LMR62421 SW waveform

tateo yamashiro

Guru 10890 points

Other Parts Discussed in Thread: LMR62421

Hi,

We got a question from customer about LMR62421.
Could you help us?

[Question]
- My customer measured SW waveform on EVM. Please see figure below.

   Vin=4.2V
   Vout=5V
   Iout=500mA

When internal FET is turned on, ringing on SW is big. How can we reduce this ringing?
Why does the ringing become bigger?

Best Regards,
tateo

over 8 years ago

0 Frank De Stasi over 8 years ago

TI__Guru 72075 points

Some of this ringing may be coming from your scope probe measurement. Please see app note SNVA494 for details about probing.

If you still see too much ringing you may need a snubber.

0 tateo yamashiro over 8 years ago in reply to Frank De Stasi

Guru 10890 points

Thank you for your support.
If they need sunubber, which is better, added to diode or added to IC(SW - GND)?

Best Regards,
tateo

0 Frank De Stasi over 8 years ago in reply to tateo yamashiro

TI__Guru 72075 points

I would try from SW to GND.

0 tateo yamashiro over 8 years ago in reply to Frank De Stasi

Guru 10890 points

Hi, my customer tried to add snubber(2,200pF + 3.3ohm) to diode. And they could reduce ringing on SW.
I appreciate for your kind support!

So they have addtional questions about snubber.
Could you help us.

- How is the estimation to loss of snubber resistor?
- Is there recommended snubber capacitor range?
They considers to use 2,200pF.

Best Regards,
tateo

0 Arief Hernadi over 8 years ago in reply to tateo yamashiro

TI__Mastermind 23845 points

Hi Tateo,

Here is a guideline on how to select your snubber and your power dissipation on your resistor

6332.RC Snubber.doc

Thanks

-Arief

0 tateo yamashiro over 8 years ago in reply to Arief Hernadi

Guru 10890 points

Thank you for your reply.
We can estimate loss of snubber(across the diode) resistor by below equation.

Ploss = Fsw * Csnb * Vout^2

Is it right?

Best Regards,
tateo

0 Arief Hernadi over 8 years ago in reply to tateo yamashiro

TI__Mastermind 23845 points

Hi,

Close, it will be VOUT - Vdiode instead of just VOUT

Thanks

-Arief

0 Arief Hernadi over 8 years ago in reply to Arief Hernadi

TI__Mastermind 23845 points

Correction, it should be VOUT + Vdiode

Thx
-Arief

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LMR62421 SW waveform