LM3478: Design not working and input supply goes into CC mode

Deepak Bansal1

Part Number: LM3478

I am using Lm3478:

1. Input: 11V to 14V

2. Output :12.6V, current to be limited at 2.5Amps by Lm3478 as its used for battery charging

PFA the ckt. Issue is as soon as I connect the power supply at 12V, board makes power supply to CC mode, current goes more than 0.30Amps , nothin is connected at the output, just a output voltmeter.

What could be issue, I trippled checked all component values again and again, nothin found at the moment,

V1-sch.pdf

over 4 years ago

0 Yinsong Cai1 over 4 years ago

TI__Expert 7450 points

Hi Deepak,

Thank you for choosing LM3478.

Do you mean you set the power supply current limit to 0.3A? Have you tried a little higher limit? The startup could have some current overshoot.

Thanks,

Yinsong

0 Deepak Bansal1 over 4 years ago in reply to Yinsong Cai1

Intellectual 300 points

Hi Yinsong,

1. I opened current limit completely have 2.5Amps supply. Now its random, sometimes power supply again goes to CC mode & mosfet start getting too hot, so I turn off, current limits to 2.5Amps at that time.

2. Sometimes ckt do not go into CC mode & input voltage is maintained, at that time, output is like this, it keeps on risin and then falling as in pic below.

Each Y axis each block is 500mV. Probe is 10x, x-axis is 2seconds/block.

0 Deepak Bansal1 over 4 years ago in reply to Deepak Bansal1

Intellectual 300 points

1. Removed all the electrolytic caps from output. I had 22uF ceramix and 1000uF ceramic.

2. Now I have connected 5 caps of 22uf ceramic in parallel. Here is output Each Y axis each block is 500mV. Probe is 10x, x-axis is 2seconds/block.

3. Then tried connecting 5x22uF at ceramic output and 1000uF electrolytic in parallel. Power supply again went to CC mode with 2.5Amps, so removed it.

0 Deepak Bansal1 over 4 years ago in reply to Deepak Bansal1

Intellectual 300 points

Pin wise detail , gnd and Vin left since they are clean , checkd on CRO also.

Rest screenshots are in pdf attached.

Cout = 5x22uF , No load connected, Vin: 12V , Cin: 470uF(electrolytic) + 0.1uF(ceramic)

pin-wise.pdf

0 Yinsong Cai1 over 4 years ago in reply to Deepak Bansal1

TI__Expert 7450 points

Hi Deepak,

Thank you for the information. What is the input voltage for the conditions above? And they are all at no load, correct? Just want to opt out the current protection.

Thanks,

Yinsong

0 Deepak Bansal1 over 4 years ago in reply to Yinsong Cai1

Intellectual 300 points

Hi,

Input voltage is 12V around for all.

Yes no load is connected.

0 Deepak Bansal1 over 4 years ago in reply to Deepak Bansal1

Intellectual 300 points

tried connecting a 3S battery 11.1V at the output, even if output is fluctuating.

1. Vin: 12.0V, Iin: 4.38Amps

2. Iout in battery : 3.63Amps

But after 2-3seconds mosfet gets heat up so removed ASAP.

We want to limit current to 2.5Amps.

0 Yinsong Cai1 over 4 years ago in reply to Deepak Bansal1

TI__Expert 7450 points

Hi Deepak,

It looks like there is a high current although there is no load connected to absorb this power. I would like to check the Isense and SW node waveform to see

1 if the current limit is functional or not

2 if the MOSFET if working and what is the duty cycle

If possible, please show both zoom in (to see several switching period) and zoom out.

By the way, do you see this on one or more boards?

Thanks,

Yinsong

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LM3478: Design not working and input supply goes into CC mode