LM3475: Output is not as expected

Manali Durgule

Part Number: LM3475
Other Parts Discussed in Thread: CSD25402Q3A,

Output of the ic is not as per expectations. According to calculations it should be 4.2V but I'm getting output around 5.3V.

Output voltage waveform nature in no load condition is as per expectations but during load condition it's entering into soft start.

Voltage at feedback pin is around 1.5 V whereas it should be between 0.8 V and 0.82 V.

Components used:

MOSFET used is CSD25402Q3A

Diode used is SD1206S040S2R0

Rfb1 = 47k ohm

Rfb2 = 11k ohm

input capacitor = 470 uF (electrolytic) + 47uF (ceramic)

Output capacitor = 220 uF (aluminium electrolytic) + 47 uF (ceramic)

I'm attaching snaps of output waveform below.

over 4 years ago

0 JohnTucker over 4 years ago

TI__Guru**** 191445 points

Manali,

Most likely the issue is the 47 uF ceramic output capacitor. LM3475 is a hysteretic converter. It relies on significant ripple content on the output. Try removing that capacitor.

0 Manali Durgule over 4 years ago in reply to JohnTucker

Prodigy 95 points

Thank you sir. That answer resolved the issue. Can you explain how?

But there are some more issues.

In no load condition output is around 5V whereas it is expected to be 4.2V.

And the switching frequency of the gate observed is more than 2 MHz whereas the maximum limit of the ic is 2 MHz.

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LM3475: Output is not as expected