LM3489: Asking for LM3489 slew rate design review and suggestions

SHH

Part Number: LM3489

Hi Team,

Customer designed LM3489 in new project.

Would you please help review slew rate question and provide your comments?

During voltage transition, the slew rate should not exceed vSrcSlewPos (vSrcSlewNeg), which is 30 mV/µs according to the PD specification.

vSrcSlewPos ≤ 30mV/us

customer measure the 5V to 20V case, slew rate is over 30mV/us.

15.2V/340us=44.7mV/us

If need to add bulk cap, would you please provide suggestion and how to add the bulk cap to slow the slew rate?

Thanks for help!

SHH

over 2 years ago

0 Orlando Murray over 2 years ago

TI__Mastermind 22301 points

Hi SHH,

You can increase the feedforward capacitance (capacitor from VOUT to FB).

This capacitance will slow down FB response time which will slow down transients and slew rates.

Be sure to check switching stability afterwards.

Hope this helps,

-Orlando

0 Jimmy Hua over 2 years ago

TI__Guru 50120 points

Hello SHH,

For the waveform that you've attached, is this measuring VOUT? What is your application condition?

If you are asking about output voltage soft-start, then you can try changing the CADJ capacitor to values higher than 1nF to create a soft-start function as detailed in the datasheet Section 7.3.3.

Regards,

Jimmy

0 SHH over 2 years ago in reply to Jimmy Hua

TI__Mastermind 32156 points

Hi Jimmy,

customer's application Vin is 20V.

Vout will be 5V/9V/15V/20V by selecting different FB resistor.

In type C definition, the slew rate vSrcSlewPos ≤ 30mV/us need to meet.

So customer test 5V to 20V case and customer feedback the slew rate is too fast that cannot pass spec.

CAdj is 1nF in schematic. Do you have suggested value for Cadj?

BR,

SHH

0 Orlando Murray over 2 years ago in reply to SHH

TI__Mastermind 22301 points

Hi SHH,

C7205 feedforward capacitance will slow down FB node.

You can increase that capacitance to slow down the transient response.

Hope this helps,

-Orlando

0 SHH over 2 years ago in reply to Orlando Murray

TI__Mastermind 32156 points

Hi Orlando,

thanks for comment.

do we have any simulation tool that can simulate the cap value?

Do you have suggested value for customer?

BR,

SHH

0 Orlando Murray over 2 years ago in reply to SHH

TI__Mastermind 22301 points

SHH,

You can try increase the feedforward C7205 to maybe 220pF or 1nF.

I can run some simulations, what is the load current? Is the load current the same for both output voltages?

-Orlando

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LM3489: Asking for LM3489 slew rate design review and suggestions