TPS25944L: protecting the LMR14030

Hi Rakesh,

Simulation- I do not have.

This solution is not very appriciated. My idea is to use PGTH as protection against overvolatage at LMR14030 output.
Currently the PGTH it is connected(wrongly) to the input of LMR. instead of its ouptut. I will check this way this might be a reason.
My idea is so, If the 3.3 goes to hight let say to 3.5V this will switched off the TPS25944. So I will have overload and overvoltage protected 3.3 output, thats the wish.
The LMR14030 has 22uF + 47nF inpuyt capacitance, it is placed between output off TPS and Input of LMR. I have increased even 220uF, but the problem was too. There is no input capacitance for the TPS it was straight to power supply ( at the moment).

PGTH  is not a problem, actually it is used as a flag only, I wrongly tought it is restting the TPS.

I enclose a picture,  of FLT PIN yellow, Pink is output of the TPS. This fault beahaviour :I think is due to the overshoot after switch off. When I add an extra 220uF to the input of TPS overshoot is disapearing, but circuit ist still oscilating after restart.

Restart with 1R5 load at the output( almost short) Yellow 3.3 output Pink output from TPS

Restart with 1 Ohm Load Yellow Vout 3.3 Pink out from TPS

What we can note from above is that the circuit is corectly switched off ( after restart which is needed) till the load of 1.5 Ohm, if there more low ohmic load circuit is starting oscilating. I guess this is all because of that that the TPS is not acumulating those pulses. It can be trigered either by the Itrip which in this case is to low or if the overcurrent is higher than the Ilim and longer than 4ms. Both cases are not true and...is oscilating.

And all this strange behaviour is due to the LMR behaviour which is dynamic object.

Rakesh:

Let assume, I connect PGTH to the output of 3.3 converter (through the divider clear), and the Drain of PGD FET of TPS25944 to EN pin of the LMR14030 Pin ok. Now is such case:

LMR is overloaded comparator in TPS will keep the LMR off, and somehow overload disapears, what will triger LMR to on state if TPS is keeping it off?

I am afraid it will never start until you provide some voltage to the 3.3 output. Again. It is off, load removed , its still off becasue TPS is locking it  through EN. Or am I wrong?:

Css capacitor in LMR was 10nF and 100nF, for 10n the output volateg is rising up in 2ms for 100n its about 20ms. 20 is to muchfor me,  there will be mcu at 3.3 output. But I tested it as well.

This is enable pin of LMR -yellow, pink is out from TPS- Treshold 1.2V +-50mV hyateresis seems ok

And this is Enable Pin of TPS- falling treshold 0,92V - seems ok

OVP Pin of TPS, rising treshold 0,99V -seems ok

I did a test with the LM2596 converter connected to the output of TPS25944.When I have a full short or partial short on the output set to 3.3V ( as in LMR) the TPS is correctly switched off after 4ms.
Pink is output from TPS yellow is the current information from Imon pin(TPS). 0.5V/0.5A roughly

I am still waiting for the current probe, however current information from the Imon pin seems correct, my limitation is set to 0.75A and it seems so during this 4ms period.
What I do not like is that the LM2596 consumes about 6mA current in standby mode/no load. Therefore the LMR   was my choice as it is very good less than 1mA stdby current, what for my battery application is important.

Now i need to find a way, if its possible, to connect the LMR and TPS correctly. I am afraid this is not possible because of the LMR behaviour, Do not you think so?

I still do not have current probe. Maybe it will arrive in next week.

But i did another test with TPS56321evm board which has also this

Hi Christopher,

I have reached out to LM2596 team to suggest a Switcher with Low operating quiescent current like LM2596 but without cycle-by-cycle current limit. The Engineer will get back to you shortly.

Best Regards,
Rakesh

Yes I did C41 was 220uF, and even 2x220uF. However it was still oscillating with one C=220uF. Somehow it helps with 12V input, but if the input is 9V (battery low) it does not ( bigger input C needed in this case).
And I can not increase this capacitance so easy, limit with space and some another critical requiments.

I must repeat tests with current probe. I am still waiting for a current probe, its ordered and there are some customs declarations needed as it was imported from US. Takes time..

As so far my current information was taken from the Imon output. I have a one idea how to avoid this. The TPS25944 would be a very good chip if ( it is partially)

if it would integrate these currents spikes. Because if your system will try to start with short on the output it will jump in neverending loop.

So my solution is so: i will take a signal from Imon pin and this will feed  through a diode (1N4148) to RC filter. ( simple peak sampling net) The voltage on the Capacitor should be equal to the current spike  amplitude, and this voltage will trigger some additional circuit which will pulls down the EN Pin, perhaps a delay is needed to not jump in another never-ending loop.

I am trying to perform some simulation in PSpice , I am not sure if  TImodel is really so realistic. But i can verify this in hardware to its faster and more acqurate.

It has nothing to do with the main problem. But there is a Failure in datasheet,:

SLVSCE9D–JUNE2014–REVISED OCTOBER 2017

But...The problem is that , from the LMR14030 uC is supplied and usually the rising time for uC should be less than 2ms.

I have designed a circuit which based on the information from Imon pin is switching off the TPS25944. However I need several components for that, no place on board,

and the other problem is that this puls down the EN pin, so without latch function it will jump in another never-ending loop. Long dealy is needed..

The best solution would be if TPS would be latched off by itself, based on the Itrip current. Thus, LMR should sink high current spike, everything will be more clear with current probe which is still on the way..

If we are lucky. It must protect the battery and the common 12V battery bus, not the dc-dc converter. To protect the dc-dc converter cycle by cycle limiatation is good. But when it fails, system is dead, nobody will guarantee that. Thus, additional separated protection is needed on the input. If it fails redundant converter is useless.

I have ordered additional EVM with TP65533 which should pulls much higher current  during short on its output, hopefully, this should trigger the TPS25944.

Regarding to the increasing capacitor between TPS and LMR. If the capacitor is Big then during short on the output of LMR most of the energy/current during first cycles will come from this capacitor, and the current will flow not through TPS. Thus the big capacitance should be on the input of TPS to trigger it and latched of. Do not you think so Rakesh? I think so.

The input current of LMR measured with the current probe is similar to that one taken from the IMON pin, no big differences.
0,5A/div pink, yellow  is12V Input voltage.

TPS53353 sorry for typo error. Iq is less then 1mA, its about 0,5mA.

The peak current (during restart on the shorted output)  was in caseof the LMR14030  -5Amps and for the TPS53353 is 15A.