LM5145: Vin Capacitor Capacity

CHO SR

Genius 3415 points

Part Number: LM5145
Other Parts Discussed in Thread: CSD19534Q5A, LM5146-Q1

Hi,

The customer wants to change the vin capacitor due to supply and demand issue.

Please give us your opinion on the choice of Spec.

2744.201013_Main_Power_LM5145 Schematic.pdf

Vin Capacitor Available Items

1. 1000 pF C0G 100 Vdc

2. 1500 pF C0G 100 Vdc

3. 22nF X7R 100Vdc

over 4 years ago

0 Timothy Hegarty over 4 years ago

TI__Guru 74002 points

Hi Cho,

Here are some comments on the schematic:

No need for 2 x 470uF electrolytic caps on the input (one is sufficient). However, add more ceramic capacitance to provide the switching current to the power stage. 2.2uF or 4.7uF/100V/1210 is suitable with one 22nF installed for high frequencies. See app note snva803 for power stage capacitor layout and placement.
Depending on the output current, change the high-side FET to the 5 x 6mm equivalent, CSD19534Q5A, for better thermal performance
Tie the NC pins of the controller to GND at the DAP.
No need for a 20k pulldown on SYNCIN (an internal 20k pulldown is provided).

Please fill out the Excel quickstart file for this design (the file is available from the product folder).

Regards,

Tim

0 CHO SR over 4 years ago in reply to Timothy Hegarty

Genius 3415 points

Hi Tim,

Please help me output 12V.

The customer made a board and tested it.

However, the output voltage operates at 4.7V.

Spec : Vin : 43 ~ 60V, Vout : 12V, Iout max : 10A

We tested it with the settings of the webbench simulation, but the output voltage is 4.7V the same.

8688.201201_WBDesign_NEWCOCO LM5145.pdf

Schematic & Calculator Attachment

8562.201201_Main_Power_LM5145 Schematic.pdf

3652.LM5145 Wide Vin Synchronous Buck Controller Quickstart Calculator r12-7.zip

0 Timothy Hegarty over 4 years ago in reply to CHO SR

TI__Guru 74002 points

Hi Cho,

Increase the soft start (SS) cap to at least 100nF. 18nF is too low, especially given the large 1000uF electrolytic output cap. In fact, for a 12V output you only need 20-100uF -- see the LM5146-Q1 12V/8A/400kHz design as an example.

If the Vout rise time is too fast with a low SS cap value, current limit is likely engaged.

If the problem persists, please send some waveforms (Vout, Iout, inductor current, LO, SW, etc.) during startup and steady state.

Regards,

Tim

0 CHO SR over 4 years ago in reply to Timothy Hegarty

Genius 3415 points

Hi Tim,

Changing to a soft start cap 100nF does not work.

1000uF removal test result is the same.

I have attached some additional waveforms.

201202_LM5145 Waveform.pptx

0 Timothy Hegarty over 4 years ago in reply to CHO SR

TI__Guru 74002 points

Hi Cho,

The SW and HO waveforms should be rectangular - please check your design as well as the scope probing technique. Also, if such a large output cap is removed, you will need new compensation values (refer to the quickstart file).

Regards,

Tim

0 CHO SR over 4 years ago in reply to Timothy Hegarty

Genius 3415 points

Hi Tim,

Thank you for your support.

We found the cause of the failure.

The cause is the customer's PCB Artwork problem.

Pin 17(BST) and pin 19(SW) were connected in the opposite direction in the artwork.

After modification, the output voltage is normal with normal switching.

Q1. CLIM with 100V capacity is recommended for calculator.

The customer wants to use CILIM 47pF/50V, is there any problem with applying it?

0 Timothy Hegarty over 4 years ago in reply to CHO SR

TI__Guru 74002 points

Hi Cho,

If Vin is 60V, then you'll need a 100V cap on ILIM.

Regards,

Tim

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LM5145: Vin Capacitor Capacity