LTC3531-3.3V with low voltage sense and regulator shutdown

[jimbostlawrence]

Hi all,

I tried to come up with a circuit on this before with an inappropriate regulator (buck only) and have now switched to a buck/boost solution.

What I'm trying to do is generate a power supply for a low current circuit using a buck/boost regulator (3.3V out) driven by a 3.6V SAFT non-rechargeable battery. I've chosen the LTC3531-3.3V fixed voltage output chip.

I've also been looking at the MAX6328 voltage sense chip which when given a potential and ground, will output a low when then potential falls below its inbuilt reference (in my case 3.1V).

What the circuit below 'should' do is power up and drive the LED until the battery voltage falls to 3.1V at which point, the MAX6328 outputs a low, shutting down the regulator, and at the same time, switches on the PNP transistor through the 2.2k resistor and drains the battery through the 100 ohm resistor to ground, so the regulator cannot switch back on again.

What seems to be happening is that, if there is no load, i.e. the resistor/LED combination is disconnected, and the battery is connected, the regulator does indeed switch on and the resistor/LED can then be connected and illuminates until the battery goes low and then the regulator is switched off etc...

However, if the resistor/LED is connected (about 30mA), then the regulator never switches on, and the MAX6328 outputs a low (!reset) which switches on the PNP transistor draining the battery!

Is there some issue here with the regulator not being able to handle an immediate load or something? The circuit seems correct, just as long as there is no load on the regulator before switching on power.

Please ignore the PNP component type, it's not what it says, just a simple small 100 mA PNP. Same with the LED.

Might it be something to do with the 10k pull up on the !reset line together with the 2.2k base resistor on the PNP to VCC?

Cheers all

Jimbo