Using a larger battery

[Mike Warren]

I want to run a 1 cell Li-ion powered video camera from a 2 cell Li-ion battery. The idea I have is to use a LT3995 to drop the voltage . I would like the existing battery meter system in the camera to remain operational with this larger battery, so I would need the output to track the input. Basically, the output voltage should always be half the input voltage.

My first thought was to use a transistor driven from the battery to offset the adjust pin on the regulator, but that seems like it might be a bit difficult to get to track properly.

I have a small microcontroller (ATTiny24A) which I'm using to do a couple of minor things and I could use the PWM output to control the regulator and an ADC to read the battery voltage. That way I could fine tune it in software to get it to track.

Anyway, the reason for this post is in case I've missed an easier way to do this.

Anyone have any thoughts on the subject?

[NiHaoMike]

Put the cells in parallel.

[Mike Warren]

Put the cells in parallel.

I can't get access to the cells as they are in a sealed pack.

[staxquad]

I want to run a 1 cell Li-ion powered video camera from a 2 cell Li-ion battery. The idea I have is to use a LT3995 to drop the voltage . I would like the existing battery meter system in the camera to remain operational with this larger battery, so I would need the output to track the input. Basically, the output voltage should always be half the input voltage.

My first thought was to use a transistor driven from the battery to offset the adjust pin on the regulator, but that seems like it might be a bit difficult to get to track properly.

I have a small microcontroller (ATTiny24A) which I'm using to do a couple of minor things and I could use the PWM output to control the regulator and an ADC to read the battery voltage. That way I could fine tune it in software to get it to track.

Anyway, the reason for this post is in case I've missed an easier way to do this.

Anyone have any thoughts on the subject?

internal or external battery?  (picture)

why not use a larger capacity 1S Li-Po?  (assuming you're trying to add capacity and will be running the battery externally)

[Mike Warren]

Because of physical restrictions and using batteries that I use on other equipment.

[peter.mitchell]

just use a high value resistive divider with 2 small caps on it to the battery gauge; i assume you're using the batteries at a reasonably low discharge rate so they should discharge fairly evenly.

[Mike Warren]

Unfortunately, I can't do that either. I have no access to the battery gauge. It's built into the camera and also includes graceful shutdown when the battery is exhausted, which is the main reason Id' like to simulate the original battery. The camera draws between 700mA and 1.1A depending on what it's doing.

[dr_p]

From the 7.4V battery, two 100k resistors to ground with a 1K trimpot in the middle, adjust to precisely half.


Feed this in the Positive input of an op-amp, feed the regulator's output in the negative input, and op-amp output goes to feedback pin of the regulator.

So the op-amp does whatever's neccessary to keep the output of the regulator at 1/2 of the input voltage


edit:


I'm not sure if you should disconnect the feedback path before or after the RC stuff connected to the feedback pin of the LT3995.

[Mike Warren]

Hmm, that's something to consider. Anyone know of a readily available op-amp with extremely low quiescent current?

[Mike Warren]

I'm not sure if you should disconnect the feedback path before or after the RC stuff connected to the feedback pin of the LT3995.

The feedback resistor and capacitor will need to remain connected because this regulator uses extremely small pulses during low current mode and I expect it would either be unstable or the ultra low quiescent feature wouldn't work properly. So using your op-amp idea would need to be an offset situation much like my transistor idea, but it should be easier to get it to track.

Unfortunately, I don't yet have a LT3995 to experiment with.