LM3481SEPICEVAL: Imax - SEPIC current limit behavior

Sergey Matkovsky

Part Number: LM3481SEPICEVAL
Other Parts Discussed in Thread: LM3481-Q1, LMP8646

Hello everyone,

We are using the LM3481-Q1 in SEPIC topology in our designs.

Vin=7-24V
Vout = 12V
Iout=0.75A - set by Rsen to 0.063 OHM
f=1Mhz

I want to ask about the Iout behavior we encounter. When I choose Rsen that defines Iout to be 750mA, then why when Vin=12V the output current is indeed 750mA, but when Vin=7V, Iout can't be set to 750mA, going above 200mA will lead to Vout drop. Moreover, when Vin=24V, I can draw 1.2A (when Rsen was set to supply 750mA). So why Iout varies with Vin, and why it allows me to exceed 750mA which is the limit set by Rsen.

The current is drawn by adjustable DC load.

Thanks.

over 5 years ago

0 EM over 5 years ago

TI__Genius 11215 points

Hello Sergey,

Thanks for your question. Let me clarify if I understood your request: you want to limit the output current of your SEPIC design to 750mA via the Rsen resistor, but when Vin is 7V or 24V you are not able to set the load current that you wanted, correct?

Other than this, I see that the voltage on the UVLO pin with 7V Vin is below the 1.43 V (typical) threshold, and therefore the converter will be disabled (see page 17 of the datasheet https://www.ti.com/lit/ds/symlink/lm3481-q1.pdf?ts=1615831343915&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM3481-Q1 ). This might be one of the problems. I am looking forward to more details.

Kind regards,

Elisabetta

0 Sergey Matkovsky over 5 years ago

Prodigy 30 points

That is correct, I'm not able to set the output current that I want in the whole range of Vin.

Regarding the UVLO, yes I know that when Vin is ~7V the SEPIC shuts down, so for this discussion, I will rephrase, I'm not able to set the current that I want when 7.5V<Vin<24V. I only get the desired output current when Vin = 12V, by setting Rsen to 0.063ohm

+1 EM over 5 years ago in reply to Sergey Matkovsky

TI__Genius 11215 points

Hello Sergey,

Thanks for the clarification. Your resistor Rsense is placed on the primary side of the transformer, therefore it is sensing the input current of the SEPIC. For example, if you have 24Vin and 0.75A input current you also have Pin=24*0.75=18W, with 12Vout and supposing an efficiency somewhere around 80% you will have Iout = Pout/Vout = 14.4/12 = 1.2 A . This is why you are seeing these values for the load current. If you want to limit your output current, you might think of using a current limiter like the LMP8646 www.ti.com/.../LMP8646

Let me know if I can help you furthermore.

Kind regards,

Elisabetta

0 Sergey Matkovsky over 5 years ago

Prodigy 30 points

Thank you, makes sense.

0 EM over 5 years ago in reply to Sergey Matkovsky

TI__Genius 11215 points

You're welcome.

Have a nice day,

Elisabetta

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LM3481SEPICEVAL: Imax - SEPIC current limit behavior