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Using dc/dc converter to power a board

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wanchope

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I have a pic based device with an rtc inside. This device is powered by a piece of LR44 (1.5volt). As the rtc minimum operating voltage is 2.7 volt, I am thinking of using a dc/dc converter to power it. The whole board's quiescent current at 3 volt is 4uA without the dc/dc converter. But after I added in the max1724 dc/dc converter to power the whole board (it claims to have 1.5uA quiescent current), it is around 25uA, which is a lot to me. Anyone have come accross a better converter? Or any other ideas?
Thanks.
Wanchope
 

throwaway18

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Re: dc/dc converter

Perhaps you could use a 3V battery or two 1.5V battery's in series.
 

Baiyawfan

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dc/dc converter

Yes,for so small quiescent current,I also think battery is good,especially button battery
 

Sceadwian

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dc/dc converter

It is very inefficient to convert DC voltages at these low levels, you are better off as said already to chose the battery first. An RTC should never be powered by a DC to DC converter, as efficient as they are they're still wasteful, a RTC should always be powered directly from a rechargeable cell (capacitor or battery) as close to it's native voltage as possibe. Batteries are usually best because most RTC's require so little power that the battery experie from shelf life before it does from current drain. A 3 volt Lithum cell will provide viable current for a RTC down to almost it's absolute discharge limit.
Also think about using a dedicated RTC (not a pic based one) as they sometimes come with built in batteries and lifetimes in 10's of years. The RTC on a pic or other micro controller is nice, but not always the sollution for super low power applications.
 

wanchope

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Re: dc/dc converter

Thanks, all. If using 2 LR44 in series, it may be a bit wasteful as I need to throw them away when the in-series voltage drop below 2.7 volts. In addition,, the active current is around 20mA for the device. The lifespan could be quite short if i cut it off at 2.7volt. And also there is one chip's operating voltage at 3 volt. Then no choice, i need a regulated 3 volt source. Rechargable battery is one way, but charger is needed, which could be barrier to the consumers. May not be able to go into it at the moment.
Now the quiescent current goes down to 13uA after i change the low ESR cap to a normal one at the i/p and o/p stages. But still need some ways to reduce it.
Any other ideas?

Thanks.
Wanchope
 

Sceadwian

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dc/dc converter

You could try using the DC to DC converter to charge a high value cap in bursts and then let the very slow natural discharge drain the storage cap. This would increase effciency possibly to useful levels. I'd use rechargeable batteries though, using coin cells as throw aways in this kind of device is environmentally unfriendly and really irresponsible. N cell sized Nicads can be charged in less than a minute and provide 30-70mah's of power using only free PIC resources (ADC) and a couple transistors for charging circuitry. Power can be supplied by a generic low voltage DC power supply.
 

wanchope

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Re: dc/dc converter

I ever thought of using the supercap, however i still need to turn on the dc/dc converter to power my pic (even though most of the time it is sleeping). The minimum operating voltage is 2 volt for my pic. It also need to wake up from time to time to charge the cap also.
I used lithium polymer paper battery before, but the self discharge is a concern.

regards
Wanchope
 

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