the LED combination. All LEDs are red.
See you've fallen into the trap of saying how it works rather than what its pure function is...

But no matter, lets run some math...

18 x 5 red LED @ forward voltage of 1.7v and current of 2mA with, say, on average 12 of the 18 on at once (strobing on the 5-led direction) requires 1.7v * 2mA * 12 mW = 41mW. Driven from 3.3v via resistors loses another (3.3 - 1.7) * 2 * 12 mW = 38mW, so doubling the losses - better to drive from buck converter 3.3 -> 1.8v at 80% efficiency gives only 8mW loss. A suitable microcontroller sinking the 18 LED and driving 5 p-channel high-side mosfets on 3.3v will consume around 20mA on top of the display, or another 66mW (led matrix drivers like MAX7219 consume too much power and are physically too large). So allowing an extra 15% we need around 133mW from 3.3v or 40mA. A 150mAH Li-Ion pouch battery will give 2 - 2.5h of operation in a package around 17mm wide x 53mm high x 15mm deep - using 2mm x 2mm chip LED and mosfets on one side of PCB and everything else on the back.

Its doable - but not for a novice... the uController is a 10mm x 10mm 44pin leadless package on a 6 or 8 layer PCB

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#### Exjay

Joined Nov 19, 2015
94
See you've fallen into the trap of saying how it works rather than what its pure function is...

But no matter, lets run some math...

18 x 5 red LED @ forward voltage of 1.7v and current of 2mA with, say, on average 12 of the 18 on at once (strobing on the 5-led direction) requires 1.7v * 2mA * 12 mW = 41mW. Driven from 3.3v via resistors loses another (3.3 - 1.7) * 2 * 12 mW = 38mW, so doubling the losses - better to drive from buck converter 3.3 -> 1.8v at 80% efficiency gives only 8mW loss. A suitable microcontroller sinking the 18 LED and driving 5 p-channel high-side mosfets on 3.3v will consume around 20mA on top of the display, or another 66mW (led matrix drivers like MAX7219 consume too much power and are physically too large). So allowing an extra 15% we need around 133mW from 3.3v or 40mA. A 150mAH Li-Ion pouch battery will give 2 - 2.5h of operation in a package around 17mm wide x 53mm high x 15mm deep - using 2mm x 2mm chip LED and mosfets on one side of PCB and everything else on the back.

Its doable - but not for a novice... the uController is a 10mm x 10mm 44pin leadless package on a 6 or 8 layer PCB
Thanks for the analysis. Can't I use a PIC with 16 pins? The pins to be used is just 10 pins. I want the design to be simple.

#### Irving

Joined Jan 30, 2016
2,197
Thanks for the analysis. Can't I use a PIC with 16 pins? The pins to be used is just 10 pins. I want the design to be simple.
You could but you'd need serial-to-parallel conversion on one or both axis; thats at least 3 or 4 x 16pin SOIC chips, which will take far more real estate and power than driving direct from the uController. Also your 16pin PIC will be a SOIC package almost as big as the 44-pin device. There is no way to easily make this small and battery powered and keep it simple, indeed the solution I've suggested is probably the simplest - if you want to manufacture in quantity you have to design for ease of manufacture, and every device you add increases manufacturing and testing costs disproportionally. A nearly two chip solution is by far the most cost effective.

You have much to learn my friend!

#### Exjay

Joined Nov 19, 2015
94
You could but you'd need serial-to-parallel conversion on one or both axis; thats at least 3 or 4 x 16pin SOIC chips, which will take far more real estate and power than driving direct from the uController. Also your 16pin PIC will be a SOIC package almost as big as the 44-pin device. There is no way to easily make this small and battery powered and keep it simple, indeed the solution I've suggested is probably the simplest - if you want to manufacture in quantity you have to design for ease of manufacture, and every device you add increases manufacturing and testing costs disproportionally. A nearly two chip solution is by far the most cost effective.

You have much to learn my friend!
You're right. I have much to learn. I just don't know where to start from. I need a mentor like you and other experience people

#### Exjay

Joined Nov 19, 2015
94
You're right. I have much to learn. I just don't know where to start from. I need a mentor like you and other experience people
Charlieplexed LEDs from uC doesn't need serial to parallel converter

#### Audioguru again

Joined Oct 21, 2019
3,675
You said a 3V battery and a 5V at 300mA output. Here it is with two alkaline AA battery cells:

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#### Irving

Joined Jan 30, 2016
2,197
Charlieplexed LEDs from uC doesn't need serial to parallel converter
True, but charlieplexing is of no use to you - for reasons I will explain shortly - but first lets go back to what you are trying to achieve...

I realize now of course your proposed display is 5 high and 18 wide, so what font are you planning to use, and how fast will it scroll? Do you have a bitmap representation of the font?

Did you have a specific LED in mind or were you quoting a 'generic' example?

#### Exjay

Joined Nov 19, 2015
94
True, but charlieplexing is of no use to you - for reasons I will explain shortly - but first lets go back to what you are trying to achieve...

I realize now of course your proposed display is 5 high and 18 wide, so what font are you planning to use, and how fast will it scroll? Do you have a bitmap representation of the font?

Did you have a specific LED in mind or were you quoting a 'generic' example?
Thanks Irving. I'm trying to use text font that's readable to anyone in english. I want to achieve a 0.5second scroll i.e if an LED is on then in 0.5s the next one should one depending on the character to be displayed. I don't have a peculiar bitmap since I want the use to select any character or words of choice

#### Irving

Joined Jan 30, 2016
2,197
Thanks Irving. I'm trying to use text font that's readable to anyone in english. I want to achieve a 0.5second scroll i.e if an LED is on then in 0.5s the next one should one depending on the character to be displayed. I don't have a peculiar bitmap since I want the use to select any character or words of choice
Yes, I get you want the user to specify the words, but you have to say how each character looks... so, for example, is the font based on a 5 x 5 bitmap or a 5 x 4 bitmap? For example, in a 5x5 font the character A is represented by the 5 binary values:

00100​
01010​
10001​
11111​
10001​

while in a 5x4 its:

0010​
0101​
1001​
1111​
1001​

With the former, allowing 1 column between letters you get 3 letters at a time, with the latter you get 3.6 letters at a time

This is a 5x5 monospaced font at 10mS per column (0.5s per character). Monospaced fonts are f-ugly but easy to program...

Now consider that, in my proposed solution, each row is energised 5 times between each shift, so each row is on for 2mS in 10mS. Effectively every LED is running at 1/5 max brightness. We can compensate for this to some extent by increasing the current through the LEDs. This will give a consistent brightness across all LED as the multiplexing is at row level and is at a constant frequency irrespective of the number of LEDs switched on.

Now consider your charlieplexing approach where one LED in 90 is switched on. Each LED gets 1/90 of 10mS or 111uS. It doesn't matter what current level you set, you will never really see them on. You might consider breaking it down into 3 groups of 5 x 6 where you charlieplex the 6 and strobe across the 5. Then you need 5 + 3 * 3 = 14 pins but each LED per group of 6 is still only on for 1/30 of of 10mS or 333uS and that's still 1/30 full brightness. Arguably you could charlieplex 3 groups of 30 without strobing but that needs 18 pins and still only gives 333uS per LED. Plus charlieplexing needs the dropper resistors so still wastes battery power.

Charlieplexing is great for static displays where only a few LED are on - eg warning/alarm boards where mostly they are off, or where you have a small number of LED broken into independent non-multiplexed groups. It just doesn't work for your requirements.

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