LT1317 error in data sheet? Or, what am I doing wrong?

[HooRide]

Hi all, a project I'm working on requires 27V at 8mA to operate and plan to power it from two AA batteries. I've built a fully functioning prototype, PCB and all but the under voltage lockout is not working.

I copied the lockout design verbatim from the Linear Technology data sheet but it does not work as expected. After some quick calculations, it looks like they made a mistake.

If I understand it correctly, the comparator is on @ 200mV and off below 200mV.

It works close to how they intended if the 340K resistor is swapped out for a 34K resistor (actually, 39K). Is there something wrong with their design or my implementation?

Thanks!

[HalfSpace]

Hi HooRide,

It’s a mistake by Linear Technology it should be 34K.

If you calculate that the series 100K and 1M are in parallel with the 301K, we get (1/301K) + (1/100K+1M)  =  1/total = 236K for the upper leg of the voltage divider when the LT1317 is in running mode.

The voltage divider ratio is (236K/34K) + 1 = 7.94 so the low battery trigger level will be 0.2*7.94 = 1.58v

When the LT1317 low battery level has been triggered the 1M resistor will be pulled to ground by the LBO pin placing it now across the 34K resistor, thus changing the low battery trigger level. The effect of the 1M in parallel with the 34K and the voltage drop across the NPN open collector of the LBO pin will be small so I will leave it at 34K.

Now the voltage divider ratio will be (301K/34K) + 1 = 9.85 making the low battery trigger level 0.2*9.85 = 1.97v thus providing hysteresis to the circuit. This reduces the chance of oscillation or false triggering of the LT1317 if the battery voltage increases when it is placed in shutdown by the low battery circuit.

Hope this helps.

HalfSpace.

[HooRide]

That helps immensely, thank you! Especially useful is the explanation of the hysteresis; very clever.

I'll have to order a new sample because I can't trust this part to function 100% now that the LBI pin has seen 3x is absolute maximum.

Thanks again!

[stp715a]

Hi.

Did you get this to work?  Thank you.