LM2587: LM2587-Adj circuit is working with no load. It is not working with load at startup.

Zhongwen Jin

Part Number: LM2587

I built a circuit based on "Increasing Available Load Power in an LM2587 Boost Regulator" app note (its link is http://www.ti.com/lit/an/snva552/snva552.pdf). The difference is my input is around 24VDC, the output I want is 28VDC.

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The circuit is working fine when there is no load at all! The output is 28VDC, and when switching the circuit ON/OFF, the circuit is working fine.

But when putting a load drawing current 1A, the input voltage drops to 3.5V insteading of 24V DC.

If I switched ON the circuit without load, then connected the 1A load afterwards, the circuit is working fine and the output is drawing 1A.

But if I switch OFF the circuit and then switched it ON (with the load connected), then the circuit failed to regulate. Any thoughts please?

Thanks in advance.

over 6 years ago

0 Youhao Xi over 6 years ago

TI__Guru 68655 points

Sounds like your input source power supply hits the over current and clasps the source voltage. There is inrush current to charge up the 1000uF output capacitor. Applying the load earlier makes the total current from the source supply higher. Can you try to put 0.1A, or 0.5A and see if there is any improvement at startup?

0 Zhongwen Jin over 6 years ago in reply to Youhao Xi

Intellectual 385 points

I have tried 0.4A, still no luck yet.

0 Zhongwen Jin over 6 years ago in reply to Youhao Xi

Intellectual 385 points

Can I use some Inrush current limiter to fix this problem? where shall I put?

0 Youhao Xi over 6 years ago in reply to Zhongwen Jin

TI__Guru 68655 points

You may add an external soft start circuit to slow down the startup and hence the inrush current. The concept is presented in a TI app note at the following link:

www.ti.com/.../slva307

0 Zhongwen Jin over 6 years ago in reply to Youhao Xi

Intellectual 385 points

lm2587 has got a soft-start already? My issue is the output current is much less than expected. The drawing is saying 36V 2A. But mine is probably 500mA.

0 Youhao Xi over 6 years ago in reply to Zhongwen Jin

TI__Guru 68655 points

That soft-start may not be "soft" enough. Let me put this in another way: after starting up with light load, say 100mA, and see the output voltage is established, then, you gradually increase your load up to 2A, and do you see if the output voltage stays at the regulation level?

0 Zhongwen Jin over 6 years ago in reply to Youhao Xi

Intellectual 385 points

I can see the application note for extending soft start time for TPS6107x. But for LM2587, it does not have Vout pin. How to add this soft-start circuit please?

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LM2587: LM2587-Adj circuit is working with no load. It is not working with load at startup.