TPS61241: Current limit

Julian Chen

Expert 2550 points

Part Number: TPS61241
Other Parts Discussed in Thread: TPS61240,

Hi E2E,

I use IOUT(CL) formula which in data sheet to calculate the IOUT(CL) as below.

VIN=4.7V, VOUT=5V

Finally, the IOUT(CL)=0.626A

BUT the figure 1 shows the Iout can't over 0.5A at Vin=4.7V

Could you let us know the detail why Iout can't over 0.5A at Vin=4.7V ?

over 4 years ago

0 Zack Liu over 4 years ago

TI__Mastermind 34201 points

Hi Julian,

Efficiency should be took into account when calculating duty cycle. I'd prefer using D=1-Vin*eff/Vout. Assuming efficiency is 0.87. So duty cycle is 0.21. Then inductor current ripple is 4.7*0.21/(1.5*4.2)=0.156A. Maximum output current is then (1-0.21)*(0.6+0.5*0.156)=0.535A.

0 Julian Chen over 4 years ago in reply to Zack Liu

Expert 2550 points

Hi Zack,

Thanks to your reply.

So what the figure 1 mean?

Should I ignore it?

0 Zack Liu over 4 years ago in reply to Julian Chen

TI__Mastermind 34201 points

Hi Julian,

Figure 1 is the bench test result, which is trustworthy.

0 Julian Chen over 4 years ago in reply to Zack Liu

Expert 2550 points

Hi Zack

Our customer IOUT peak reach high to 0.54A

So, Should I ask for them change the Choke to 1uH or change to another part?

0 Zack Liu over 4 years ago in reply to Julian Chen

TI__Mastermind 34201 points

Hi Julian,

I would suggest you choose another device using WEBENCH.

0 Julian Chen over 4 years ago in reply to Zack Liu

Expert 2550 points

Hi Zack,

The figure.1 is base on 1uH, right?

Could you Share the figure1’s table let our customer to refer it to assess his project

If we use 1.5uH or 0.47uH, Does the maximum output current will be change?

0 Zack Liu over 4 years ago in reply to Julian Chen

TI__Mastermind 34201 points

Hi Julian,

Yes. It is based on 1uH.

1.5uH or 0.47uH do little changes on maximum output current because inductor current ripple is small comparing to 0.6A.

0 Julian Chen over 4 years ago in reply to Zack Liu

Expert 2550 points

Hi Zack,

I have a question, using D=1-Vin*eff/Vout. Assuming efficiency is 0.87. So duty cycle is 0.21.

Vin=4.7V, VOUT=5V, Eff=0.87, the answer is 0.1822 not 0.21.

Is the formula wrong or the calculate miss?

0 Zack Liu over 4 years ago in reply to Julian Chen

TI__Mastermind 34201 points

Hi Julian,

You are right.

0 Julian Chen over 4 years ago in reply to Zack Liu

Expert 2550 points

Hi Zack,

Thanks for you always reply fast.

My customer want to reduce the value of choke for increase the IOUT maximum current.

So ask for me build a table which follow your formula. BUT the result is difference with figure1 of datasheet.

However, I Think when Vin=4.7V, use 0.47uH will reach to OTP first.

So, Could you share or mail to me what TI suggest IOUT when L=0.47uH / 1uH/ 1.5uH @ 3.4V/ 3.7V/4.2V/4.7V

They want to know the maximum IOUT to assess the project.

0 Zack Liu over 4 years ago in reply to Julian Chen

TI__Mastermind 34201 points

Hi Julian,

It's not very easy to correlate the calculated maximum Iout with bench test result. Because when calculating the maximum Iout using the formula in datasheet, you should take care of several things:

1. Inductance. The actual inductance has variation, normally +-20%.

2. Efficiency.

3. Switching frequency. The switching frequency is not a constant value. Please refer to figure 7 in datasheet.

And I forgot to remind you that figure 1 curve is based on 1uH inductor and it's the for TPS61240, not TPS61241. TPS61240 valley current limit is 500mA.

You excel tool seems has a mistake in the column of current ripple when L=1uH & 0.47uH. Please take a look it more carefully.

Below table is calculation result for TPS61240:

Here the valley current limit is 0.5A. Fsw is 4.5MHz. It matches well with Figure 1.

To get the estimation Iout for TPS61241, change the valley current limit value to 0.6A. Below is the calculation result:

0 Julian Chen over 4 years ago in reply to Zack Liu

Expert 2550 points

Hi Zack,

Thanks for you great support!!

Hope we can meet in the future, I will thaks you personally.

Last Question,

So Theoretically, TPS61241 can operate under the IOUT(CL), right?

0 Zack Liu over 4 years ago in reply to Julian Chen

TI__Mastermind 34201 points

Hi Julian,

Yes. But I'd recommend customer not using it at Iout(CL). It's too marginal.

In load transient conditions, from 0A or half load to full load current, device will triggers switch current limit and a big output voltage drop may exists.

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TPS61241: Current limit