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# TPS61046: two questions, one is related to the input peak current,the other is related to overvoltage possibility of the output voltage

Part Number: TPS61046

For one question,

We use 10UH inductor in TPS61046 circuit, and we have encountered that input peak current will go up to 680mA when the loading is coming.

After we changed inductor to 1UH,the input peak current would go down to 330mA. I checked the datasheet,it gave the formula on page 12.

IL(P)=IL(DC)+ΔIL(P-P)/2

ΔIL(P-P)=Vin*(Vout+0.8V-Vin)/L*fSW*(Vout+0.8V)

From the formula,if L is bigger,ΔIL(P-P) will be smaller，and the inductor peak current IL(P) will be smaller, but we see the opposite result.

Can you help to explan the principle and give us suggestion?

the other question,

Whether the output voltage is fully controlled by feedback,I mean it will not give a bigger voltage than we set in any time,whether it is possible to output a 28V voltage although we just set the output voltage to 9V?

• Hi Sir:
May you share the waveform about the Icoil and Vin/Vout?
And did the load increase smoothly?

Best Regards

Minqiu

• In reply to Minqiu Xie:

Hi Minqiu,

Thank you for your quickly response,I send you the waveform about the coil's voltage and current for double checking.

About the load,it will raise quickly when it power up,the load is OLED panel which's max sink current is 20mA more or less.

TPS61046 issue.doc

Dear sir：
The peak current will be limited by the converter if it catches.(Ipeak Min is 600mA)

So IL(P)=IL(DC)+ΔIL(P-P)/2 ==> IL(DC)=IL(P)-ΔIL(P-P)/2; Larger ΔIL(P-P), Smaller IL(DC) .

For the second question, if the FB divided resistance is designed well, then it wont' exceed the target too much. If the FB voltage rise to 1.2V for some reason, the Vout target will be fixed at 12V.

Best Regards

Minqiu

• In reply to Minqiu Xie:

Hi Minqiu,
1, Do you mean the IL(DC) is equivalent to surge current?I just think the surge current use IL(P) as reference,because datasheet define IL(P) as inductor peak current and IL(DC) as inductor dc current.
2, so if the feedback is normal,the output will still is normal.we want to know which possibility the FB voltage will rise to higher voltage than the setting we have done, customer have concern about this,it affect safety.
• In reply to Minqiu Xie:

Dear sir:
Please let me know if you have other questions.

Best Regards

Minqiu

• In reply to Minqiu Xie:

Hi Minqiu,

I have no other questions besides the first question which I have given comment yesterday,because the IL(DC) have formula to calculate(on datasheet page 12 ):IL(DC)=VOUT*IOUT / VIN*η,it is not related to inductance.

Thank you very much for paying much time to support.

HI Sir:

But the ΔIL(P-P) is related to the inductance.

TPS61046 is a fixed switch frequency part with 1Mhz. And for the constant Vin, Vout, the duty cycle could be calculated. You could check the Equation 2. in datasheet.

The equation you mentioned :IL(DC)=VOUT*IOUT / VIN*η,is also right for almost case. But if it meet the limit of the converter, it couldn't supply with the target Iout and Vout.

Best Regards

Minqiu

• In reply to Minqiu Xie:

And the minimal value for peak current of the inductance listed in datasheet is 600mA.

Best Regards

Minqiu