Battery Powered Supply

[jsherman]

Hello to everyone,

I'm planning a microprocessor project that requires 5V at approximately 1A (in the worst case, when the motor and GPS are on, more like maybe a few hundred mA in general, non-worst case). Maybe that's a bit misleading, then; it should be able to tolerate spikes of about 1A. In any case, I want to use a 9V battery as a supply so that it's portable. I'm very new to power supply electronics, but I did find a lot of information on the DC to DC conversion of Buck Converters, which seemed to be what I wanted. However, battery voltages tend to decrease after a few hours of use (depending on the load).

Let's say I designed a 9V to 5V converter, so the inductor, cap, and diode values are set. The only other portion controlling the output voltage is the duty cycle of the switch. Is there any easy way to read the voltage off of the battery and change the duty cycle to continuously approximate 5V?

Thanks for your insight!

 

[halls]

Hi jsherman,

I am not sure if I understood correctly, but your problem seems to be how to supply a 5V micro with a 9V battery, right? I think one of the easiest ways is using an LDO, like LM7805A (take a look at this document: https://www.fairchildsemi.com/ds/LM/LM7805.pdf).

The LM7805A outputs 5V from an input range of up to 18V, and supports up to 1A current... take a look if it's of use for you. There are many other LDO with similar characteristics, and different prices, so this might not be the best solution for your needs...

Hope it helps

 

[jsherman]

Hi halls,

I initially was going to use an LDO, actually! I agree, it is a very easy solution and solves my issue of the draining supply voltage from the battery. That said, LDOs are pretty wasteful on power, aren't they? A 9V supply dropping down to 5V at an average of 500mA wastes 2W, doesn't it? For a battery powered system, I was hoping to waste as little power in the supply as possible.

The LDO's my backup plan, though, if I can't figure out how to fix the Buck Converter issue!

 

[halls]

Sure, didn't think of the power waste of the LDO's... Well as you said, the best option would be a Buck Converter, but not just that. You could then use an LDO once you have lowered the voltage, so the powe dissipation on the LDO is reduced significantly. However, I think the best solution in combination with the Buck Converter, would be to also use a battery supervisor. These devices let you have a constant output voltage no matter how the input voltage (battery) decreases.

What these devices do is, in stead of dropping gradually the output voltage, they mantain it, requiring more current from the battery, thus finishing a bit earlier with its charge. But most of them also have an output pin to let you know when the batt is becoming to be very low.

Hope this helps now

 

[jsherman]

Hi Halls,

That sounds like exactly what I would want! However, I'm having a little trouble finding an IC that does that. I've been searching around for battery supervisors, but I've only found battery monitors and battery chargers. Are you referring to the battery charging circuits that supply a constant voltage?

Thanks!