LM5010: What may be the problem with my design?

Hi Yuuki,

First pls remove L3 for circuit debugging

Download the calculator as below to verify the circuit design.

https://www.ti.com/tool/LM5010QUICK-CALC

B R

Andy

Hi Andy, thanks for the reply.

I tried removing L3.

The result remains the same and only outputs up to 3.5V.

Switching seems to be stopped.

After the power is turned on, switching stops near 13V.

After that, it drops to 3.5V.

The SW waveform is attached.

I used a calculator. I try check again.

Thanks a lot.

-yuuki

Let me check and update you later....

Hi Andy,

I used a calculator and checked again.
As a result of confirmation, Cin that did not meet the conditions was changed from 4.8uF to 9.5uF.
However, the result does not change and 15V is not output.
Attached the calculator used for checked.

LM5010QuickStartCalculator_211206.xls

Thanks very much
-yuuki

Hi Andy,

The positions of Rseries on the Webench design and R3 on the calculator's Type2 Reduced Ripple Configuration are different.
Is there a difference? Which is correct?

Thanks very much
-yuuki

Hi Yuuki,

There is no difference for output capacitor ESR position.

What's output cap you used on board? the effective output capacitance should be >25uF, for 15V output voltage, you should use low ESR cap like 25V polymer cap or 50V ceramic cap, C19 voltage rating also 50V.

B R

Andy

Hi Andy, thanks for the reply.

The output cap uses:
C19 : GRM188B11H223KA01D (50V 0.022uF)
C38 : GRM188R61E106MA73D (25V 10uF)
C7,C36,C37 : GRM32EB31E226ME15L (25V 22uF)
C35 : GRM155R71H104KE14D (50V 0.1uF)

Thanks very much
-yuuki

Hi Andy,

> the effective output capacitance should be >25uF
Which is correct, Webench design or calculator?
Webench:10uF
calculator:>25uF

Any thoughts or ideas for troubleshooting?

Thanks very much
-yuuki

Hi Yuuki,

Pls try to use 2X GRM32EB31E226ME15L (25V 22uF) as output cap. upload the layout of LM5010 section if problem still existing.

B R

Andy

Hi Andy, thanks for the reply.

I changed output cap to 2 X GRM32EB31E226ME15L(25V 22uF).
However, the result does not change and 15V is not output.

Attached the layout of LM5010 section.

Thanks very much
-yuuki

Hi Andy,

I was able to output 15V.
The cause seems to be the output load.
Since the load was light at startup, a resistor(500Ω) was added between P15 and GND as an emergency measure to increase the load.

Can you tell me the recommended measures in such a case?

Hi Yuuki,

you can reduce the Rfbt to 500 ohm as dummy load. increase Cff to keep same Cff*Rfbt time constant.

B R
Andy   

Hi Andy, thanks for the reply.

Is Rfbt R4 or R5?

I was able to output 15V with 24V input, but I could not output 15V with 48V input.

I was able to output 15V by increasing the load, but what was the cause?

I want to know the cause because it outputs 15V even with 48V input.

Thanks very much

-yuuki

Hi Yuuki,

Can you test the startup waveform at Vin=24V and 48V?

1. Vin

2. SW

3. Inductor current

4. Vout

B R

Andy

Hi Andy, thanks for the reply.

I will send it when the waveform is acquired.

Please take a moment.

Thanks very much

-yuuki

Hi Andy,

I test startup waveform.

ch1:VIN ch2:SW ch3:VOUT ch4: Inductor current

①Vin=24V without 500Ω
②Vin=24V with 500Ω
③Vin=24V with 500Ω
（Power supply different from ②）
④Vin=48V with 500Ω

Differences between
①-②：With or without 500Ω resistor
②-③：Input power supply
③-④：Input voltage

In the case of the power supply used in (3) and (4), P15 output may or may not be possible.

Thanks very much

-yuuki

waveform_20211213.xlsx

Hi Andy,

Did you know anything from the waveform?

Please tell me about Cff
I added a 500Ω resistor to the circuit I posted at the beginning and tried it.
VIN: 24V Cff: 0.23uF → P15 output not possible
VIN: 24V Cff: 0.01uF → P15 output possible
It changed with the difference of Cff, what does this mean?
Where do I need to change to use Cff: 0.23uF?

Thanks very much

-yuuki

Hi Yuuki,

On the waveform, the converter starts to operating at Vin ramp up, the inductor peak current is only about 400mA, but the VIN can't ramp up or even drop during soft startup, so Pls check if the input power supply has too small current limit ? 

B R

Andy