TPS92515-Q1: design review +output current not match calculation

Dongbao Zhou

Expert 4170 points

Part Number: TPS92515-Q1

Tool/software:

hi team,

May you help review the sch design and check what are the possible reason for output current not meet setting?

Calculate Iout=0.77A but actual test results is 0.5A, if we change the Rsense value, the Iout will change as well.

6 months ago

0 Daniel Louis 6 months ago

TI__Expert 5690 points

Hi Dongbao,

1. What values did your customer use for R20 and R21?
2. Where are you measuring Iout?

Please include the following wave forms:

Inductor current
Switch node voltage
Input voltage
Output voltage

3. I will modify one of our EVMs and try and replicate the issue.

Best,
Daniel

0 Nature Li 6 months ago

Prodigy 30 points

1、R20 0.25ohm R21 NC

2、behind of C37

3、Vidj Actual measurement 5.1V

4、input voltage 13.5V

5、output voltage 4.5V

0 Dongbao Zhou 6 months ago in reply to Nature Li

TI__Expert 4170 points

0 Daniel Louis 6 months ago in reply to Dongbao Zhou

TI__Expert 5690 points

Hi Dongbao,

Thank you for providing all the requested information.

The Viadj value on your excel calculation sheet is different from the one you measure and thus your Rsense value needs to be changed to...

Rsense: 0.5407Ω

Here is and image of the modified calculation sheet:

Please let me know if I can further assist you.

Best,
Daniel

0 Nature Li 6 months ago

Prodigy 30 points

Hi Daniel

The actual test proves that the current can only be increased by reducing the rsense value。

0 Daniel Louis 6 months ago in reply to Nature Li

TI__Expert 5690 points

Hi Nature,

I made a mistake regarding Viadj.

Viadj is clamped at 2.4V.

The issue lies with your inductor size as you used a 10uH inductor instead of the recommended 15uH inductor value. The sense resistor sets the PEAK current- the current sense topology is configured as (Viadj/10)/ Rsense. Therefore your peak current set point is accurate. The average current is set by your peak current, Roff, Coff, and your inductor value. The inductor value sets the positive slope of the inductor current and ROFF and COFF set the negative slope of the inductor current. By reducing the inductor value you are directly reducing your ripple.

Best,
Daniel

0 Nature Li 6 months ago in reply to Daniel Louis

Prodigy 30 points

Hello Daniel

Based on current design [10uH inductor],Is there any risk that I increase the current directly by reducing the Rsense value? I find that R20 0.25ohm R21 1.2ohm can meeting my request.

0 Daniel Louis 6 months ago in reply to Nature Li

TI__Expert 5690 points

It could cause stability issues or flicker, but you are welcome to try it

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TPS92515-Q1: design review +output current not match calculation