Low Battery Detection

[Illum]

Several DC/DC switching regulators have this function, namely the presence of "LBO" and "LBI" pins. I'm in the process of coming up with a stable circuit for the LT1308 Chip :grin2:
Info here: http://cds.linear.com/docs/Datasheet/1308abfb.pdf


I'm trying to build a portable battery pack that can provide 5V output for charging gadgets on the go.
So far, it looks something like this: :rolleyes:


[USB Male] ---> [MAX1811] ---> [AW 18650] ---> [LT1308] ---> [USB Female]


In the past I've largely ignored this "Low Battery Detection" feature because I was using primarily NIMH and battery packs far bigger than any practical charge time would require. However, lately after seeing the ECOSOL Powershot I've been wondering to myself if I have the potential to come up with my own pocket charger. :ohgeez:


Here's the block diagram of the LT1308 :eek:

According to the pin functions, the SHDN pin turns the device off when pulled low to ground.
The datasheet states that:

The LT1308A/LT1308B low-battery detector is a simple PNP input gain stage with an open-collector NPN output. The negative input of the gain stage is tied internally to a 200mV reference. The positive input is the LBI pin. Arrangement as a low-battery detector is straightforward.

As far as the block diagram shows, in the heart of the comparator there lies an NPN switch that pulls LBO low as long as LBI > 200mV, turning off only when LBO </= to 200mV. If the logic is correct, then my wiring goes as follows:

Suppose I want this to lock out at Vbat: 3V, LBI = 0.2V, R1 = 130K, R2 = 10K is a good starting point. As long as LBI > REF, SHDN transistor pulled low, keeping the NPN off. While potential of at least 1V maintained at SHDN by means of a pull up [~1K?]. When comparator turns off, the pull up resistor at the base of the SHDN pin's NPN turns it on, which causes the IC to shutdown, drawing only 1uA at the chip and whatever the pull-up will eat.... so it can't be drawing only 1uA in shutdown mode!


Obviously I'm not doing something right, because down in the middle there's this:

Okay... now I'm totally lost :shrug:
What is "To processor?" and pull up from 5V... that an external reference or is that supposed to be Vout?
R1 = [VLB - 200mV]/2uA... err, when I see that I think of [VLB - 0.2V]/0.000002A... Assuming R2 = 100K and VLB = 3V, R1 would compute to about 1.4M, gee... that seems a little high.


Am I even looking at this correctly? Please :help:


:thanks:

 

[Illum]

:anyone:

 

[anuragwap]

What is "To processor?"

Maybe it just goes to an MCU input pin which then blinks a red LED or displays 'low batt' instead of shutting the thing down? Just a shot in the dark.

 

[Steve K]

:anyone:

just to review the basics... are you trying to shut down the LT1308 when the battery is reaching zero capacity? That's what I'm going to assume. at least.

If so, I understand the intent to use the low batt output to drive the shutdown pin and quit draining the battery. Very reasonable. Unfortunately... LT tied the shutdown function into the low batt output, as they say "Low-battery detector does not function with SHDN pin grounded." Once the low batt condition is sensed and the shutdown pin is grounded, the low batt output pin is disabled and forced low. There's no way for a high battery voltage to enable the converter again... unless you disconnect the battery and basically reset the system? It doesn't look like this is your intent, though.

The "low batt output" pin is indeed intended to go to a processor; usually to tell it to start shutting down or to go to sleep. I'm going to assume that the shutdown pin is coming from an external low batt detector?? That makes no sense, though.

I'm not clear on how they intend the low batt input to be used, since they didn't include any hysteresis. Fortunately, you can add a resistor from output to input to set whatever hysteresis that you want. Hysteresis is important, because as soon as you shut down the converter, the battery voltage is going to increase noticeably. Without hysteresis, the converter would turn back on and start drawing current, which would cause the battery voltage to drop enough to shut down the converter, and this oscillation would continue until the unloaded battery voltage was below the desired voltage threshold.

So what's the solution?? Maybe you can find something good in LT's app notes? If that doesn't pan out, maybe you can use the low batt output to drive a mosfet between the converter and the load. You don't get rid of the converter's quiescent current, but at least you quit providing current to the load.

As far as the calculation of R1 leading to a value of 1.4M, that seems reasonable if you want R2 to be 100k. The bias current of the input is 100nA, so a 2uA current through the resistor divider seems reasonable, and reduces the drain on the battery when the converter is inhibited.

Is that any help?

 

[Illum]

Well, since this will be running off of an 18650, I would indeed like to have the LT power down when the battery voltage reaches 3.0V, preferably 3.2V. I figured I could use the built in comparator to do this... but not enough info exists on the datasheet as I can see for a straightforward construction

 

[Steve K]

There are some handy little devices called "voltage supervisors" or "voltage detectors" or other things.. they are small SOT devices that provide a low voltage indication to processors, similar to what the LBO does on the 1308. The On Semiconductor NCP304 and NCP305 are good examples of this:
http://www.onsemi.com/pub_link/Collateral/NCP304-D.PDF

It comes in a lot of various voltage thresholds, so there ought to be something that works. The standard hysteresis is fairly small. Maybe you can add some positive feedback to increase it as needed? If you can make it work, it's a lot simpler than adding a comparator and voltage reference.