Built circuit w/ timer IC connected to SHDN pin of a voltage regulator - not shutting down properly?

[Mahonroy]

Hey guys,
I built a circuit using a timer IC (TPL5111) connected up to the SHDN pin of a voltage regulator (MAX1724).
Here is a simplified schematic of the intended use of the timer IC:


The basic idea is that the device powers up, a microcontroller can do a few things, the microcontroller can indicate to the timer that its done doing whatever it wanted to do, and the device powers down for a few minutes with very little quiscent current drain... greatly increasing battery life.

I verified that the SHDN pin (EN pin) is held low for the duration of the timer, at which point its brought up to 3.3V to power everything up. The microcontroller does the task, then indicates to the timer that its done, at which point the SHDN pin is taken back to 0 volts again for the duration..... then repeat. This is good and is how its supposed to work.

My problem is that I can see the power rail gets 3.3V when powered up, but when powered down its held at a steady 2.8 volts. I can see that during this period of time current draw is still 4mA. It should be 0.0002mA when powered down (quiescent current of the voltage regulator in shutdown mode, and quiescent current of the timer IC).

Do you know what might be causing this behavior? Or how I should go about troubleshooting it? Any help/advice is greatly appreciated!

Here is my actual schematic for reference:

 

[Alec_t]

Do you know what might be causing this behavior?

Yes. The 1724 is doing what it says on the tin. Read the datasheet's description of the shut-down process.

 

[Mahonroy]

Yes. The 1724 is doing what it says on the tin. Read the datasheet's description of the shut-down process.

Thanks for the info. I didn't understand the difference between "shutdown" and "true shutdown", I was comparing so many different boost converters that I assumed the shutdown all worked the same, and missed that piece of the datasheet.

So with that in mind, I am thinking I can just use the MAX1795 instead. Its shutdown pin is the opposite though (high is off, low is on). Can I just use an inverter such as 74LVC1G14SE-7 to handle this? I am slightly worried that the inverter would end up using too much power when it is holding the pin high for shutdown?

It would be great to get an opinion on this so I don't make another stupid mistake, and thanks again.

 

[Alec_t]

That inverter is a CMOS one, so draws just a few micro-amps when pulling high or low, hence dissipates only a few micro-watts. In my book that's negligible, but I don't know what you consider "too much" power?

 

[Mahonroy]

That inverter is a CMOS one, so draws just a few micro-amps when pulling high or low, hence dissipates only a few micro-watts. In my book that's negligible, but I don't know what you consider "too much" power?

Maybe this would be a better choise then?
NC7SV04P5X
Its for a battery powered device with long duration sleep modes. In sleep mode the only thing being powered is the timer IC (TPL5111), the boost regulator (MAX1795 in shutdown mode), and the inverter.

 

[Alec_t]

I replied at AAC.

 

[Mahonroy]

I replied at AAC.

Thanks again for all the help!
So I re-designed this thing, only using the MAX1795 instead, and an inverter for the SHDN pin since it works the opposite as before.

I am still getting a similar problem? When the SHDN pin is pulled low, this enables the MAX1795, and I see the 20mA @ 3.5 volts. When the SHDN pin is pulled high to 3.5 volts (same voltage as the battery supply, which is supposed to put the MAX1795 in shutdown mode), I see 5mA @ 2.55 volts.

How can this be?

Thanks again and any help is greatly appreciated! Below is the new schematic:

 

[Mahonroy]

Do you think this means that power is getting drawn from something external? Or is my shutdown circuit malfunctioning?

 

[Mahonroy]

Ok I de-soldered the IC1 (R-78E3) to remove that from the equation and nothing changed. Still getting about 5mA when shutdown mode is enabled.
FYI, I am taking the current reading at the batteries.

However... I tried with only a single battery and the thing now works. These batteries are AA (Energizer Ultimate Lithium) and the damn things are reading 1.75 volts each. So I guess the 3.5 volts is too much for the MAX1795 and the shutdown does not work.... ugh. I was hoping the 2x batteries together would be under 3.3 volts, but that is not the case.

I found 2x batteries that were reading 1.475 (2.95 together), tried them and I get the same problem with the 5mA current draw in shutdown. So even with the battery level is under VOUT of 3.3V the shutdown still does not work properly, but it does when its under 2 volts? Man I am stumped

 

[Mahonroy]

I built a PCB with only the MAX1795, the input capacitor & inductor, the 2 output capacitors, and a 100 ohm resistor for a load. Like this the thing works perfectly. I get 40mA current draw when SHDN is grounded, and when I connect SHDN to battery I get only 0.01mA current draw. This was with 3.5 volts input too. I tested with a single battery of 1.75 volts the shutdown continues to work properly.

So I guess the next step is that I should solder on the timer, inverter, and those components corresponding circuitry? Is there something I should try first you think?