TPS923655DMTREVM: Need a TPS923655DMTREVM

Hi

Due to Chinese New Year, our expert will reply you on 2/5

Hi Richard,

I am currently out of office for Chinese New Year Holiday. I will reply to you on Feb.8 when I am back to office. Thanks for your understanding.

Best Regards,

Steven

Hi Richard,

I am back to office earlier than expected :)

Sorry. The TPS923655DMTREVM is currently undergoing a revision, so there is no plan to restock the current version of EVM on ti.com. Sorry for the inconvenience it brings.

The new version of TPS923655DMTREVM will be available on ti.com in around 1.5 months.

Let's talk through private message on this E2E forum to see if there is anything that I can help. Thanks.

Best Regards,

Steven

Hi Steven,

We made a board using the LP8865X.  I have it running, but it is exhibiting a strange behavior.  The full brightness decreases with time.

We run it in Analog Dimming mode (Table 7-2). 

When it is initially operated it gets to the maximum brightness I expect.  Measuring the voltage across the current sense resistors I see the 200mV I expect.  As it runs this voltage, and the brightness, steadily decreases.  It stabilizes out at 152mV.

If I dim it down to a low PWM for a couple minutes, 10% for example, then run the PWM back to 100% the sense voltage returns to the full 200mV,  It then decreases again.

If I dim it down to a low PWM for a shorter time then run the PWM back 10 100% the sense voltage goes to a value less than 200mV and then decreases again.

It appears to be thermal.  I freeze the chip and it returns to full brightness.  We are running the chip in boost with 12V in and approximately 46V out, delivering about 500mA to the LEDs.  It's tripping the thermal foldback. 

I can increase the threshold but the data sheet is unclear on what is allowable.  I currently have it set for 100C. The data sheet table 6.3 says that the Recommended maximum Tj is 125C, yet the threshold can be set much higher than this and the thermal foldback runs 25C above this number.

Hi Richard,

Are you going to use LP8865X-Q1 or TPS923655? If this thread is related to LP8865X-Q1, then I will assign this thread to the corresponding engineer to help you.

Best Regards,

Steven

We are using the LP8865X-Q1.  The TPS923655DMTREVM is what's recommended on the LP8865 web page.  I was going to replace the chip in the EVM with a LP8865X.

Hi Richard,

Understood. I will assign the engineer who covers LP8865X-Q1 to this thread. He will reply to you later. Thanks for your understanding.

Best Regards,

Steven

Hi, Richard

As your output is 46V/500mA, it may not enough for the switching current limit of LP8865. Pls consider LP8868, a high current version.

Hi Zhiqian

How will the LP8868 change anything?  The MOSFET resistance is 150mOhms, same as the LP8865.  Therefore the dissipation will likely be the same, so it won't improve the thermal issue.

You didn't answer the question in my last post "I can increase the threshold but the data sheet is unclear on what is allowable.  I currently have it set for 100C. The data sheet table 6.3 says that the Recommended maximum Tj is 125C, yet the threshold can be set much higher than this and the thermal foldback is operational through 25C above this number."

Based on the 100C thermal threshold I have set and the fact that the current decreases approximately 20% when it heats up I estimate that Tj is reaching 110C.  My lab bench is at 25C.  Since I need the product to properly function up to 50C I can expect Tj to reach 135C with the current board design.  That's still under the 150C in table 7-5

I will be revising the board layout to reduce thermal resistance.  I don't know how much of an improvement I can make.  However, I would like to understand the junction temperature recommendations and their apparent conflict with the entries in Table 7-5.

Also, is the power dissipated by the LP886x dependent on the inductor value?  There's no mention of it in the data sheet equations but it seems like reducing the peak inductor current would reduce the power dissipated by the switch.

Rich

Hi Richard,

Our expert is currently out of office and will reply to you later. Thanks for your understanding.

Best Regards,

Steven

Hi, Richard

Let me clarify what I am thinking:

• There are two part limit the power output, one is the thermal foldback, another one is the current limit.
• For the current limit, increase the Tj will increase the Rdson and limit the output power
• For the thermal foldback, Pls see the thermal foldback introduction in below. Pls set with the target value as you want to have a test again,

For the operation temperature, our device can work as 150 for Tj.

Is the power dissipated by the LP886x dependent on the inductor value?  There's no mention of it in the data sheet equations but it seems like reducing the peak inductor current would reduce the power dissipated by the switch.

Hi Richard,

Our expert is currently out of office and will reply to you later. Thanks for your understanding.

Best Regards,

Steven

Hi, Richard

Sure, it is the basic performance for the switching device. For the same current rms, a high peak current means high dissipation.

Please send me a copy of the design calculator mention in the thread
e2e.ti.com/.../lp8865y-q1-design-calculator

Don't bother if it is Power Stage Designer at https://www.ti.com/tool/POWERSTAGE-DESIGNER
I already have that.

Hi, Richard

Pls left your email here, and I will send it to you.

I'll DM it to you

Hi Richard,

Our expert will reply to you later. But I suggest you send your email address in private conversation.

BR, Gracie