TPS61081 5V output problem

Roman Yudin

Prodigy 30 points

Other Parts Discussed in Thread: TPS61081

Hello. I built DC/DC converter based on TPS61081.

The input voltage is about 3.5V. Output divider is calculated for 5V output.

In fact I have unregulated output voltage about 9V & it depends on input voltage.

Similar converter calculated for 12V output works fine.

Device datasheet says that output voltage range starts from VIN.

What the problem?

over 10 years ago

0 Jasper Li over 10 years ago

TI__Mastermind 46905 points

could you share the output waveform. and please select the feedforward cap based on page 13 in datasheet. thanks

jasper

0 Roman Yudin over 10 years ago in reply to Jasper Li

Prodigy 30 points

The output waveform - is constatnt voltage (about 9V) with switching ripple (50mV).

I tried adjust output capacitor, feedforward capacitor and also SS capacitor. It gave no result.

Then I added load resistor 4K7 to the output and converter began to work properly!

Some experiments & I found out, that correct operating is possible only with load about 30KOhm and less.

12V converter with same schematics works properly with no load.

0 Jasper Li over 10 years ago in reply to Roman Yudin

TI__Mastermind 46905 points

Yes , you are right. the converter based on fix PWM control, and has a minimum duty circle about 5%. so it always transfer energy to the output. in 5V output this transfere energy is higher than that at 12V output condition, as the inductor current decrease slower at 5V output.

a small load can solve this problem. thanks

jasper li

0 Roman Yudin over 10 years ago in reply to Jasper Li

Prodigy 30 points

Thanks, Jasper.

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TPS61081 5V output problem