LM3478: Output Current Limit Mechanism

Bhargav Jyoti Saikia

Part Number: LM3478
Other Parts Discussed in Thread: PMP9779

Hello,

I have used LM3478 as a boost converter which takes 5V as input and produces 12V @ 3.6A. I want the output current limit to be 4A, so I calculated Rsen as 10milliohm. The inductor value used is 2.2uH and switching frequency is 500Khz. Additional slope compensation resistor (Rsl) of 100 ohm was used along with 10nF capacitor (Csl). I have observed the following:

1) The output is regulated at 12V till 3.8A which is desirable.

2) As 4.5A output load is applied through 2.667 ohm resistor, the voltage drops to 11.6V instead of dropping to near 0 volts. The output current was 4.23 A.

3) As 5.5A output load is applied through 2.2 ohm resistor, the voltage drops to 10.6V instead of dropping to near 0 volts. The output current was 4.78A.

4) As 6A output load is applied through 2 ohm resistor, the voltage drops to 10.2V instead of dropping to near 0 volts. The output current was 5A.

So, is the above behaviour normal? I was expecting the output voltage will drop considerably to almost 0 volts as soon as current limit is exceeded. Is there any error in the design?

Thanks,

Bhargav Saikia.

over 6 years ago

0 Helen chen over 6 years ago

TI__Mastermind 40080 points

Hi Saikia,

This phenomenon is normal, this is cycle by cycle current limit after the peak switch current exceed the limit. You may add an extra OCP circuit if you want an immediate shutdown.

0 Bhargav Jyoti Saikia over 6 years ago in reply to Helen chen

Intellectual 420 points

Hi Helen,

So that is normal even if the voltage doesn't drops to zero after current limit is triggered. But the output current should be limited to near 4 amps if designed to do so, correct? But, when I loaded with 2 ohms, the output current obtained was 5 Amps instead of 4 amps; the output voltage dropped from 12V to 10V, instead of dropping to 8V (which would then send 4 amps through the 2 ohm load).

So, can I conclude that there might be some problem with my design? The output current should not exceed the maximum current limit value of 4 amps correct?

0 Helen chen over 6 years ago in reply to Bhargav Jyoti Saikia

TI__Mastermind 40080 points

Hi Bhargav,

For LM3478, actually, it just limit the peak switch current, not the output current. So when the peak switch current exceeds the limit, the output power (VoXIo) will not increase any more, but the output current may still increase.

0 Bhargav Jyoti Saikia over 6 years ago in reply to Helen chen

Intellectual 420 points

Hi Helen,

Thanks for the clarification. Yes, I can observe that the output power was limited to around 48 to 50 watts. Can you suggest me any alternative part number (boost converter with input 5V and output 12V @ 3.6A ) that limits the output current to 4A and doesn't allow more output current than it?

Thanks,

Bhargav.

0 Helen chen over 6 years ago in reply to Bhargav Jyoti Saikia

TI__Mastermind 40080 points

Hi Bhargav.

Usually extra over current protection circuit need to be added. like what in the reference design PMP9779 do.

0 Bhargav Jyoti Saikia over 6 years ago in reply to Helen chen

Intellectual 420 points

Hii Helen,

Thanks for your support.

Regards,

Bhargav.

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LM3478: Output Current Limit Mechanism