LM5145: LM5145

inno

Expert 1700 points

Part Number: LM5145
Other Parts Discussed in Thread: CSD19534Q5A

I have a problem with my DC to DC, based on LM5145 TI PWM controller.

Design input voltage is between 36V - 63V (nominal 48V).

Design output voltage is 24V, 4 Amp.

Design PWM frequency is 350 Khz.

When I measure the output voltage, I measure 19V instead of 24V (at no load at all, and at 0.5 ampere load) , the signal at pin 19 (SW), is not stable, it skip cycles, and also the duty cycle at that pin is not stable.

I can hear a small acoustic noise ( like Ashhhh noise, probably comes from the output inductor).

I did the design with the excel simulation tool of TI

Attached the Schematic design, and a scope waveform at pin 19 (SW)power supply calc.xlsm

DC TO DC.pdf

over 4 years ago

0 Timothy Hegarty over 4 years ago

TI__Guru* 76592 points

Hello,

This may an instability. Please check the compensation values with a correctly derated value for Cout (a 10uF/50V/1206 ceramic likley derates to 3-4uF when 24V is applied - check the capacitor datasheet).

Regards,

Tim

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

TI__Guru* 76592 points

Here are some additional points:

Tie the NC and EP pins to GND to allow larger copper area for heat spreading.

CSD19534Q5A also works for the high-side FET given the max load current is only 4A.

0 inno over 4 years ago

Expert 1700 points

Hi Timothy

What value of capacitors it should be after the voltage derating, i am using 10 capacitors of 10uF/50V/1206.

The capacitors are GRT31CR61H106KE01L from Murata

Thanks

0 Timothy Hegarty over 4 years ago in reply to inno

TI__Guru* 76592 points

See these links:

https://www.murata.com/en-us/products/productdetail?partno=GRT31CR61H106KE01%23

https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us

The cap goes to -83.5% of its nameplate value at 24V, so about 1.65uF.

Regards,

Tim

0 inno over 4 years ago in reply to Timothy Hegarty

Expert 1700 points

I understand the derating, but i am not understanding, from the simulation tool what equivalent capacitor i need after voltage derating.

suppose the derating is 83.5%, i still have 1.65uF X 10 = 16.5uF rated capacitor, is it not enough ?

from the simulation tool " Minimum ideal output capacitor is 4uF "

and after derating i have 16.5uF.

Thanks

0 Timothy Hegarty over 4 years ago in reply to inno

TI__Guru* 76592 points

Hi Inno,

The required output capacitance is set by the output ripple and load transient specs (see equations in the LM5145 datasheet).

However, note that the effective Cout value has a major impact on compensation values. Please use the correct derated Cout value in the quickstart to derive the compensation components and ensure stability.

Regards,

Tim

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LM5145: LM5145