Automatic Battery switching IC

[D. Collin]

Hi all,

Is there any IC which handles automatic switching power supply from a battery if the master power input detected failed?

I am trying to have a final result such as:

PIC and whole board are powered normally with 12V DC which at the end goes to a 7805 for 5V regulated rail.
Somewhere before the 7805 goes the desired IC which checks the 12VDC input and pass it to the 7805 if detected, otherwise if removed, it will supply the 7805 with a 9V battery (or so) also connected to this IC.
The IC should be able to notify the PIC by resetting a pulled-up pin (for example) that the board is running on backup battery. I think this can be designed using a low-cost low-pin count PIC running on internal clock with some resistors, switching transistors and capacitors.

Battery does not need to be charged. I have been looking at those power management IC�s that are used in mobile phones and hand-held designs but these IC are mainly for recharging and some are even more qualified that needed.

Can any one recommend a low cost IC that does this? or maybe any other method?

Thanks.

D.C.

[libor]

You can simply connect both supplies (12V DC and battery) each thru a diode to the input of the 7805. The source having the higher voltage will feed the regulator. You can use schottky diode to lessen the forward voltage drop on the battery side (to get some more energy out of the battery)
You can check the voltage input from the 12VDC source before the diode by dividing it to 5V level with 2 resistors (use high values not to load unecessary amps) and feeding it to the PICs input pin.
As an option I would also use a similar circuit on the battery side, and feeding it to an A/D input, you can always check the status of the battery in software.

[libor]

A follow-up:
Even if you are not going to use a rechargeable solution, I would suggest you to tickle-charge the battery with some minimal current (below 1mA) from the 12VDC, so you can compensate the battery's self-discharge phenomenon, and have a much longer battery life on standby. A lithium battery would however not need this.

[D. Collin]

Hi libor,

Thank you for your input. Yes, the plan is to use lithium batteries.

I am more into software than hardware, especially electronics; are the diodes prevent both of them being used because both sources are not the same voltage? which leads to me to a question if both sources (12V & 9V) share the same ground and connected on the positive rail end with both of them being used for higher amp? Was just comparing this to connecting two batteries of the same voltage in parallel to end up with one battery of the same voltage but with double the current capacity, i guess the diodes must resolve this issue.

Thanx again.
D.C.

[libor]

The diodes will prevent each source to 'charge' the other, without the battery's diode the 12VDC would roast you battery.
The diode on the 12VDC side could be left out however if you are absolutely sure that your 12VDC source will not act as a load when feeding its inputs in switched-off state. (e.g. if you have az unregulated 12VDC supply with a Greatz diode-bridge with no other semicondutor in it, you can leave your diode out and use these diodes (those in the bridge) for the same purpose. But for added safety I would suggest you to have two diodes: one on each power source
This two diodes are anyway way much cheaper than the IC solution you were contemplating. Those fancy ICs are made mainly for smart-charging the batteries, counting the charge/cycles left in the battery, etc.

Measuring battery state: wise thing to use lithium, so it is also advisable that you put absolutely no load on the battery when on standby, so I would suggest you to feed the PICs A/D right from the input of the 7805 (after the diode) using the voltage divider resistors, so they will not load the battery when powered from 12VDC.

There is however a pitfall in this whole solution: the 7805 will make a load on the battery when your whole circuit is not in use (lying on the shelf) either you (the user) must remove the battery or add a switch to do this (however it will be always forgotten to be switched off) To make your circuit fool-proof in this regard, you must add an active switch (controlled by the PIC) on the battery line going to the 7805, so you can make your circuit to start only from the 12VDC, and be able to cut off the battery supply programmativcally to unload the battery feeding the 7805. (PIC commiting suicide) To elaborate more information would be useful on how your circuit would be used.

[D. Collin]

Thank you libor for this valuable explanation about the electronics side.

Would you suggest any Diode such as 4004? Or 4008 or signal diodes?

The board itself is going to act as a master controller which needs at least an hour to operate if the main source got switched off (rare case! but just in case). Also once the board detects main source out, it will save all RAM into EEPROM

D.C

[libor]

Yes, a 4004 will do, it can handle up to 1A, a small signal diode like the 4148 with 100-200mA limit would be too small.

In case you want to squeeze every drop of power out of the battery, I would suggest to use a low-drop version instead of the normal 7805. The 7805 operates from 7-7.5V input and above, some low-drop regulators will do the same even at 5.5-6V input. This would also justify using a Schottky instead of the 4004, to lessen the voltage drop on the diode also. But if you are not concerned with the possibly longest operation on battery, the 7805 will be fine also.

If you would not want functional operation from the battery, you just need enough time to save RAM data to EEPROM before the PIC dies in lack of power, there is a more simple solution: use a larger capacitor, a 4700uF can power your PIC for several seconds after the power fails. (also can be built with a diode before the large cap, measuring the voltage before the diode to detect power loss immediately to begin saving data in software)

[Guest]

Thank you libor for all your help.

Regards
D.C.

[Christophe]

OR'ing diodes is ok, but you have a Vf drop. I had the same problem as you.. The solution is powerpath control.. use the LTC4411 or LTC4412 from www.linear.com

gl!