LM5164: Strange switching behavior

Obinna Ezeilo

Part Number: LM5164

Hi Team,

Can someone help out with the following problem:

Currently i am looking to the LM5164 synchronous buck converter IC to make a 14V 600mA low voltage supply. Input voltage from 20 to 80V DC

At the moment we use the LTC8631 for this supply and would like to switch over to the LM5164.

However I have some problems with the LM5164 when I simulate in LtSpice

I have copied the model from TI website in LtSspice.

Output voltage regulation and startup is correct, however I see some strange switching behavior.

When the upper fet is turned on to make current in the buck coil, the fet is several times turned on off with delay of 50nsec (minimum off time)

See below screen shots

Is this normal behavior or did I something wrong ?

over 6 years ago

0 Orlando Murray over 6 years ago

TI__Mastermind 22302 points

Hi Obinna,

That undesirable behavior is expected when using ceramic output capacitors with no ESR.

You should use Type-2 or Type-3 ripple injection networks, described by the following appnotes:

Type-3: http://www.ti.com/lit/an/snva874/snva874.pdf

Type 1 or 2: http://www.ti.com/lit/an/snva776a/snva776a.pdf

Hope that helps,

Orlando

0 dirk romeyns over 6 years ago in reply to Orlando Murray

Prodigy 40 points

Hello Orlando

Thx for your help.

When i place the type 2 or 3 network i see no improvement of the switching behaviour.

Also i have placed an elcap at the output of the switcher.

See below screen shot

Thx and Best Regards

Dirk

0 Orlando Murray over 6 years ago in reply to dirk romeyns

TI__Mastermind 22302 points

Dirk,

Your inductor is too small for a 12V output at 300kHz, it is triggering current limit (1.5A). The current limit causes early termination of that second pulse.

You need to increase that to ~68µH or alternatively increase the switching frequency.

Also I cannot see if this a type-2 or a type-3 network.

If it is a type-2 network the ESR must be in series with the total capacitance, having the ceramics in parallel with the electrolytic, the ceramics will dominate the electrolytic and the effective ESR will be insufficient.

Hope this helps,

Orlando

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LM5164: Strange switching behavior