TPS65279RHHR - Driving current support Issue

Jagdish Gundu

Other Parts Discussed in Thread: TPS65279

Hello,

I am in an process of evaluation of my design, i have attached my schematics and the problem is as defined below. I am looking for technical expert support.

Problem: Circuit is not able to deliver 5A current at each channel. Instead it is only delivering ~3A.

Please check what could be technical error in the circuit,.. eg Inductor selection, Switching frequency selection, Rlimit resistor selection.

Regards,

over 7 years ago

0 Jagdish Gundu over 7 years ago

Intellectual 390 points

Help Help..

0 JohnTucker over 7 years ago in reply to Jagdish Gundu

TI__Guru**** 191445 points

TPS65279 is supported out of our MCP team in China. I will alert them of your request. You should see some response in the next 10 - 18 hours.

0 Jason Wang (BSR-MV) over 7 years ago in reply to JohnTucker

TI__Genius 9550 points

Gundu,

change R111 and R112 to about 60kohm.

100Kohm is larger and limited the peak current in inductor.

Jason Wang

0 Daniel Jing over 7 years ago

TI__Prodigy 515 points

Rlim is not correct.
This is a peak current limit. 100K Rlim sets inductor peak current to 4.4 A. output could be only have 3A continous out.
To have 5A output current, please change Rlim from 100K to around 60.4K Ohm.
details, you can check datasheet page 14, figure 18.

0 Jagdish Gundu over 7 years ago in reply to Daniel Jing

Intellectual 390 points

Thank You Daniel,

Yes I agree that Rlimit incorrect, I have replaced that resistor to even ~60K and nothing works. On 3.3V buck side i am getting an current of ~2.8-3.0A and When I apply more load, the O/P V is folding back and shows 2.6V@4.5A.

However I have suspect that.. is my PIN4(MODE PIN) of IC should be to connected to GND pin instead V7V. Would like to Know what circuit level difference that you observe when compared to evaluation board.

I know it is bit hard to see/understand what i posted here below. But this my layout Followed all guidelines mentioned in AN.

Please support thx

Regards,

0 Yuchang Zhang over 7 years ago in reply to Jagdish Gundu

TI__Mastermind 18510 points

JG,

Just suggest you to upload some waveform include Vin/LX/Vout/IL signal, both normal and abnormal working.

Another question, Why the SS cap value in your schematic is 10UF?

About layout, How do you deal with chip ground? Is there an integrated gound plane connected with chip ground by some vias?

Regards,

Yuchang

0 Jagdish Gundu over 7 years ago in reply to Yuchang Zhang

Intellectual 390 points

Testing_Buck.docxThanks Yuchang,

Please see document for the requested waveforms. Not sure about SS capacitor being of 10uF. However do you recommend any value for SS to change and test it.
Please suggest.

Regards,
JG

0 Yuchang Zhang over 7 years ago in reply to Jagdish Gundu

TI__Mastermind 18510 points

JG,

From your waveform Fig7, Vout@max load is around 3.3V, it looks right. so it is werid your Multi-meter test is 2.6V, could you please test the same point as the probe to double check the Vout?

Regards,
Yuchang

0 Yuchang Zhang over 7 years ago in reply to Yuchang Zhang

TI__Mastermind 18510 points

JG,

About SS cap, Refer to EVM, it is 10nF.
and also you can calculate Css based on the formula at 8.3.3 soft start page in datasheet.

Regard,
Yuchang

0 Jagdish Gundu over 7 years ago in reply to Yuchang Zhang

Intellectual 390 points

Testing_Buck_V1.1.docxYuchang. I did verified again.. puzzled. I read again same ~2.67V at OP on MM and >3.3V on scope window. Please see attachments latest. And also i did captured Vripple on Vout side.

Is Oscope measuring Vout along with ripple voltage?

Regards,
JG

0 Yuchang Zhang over 7 years ago in reply to Jagdish Gundu

TI__Mastermind 18510 points

JG,

First, If you want to check ripple, better use AC with 20M BW, shortest probe ground, Provide a link for your reference.
e2e.ti.com/.../2243.understanding-measuring-and-reducing-output-voltage-ripple
Actually I dont think it is related with ripple, because from your fig1, we can get the mean value is 3.31, and even the min value is 3.04>3V>2.67V. so I suspect you measure the Vout after a long line which maybe cause some line loss(Rline*Iout), so could you please check the Voltage at output capacitor directly without the long line? or test at the same point as probe's.

Regards,
Yuchang

0 Jagdish Gundu over 7 years ago in reply to Yuchang Zhang

Intellectual 390 points

Thanks Yuchang, Pls see snap. I measured along two different locations which is marked in the schemaitcs.

Regards,JG

0 Yuchang Zhang over 7 years ago in reply to Jagdish Gundu

TI__Mastermind 18510 points

JG, I am confused, it doesnt make sense, from your Lx waveform@ max loading you sent, we can't see abnormal, and you scope test also showed Vout 3.3V, why just use DMM get lower voltage. please check two more things:
1, check FB1 Voltage.
2, set sope horizontal scale to ms level, check the Voltage is?
3, test 5V power rail, compare the DMM value with oscilloscope value.

Yuchang

0 Jagdish Gundu over 7 years ago in reply to Yuchang Zhang

Intellectual 390 points

Yuchang, Something New Observation I have. From the Snap attached.. I could able to measure 3.3V@4.4A wr.t board digital ground.
However when i do the measurement wr.t RAW_GND at Tespoint .. I get 2.7V@4.4A.

So does it mean that... my power choke (CMM) is creating some ground bounce and I see a 0.6V shift between RAW_GND & GND wr.t 3.3V node.

JG

0 Yuchang Zhang over 7 years ago in reply to Jagdish Gundu

TI__Mastermind 18510 points

JG,

Yes, i think it is possible. big DCR of common choke maybe caused the big different ground level, and all DCDC circuits refer to GND not RAW_GND, so you must refer GND to test DC/DC output, of course if you want to check the R-GND, you can just remove it and re-check.

Regards,

Yuchang

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TPS65279RHHR - Driving current support Issue