TPS27S100: TPS27S100A Fault activation problem

Hi Renato,

When ULN2003 is always on and you turn off TPS27S100A, there is no FLT reported but what about the situation where ULN2003A is always on and you turn on TPS27S100A, does the FLT signal go low? 

Hi Shreyas.

When ULN2003A is always on and I turn on TPS27S100A, the FLT signal goes low, with same timing as both components turn on at same time.

Fault signal remains in low state for 250 us.

Best regards

Renato Colombo

Hi Renato,

Keeping ULN2003A turned on so that there is a path to GND, could you please share the following scopeshot during the turnon event:

Ch1: VIN

Ch2: VOUT

Ch3: FLT

Ch4: IOUT

What is your current limit set at? And what is the nature of the load?

Hi Shreyas,

Sorry, but today I was involved in other problems and I had no time for this. I hope tomorrow I could send you the scopeshot you're asking.

For the current limit, there is a 1.21 KOhm resistor between pin 13 and pin 2.

The load actually is only a resistor, 1 KOhm or 75 Ohm (depending on test I was performing).

No inductive load, at the moment.

Best Regards.

Renato Colombo

Hi Renato,

Thanks for confirming. That current limit resistor lines up with a 2A external limit. I wonder if there is any inrush current during turn on that is triggering the FLT for a while. 

Hi Shreyas

I think I partially resolved.

I changed the "ILIM" resistor, from 1.21 KOhm to 681 Ohm (I do not calculate the new limit current).

In this way, taking ULN2003 always on, the Fault signal never activates, neither at turn-on nor at turn-off.

Even driving ULN2003 on and off, the Fault signal do not activate at turn-on.

But the Fault signal activates at turn-off. So I think that TPS27S100A sees an open load circuit before current is zero (due to opening of ULN2003).

TPS27S100A and ULN2003 turn-on and off at the same time. Maybe, if ULN2003 turn off a little bit later than TPS27S100A, the Fault signal do not activate.

But it is not so simple to perform a test like this, with my actual circuit. I need to thik about this.

Do you still need the scopeshot you asked me? Now I have it.

Best Regards,

Renato Colombo

Hi Renato,

Please share the scopeshot if you have it as it will help us understand the situation better. With 681Ohm I think the CL is around 3.5A.

My initial suspicions for turn on failure is that inrush current is clamped by the device and then the current drops.

For turn off, I suspect the same as you. The device is seeing an open load and I think if you could turn off TPS27S100 first and then turn off ULN2003A the FLT will not be reported.

Hi Shreyas

Here are the scopeshots.

Unlucky, I'm using a two channel scope, so I must post two images.

In the first, you can see the TPS27S100A command (EN, pin 3) in yellow and the VOUT (pin 5,6,7) in green.

In the second, you can see the TPS27S100A command (EN, pin 3) in yellow and the /FLT out signal (pin 14) in green.

Using 861 Ohm resistor on ILIM, the second image becomes as follows:

I would like to turn-off ULN2003 some time after TPS27S100A, but it is not so simple to realize in my actual board.

But this is only a prototype board, so I think I could do this when remastering the board.

Best Regards,

Renato Colombo

Hi Renato,

Do you have a current measurement that shows the current flowing through the device on turn on?