Need help with turning on voltage regulator

Here is the situation I've run into. I have a device that had a voltage regulator that didnt work to well. There for I switched to a diffrent regulator, and it seems to work really well. But when I built the prototype power supply, I ran into a little issue.

The insturment uses a single button on/off. This was done using a MAX1672 that uses ONA and ONB. This IC would turn on when (ONB not) went low, and as soon as the microcontroller got power, it took over and put ONA high, which keep the powersupply on. When shutting off, the microcontroller detected (ONB not) going low and would start a shutdown sequence, and the microcontroller would then put ONA low shutting down the supply. If this sound confusing here is a truth table.

ONA---(ONB NOT)---MAX1672
0-----------0----------On
0-----------1----------Off
1-----------0----------On
1-----------1----------On

Now what I am using is a MAX887, which uses (SHDN Not). So when (SHDN Not) was high, the regulator was running. When (SHDN Not) went low the regulator was off.

So basicly I'm asking how I could get the MAX887 SHDN to work with something that was made for ONA and (SHDN Not)? Could I use some sort of flip flop or something? I'm very confused on what to do........

GSXR1181 said:

ONA---(ONB NOT)---MAX1672
0-----------0----------On
0-----------1----------Off
1-----------0----------On
1-----------1----------On

Now what I am using is a MAX887, which uses (SHDN Not). So when (SHDN Not) was high, the regulator was running. When (SHDN Not) went low the regulator was off.

So basicly I'm asking how I could get the MAX887 SHDN to work with something that was made for ONA and (SHDN Not)? Could I use some sort of flip flop or something? I'm very confused on what to do........

Click to expand...

You want a gate, not a flip-flop. A flip flop can remember its state, and you don't want that here as the microcontroller does that.

You can get what you want with a NAND gate and an inverter. Invert the ONA signal to give you (ONA NOT). The two inputs of the NAND gate are the (ONA NOT) and the (ONB NOT) signals. The ouput is high when either is low, turning on the MAX887

ONA---(ONB NOT)---MAX1672---(ONA NOT)---NAND---MAX887
0-----------0----------On--------------1----------1-----------On
0-----------1----------Off--------------1----------0-----------Off
1-----------0----------On--------------0----------1-----------On
1-----------1----------On--------------0----------1-----------On

If you use a CD4011 you get four NAND gates in one package. That IC will work on any voltage that the MAX887 will work on, so you don't need a regulator to supply it, and it takes just about no current at all.

You join the two inputs of a NAND to make an inverter. The unused gates should have all their inputs connected to ground.

I don't know what supply voltage you have, or exactly what feeds the inputs. There is a possible problem with levels if the supply voltage is large. If the supply is 10V, the inputs of the CD4011 have to be above 7V to be sure to be high, and this won't be possible if you are using a 5V output.

Thanks for the reply. I built the CD4011 circuit and it works great when my supply voltage is between 3.5v and 6.4v. But as soon as I go above 6.5, the unit shuts off, and if I try to power it back up, the display flashes momentarily.

I'm needing this thing to work between 4v and 7.5v. Also the inputs are 3.3v's So this is why it's a little more flaky.

I'm going to try to put a voltage divider on the VCC and see if I can find a happy point where it will work.

Just add a 5V low dropout regulator to it.