TPS5401 with low Vout under I>100mA

Roman Zavyalov

Other Parts Discussed in Thread: TPS5401

Hello,

TPS5401

Vin(min)	V	14.00
Vin(max)	V	18.00
Vout	V	3.30
Iout	A	0.50

Under Iout<70mA and no-load mode are works perfect:

When Iout>100mA, TPS5401 sets the output voltage to 1.6V:

My topology is based on datasheet:

Please, help me to solve this problem.

Calculation of the stabilizer in the attachment.DC-DC_Converter_TPS5401.xlsx

over 8 years ago

0 JohnTucker over 8 years ago

TI__Guru**** 191445 points

I do not see anything particularly wrong with your calculations, but your compensation resistor looks suspiciously high. Can you post your waveforms again including the SW (PH) node? Make sure to use a resolution where the individual pulses are visible.

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

Hi John,

Thanks for your response!

Waveforms of PH node with no-load:

And waveforms of PH node with load at 120mA:

0 Roman Zavyalov over 8 years ago in reply to Roman Zavyalov

Intellectual 865 points

I replaced the resistor R49 to 47k and capacitor C51 to 10p.

TPS5401 with load at 120 mA works, but not started with load.

With no-load it starts.

I need a help.

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

Something is not right. Those PH waveforms do not look normal. You should be switching around 870 kHz. Above about 45 mA you should be in CCM and your SW node waveform should be similar to fig. 44 in the datasheet. Below that it should start to look like fig. 45.

0 JohnTucker over 8 years ago in reply to JohnTucker

TI__Guru**** 191445 points

You might try R49 = 150 k, C52 = 1200 pF and C51 = 12 pF

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

Case 1

Waveforms on PH node with:

C51=10p

R49=47k

C52=3300p

Not started with load, but started with no-load.

Case 2

Waveforms on PH node with:

C51=12p

R49=150k

C52=1200p

Not started with load, works with load less then 3-5 sec.

what do you advise?

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

Can you post a waveform zoomed out a little? I would like to see a few more pulses. It looks to be not stable. What else is attached to the output? I simulated your circuit in webench and it should be stable with the last values. The PH waveform still looks suspect.

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

John,

Case 1

Starting with no load is carried out stably. Regardless of the level of discharge of the capacitor C46.
Anyinterference of the stabilizer at the output:

Figure1

Case 2
Starting with the load. The output capacitor C46 is discharged completely.
Startingwith no load, and the with load is carried out successfully. Any noise of the stabilizer at the output:

Figure2

Case 3
Starting with the load. The output capacitor C46 is not completely discharged.
Starting with no load is carried out successfully.
Starting with the load fails:

Figure3

Case 4
After a bad start with the load when the output capacitor is discharged is not completely without turning off the input power, connect the load, the picture as shown in Figure 2.

Case 5
The process of works. It seems that the regulator is switched off during operation - seen curves characteristic of idling:

Figure4

Note:

Yellow line - waveforms of PH node

Red line - waveforms of OUTput voltage

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

Yellow is output and red is PH , correct?

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

Yes , of course. Excuse me

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

I suggested those compensation values based on your output capacitor. However it still appears your circuit is unstable. Are you positive that you do not have any additional capacitance on the output? What is your load? If it is an electronic load, you should probably operate in "resistive", "CR" or similar not constant current. The control loop in CC mode often interferes with the IC control.

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

as the load is working chip TMS470M, which has a lot of capacitors on the power supply.

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

That would help top explain it. Can you measure the loop response?

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

how can I do it?

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

You probably cannot do it easily unless you have a network analyzer. I did not really think you would have the capability else you would already have checked it. Can you tell me how much and what kind of capacitance? how far away from the TPS5401?

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

Load at 120mA:

away from the TPS5401:

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

I'm only seeing a small amount of extra capacitance. Am I missing something?

0 Roman Zavyalov over 8 years ago in reply to JohnTucker

Intellectual 865 points

No, nothing more.

The biggest capasitor is С12=22uF

0 Roman Zavyalov over 8 years ago in reply to Roman Zavyalov

Intellectual 865 points

We have another unit in this stabilizer, its topology is almost the same (not significantly different).

We gathered on the other board, this stabilizer and run a test. It is run on idle is stable, irrespective of anything. Starts and loads up to 20mA (LED with a current-limiting resistor 100 Ohm). But when connected a to him our board with a microcontroller that requires 120mA, he start to run, but to drop the voltage to 1.9V - also stable, irrespective of anything.

0 Roman Zavyalov over 8 years ago in reply to Roman Zavyalov

Intellectual 865 points

we noticed that the frequency of PH node does not match the calculated value. We have established a variety of resistors R48 from 100k to 165k.
Get frequency of calculation we have not succeeded.

However, to obtain a stable operation of the stabilizer at a frequency of 1 MHz at R48 = 110K, but the output voltage is 2.85V:

Waveform of PH node:

0 JohnTucker over 8 years ago in reply to Roman Zavyalov

TI__Guru**** 191445 points

With that waveform you should get about 2.85 V. What are you measuring at COMP and VSENSE? This should be rather simple, but we do not seem to be making much progress on line. Who is your local FAE? Perhaps they can take a look at your circuit on site.