LM27313: Output voltage not stable issue

Kevin Huang8

Part Number: LM27313

Hi Team,

When the input voltage of LM27313 is lower than 4V, the output voltage of LM27313 can be very stable, but when the input voltage is higher than 4V, you will see the voltage bounce, The input voltage of LM27313 should be able to exceed 10V without any problem, but when I input 4V, the voltage bounces, which is quite different from the specification of LM27313. Although the PCB layout cannot be perfect, this material is very simple Boost Convertor, whether the wiring is ideal should affect the stability of the output voltage or the impact of EMI and should not be affected by changes in the input voltage, the surrounding circuit is as follows, can I get your suggestion?

11 months ago

0 Lei Zhong 11 months ago

TI__Expert 8080 points

Hi Kevin,

What's the Vin source your used? Is it possible that the maximum output power of the Vin source is not big enough?

For further check, please provide the Vin, Vout, SW waveforms when changing Vin from <4V to >4V. It will be better if inductor current can be provided also.

Thank you.

Regards

Lei

0 Kevin Huang8 11 months ago in reply to Lei Zhong

Expert 2955 points

Hi Lei,

When the voltage input is less than 4V,
the output voltage can be stable,
Otherwise, the voltage will unstable.

0 Lei Zhong 10 months ago in reply to Kevin Huang8

TI__Expert 8080 points

Hi Kevin,

Got your point already.

Normally the Vout should be stable when you adjust the Vin from below 4V to above 4V. For further check, please provide the Vin, Vout, SW waveforms and inductor current under two conditions of Vin<4V and Vout>4V.

Thank you.

Regards

Lei

0 Kevin Huang8 10 months ago in reply to Lei Zhong

Expert 2955 points

Hi Lei,

FYI.

LM27313 issue 0523.pptx

0 Lei Zhong 10 months ago in reply to Kevin Huang8

TI__Expert 8080 points

Hi Kevin,

Thanks for sharing the slides.

I saw the feedforward cap C9 is marked as "NC", so it may not be installed. Please use 68pF C9 and then measure again whether Vout is stable when Vin>4V.

Regards

Lei

0 James Chang 10 months ago

Prodigy 30 points

Hi Lei,

Please check the template and the pictures attached.

Test	Test description	Test result	Note
1	C9 移除	3.0V < V_in < 6.1V V_out 都會不穩定	C9_remove_test_01 0.png
2	C9 使用 68pF	當 V_in < 6.1V V_out 會不穩定	C9_68pF_test_01_text 0.png
3	C9 使用 330pF	當 V_in > 3.8V V_out 會不穩定	C9_330pF_test_01 0.png

https://drive.google.com/drive/folders/1dMarLXjuxp4jKTQZDl3-yWIXq-XRJOnb?usp=sharing

Thanks

James

0 Lei Zhong 10 months ago in reply to James Chang

TI__Expert 8080 points

Hi James,

We can't access to google drive through company network, could you attach the test results directly as the slides Kevin attached on May 23 in this thread?

Just for confirm, is the Iout 3mA? Please also confirm the Vin range.

One more question: when C9 = 68pF, is the Vout can't be stable for all Vin < 6.1V?

Thank you.

Regards

Lei

0 James Chang 10 months ago in reply to Lei Zhong

Prodigy 30 points

Hi Lei,

Please check the pictures as below.

0 Lei Zhong 10 months ago in reply to James Chang

TI__Expert 8080 points

Hi James,

Thank you for the pictures. I'll check and get feedback to you today.

One more check, is the Iout = 3mA in these test?

Regards

Lei

0 James Chang 10 months ago in reply to Lei Zhong

Prodigy 30 points

Yes, the Iout = 3mA in these test.

0 Lei Zhong 10 months ago in reply to James Chang

TI__Expert 8080 points

Hi James,

We can observe similar unstable waveforms without C9, when the SHDN pin is not pulled high stably.

After connecting SHDN pin to high, the device can work normally when C9 = 0(removed), 68pF or 200pF.

So please check whether the resistor between VCC and SHDN pin is properly mounted and make sure SHDN stays at high, the device should work normally at your side as well. BTW, the recommended C9 is 68pF.

Regards

Lei

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LM27313: Output voltage not stable issue