LM25119: LM25119: transient load issue

Steven He92

Expert 2125 points

Part Number: LM25119

Dear Sir,

As below transient load waveform(output current 5A to 0A transient), output voltage overshoot too high over 200mV,

Vin=20V ~ 36V, Vo=12V/34A, circuit design interleaved mode(refer datasheet page 32 circuit)

DEM pin= floating(would improve light load efficiency)

RT=22Kohm(Fosc=220KHz); Output chock(L1= L2)=3.3uF,

RAMP(R14+ C20) =73.2Kohm + 820pF ; COPM1(R7 + C17 +C16)= 49K + 682pF + 100pF

Does L1=L2=3.3uF to low? 6.8uF is OK ?

Please you kindly review & provide suggestion(how to improve transient over-shoot)..

Thanks!

Steven

over 5 years ago

0 Qian Chen over 5 years ago

TI__Mastermind 24075 points

Hi Steven,

The attached spreadsheet can be used to select component value.

1134.LM(2)5119 QuickStart ICC Modified.xlsx

Usually the inductor ripple current is 20-40% of max load current. From that point of view, 3.3uH is too small.

However, higher inductance may make the overshoot worse.

What's load step in your transient test? One method to reduce overshoot is adding more output cap.

Thanks

Qian

0 Steven He92 over 5 years ago in reply to Qian Chen

Expert 2125 points

Dear Qian,

The load transient load is 0~5A, As you see output ripple over 200mV & SW will stop.

Does this normal condition by DEMB enable diode emulation ?

I will try increase Fosc=400KHz(L=3.3uF) to see have improve.

Thanks you file.

Steven

0 Steven He92 over 5 years ago in reply to Steven He92

Expert 2125 points

Dear Qian,

DEMB(PIN17): floating is enable diode emulation mode and low side MOSFET latch-off right ?

-> that mean LM25119 into non-sync. Buck converter during output light load?

My customer try modify EVM make a 2phase output, output load transient H -> L(5A ->0A) as previously waveform SW have stop condition(make have overshoot issue).

But when we put DEMB connect to VCC1 output transient waveform(SW) didn't have stop & output voltage transient waveform OK.

It is correct behavior?

Thanks!

Steven

0 Qian Chen over 5 years ago in reply to Steven He92

TI__Mastermind 24075 points

Hi Steven,

If DEMB pin is floating, an internal 50k resistor pull the pin low and enable diode emulation mode.
Looks like overshoot is related to diode emulation mode.
After the load is released, Vout will rises up. Because the part enters diode emulation mode, Low side FET is turned off, the part is not able to pull Vout low quickly.

Is it acceptable to let the part operate at CCM mode in your application? I know the power loss at no load condition will increase.

Thanks
Qian

0 Steven He92 over 5 years ago in reply to Qian Chen

Expert 2125 points

Dear Sir,

In this project, customer accept CCM mode operating(I saw reference design also set CCM mode).

Customer wonder why DCM will cause of SW to STOP during Hi to Low transient ?(customer could accept if IC have burst mode at light load)

Thanks!

Steven

0 Qian Chen over 5 years ago in reply to Steven He92

TI__Mastermind 24075 points

Hi Steven,

During load step down (hi to low transient), once inductor valley current decreases to zero, the part will turn off low-side FET. This is how DCM mode works.

This part does not have burst mode.

Thanks

Qian

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LM25119: LM25119: transient load issue