LM5145: LM5145 frequency drops

Kevin LIN

Part Number: LM5145

Dear Ti team,

I have a new design using LM5145 and the input is 28V, output is 5V 10A and the switching frequency set to 130KHz.
But, I have to check the switching frequency using oscilloscope under no load and full load. I find that the witching frequency of the LM5145 drops to 68kHz after turned on the step down converter about 5 minutes. This state can only be reset by completely resetting the converter (turn off power longer than 5 seconds). This problem is it related to bode plot?

Please find attached for my calculation. Would you please help to review?

BR
Kevin

LM5145 Wide Vin Synchronous Buck Controller Quickstart Calculator r12-7.zip

over 4 years ago

0 Andy Chen over 4 years ago

TI__Mastermind 34625 points

Hi Kevin,

On the calculator, seems you input 7A as Max output current.

Do you use RDS(on) for current sensing? Please key in correct RDS(on) of low side FET you selected on the calculator table (cell E96).

Please also input correct output capacitance (after derating) and ESR to calculate COMP network.

B R

Andy

0 Kevin LIN over 4 years ago in reply to Andy Chen

Prodigy 110 points

Dear Andy,

Yes, the circuit is using RDS(on) for current sensing, the calculator table input value is correct.

Also, the capacitance and ESR is correct.

Kevin

0 Kevin LIN over 4 years ago in reply to Andy Chen

Prodigy 110 points

Dear Andy,

Would you please help to review on it?

Kevin

0 Andy Chen over 4 years ago in reply to Kevin LIN

TI__Mastermind 34625 points

Hi Kevin,

I re-calculate it for you, pls modify all external components accordingly, especially the COMP network

The Cout must be 5x22uF 16V 1210 ceramic cap.

Cross over freq around 13KHz

Inductor 10uH 15A--20A rated.

Low side FET RDS(on) 10mohm at 25C.

B R

Andy

0 Kevin LIN over 4 years ago in reply to Andy Chen

Prodigy 110 points

Dear Andy,

As datasheet and the calculator descried, the crossover frequency should set to 10% or 20% or switching frequency.

Would you please tell me why the crossover frequency need to set to 13kHz.

Moreover, current design is using lower Rdson MOSFET in low side, is it mean that I need to change higher Rdson low side MOSFET to solve that?

Kevin

0 Kevin LIN over 4 years ago in reply to Andy Chen

Prodigy 110 points

Dear Andy,

Would you please also share the calculator value to me.

Kevin

0 Andy Chen over 4 years ago in reply to Kevin LIN

TI__Mastermind 34625 points

LM5145 Quickstart Calculator r12-7.zip

0 Kevin LIN over 4 years ago in reply to Andy Chen

Prodigy 110 points

Dear Andy,

Thanks for your calculation. May I ask again for Low side MOSFET, is the Rdson is a critical factor affecting the system stability.

Because my selected MOSFET Rdson is 2.4mR and your recommendation for the MOSTFET is 10mR. From my understand, the Rdson is just affecting the ILIM resistor value and the efficiency for the system,

please correct me if there is any misunderstanding.

Kevin

0 Andy Chen over 4 years ago in reply to Kevin LIN

TI__Mastermind 34625 points

Hi Kevin,

Normally customer intends to use same MOSFET for both high-side and low side FET, the high side MOSFET with 2.4mR RDS(on) may not be a good choice for 28V to 5V 10A conversion.

Low side RDS(on) is critical for current sensing, it should be as higher as possible for better signal/noise ratio.

You can use 2.4mR FET as low side FET first and check if current limitation is correct and switching is stable

B R

Andy

0 Kevin LIN over 4 years ago in reply to Andy Chen

Prodigy 110 points

Hi Andy,

Thanks for your reply. May I know low side RDS(on) is critical just for current sensing (current limitation) or this value can affecting the system stability like bode plot, so, if the Low side RDS(on) value is wrong,

The system become unstable, like the frequency drops

Kevin

0 Andy Chen over 4 years ago in reply to Kevin LIN

TI__Mastermind 34625 points

Hi Kevin,

LM5145 is voltage mode control, low side RDS(on) is critical for current limitation, no impact on control loop stability.

You need to design inductor and output cap first, then design COMP network accordingly which is most critical for loop stability.

Your design has relatively high power operation, I suggest you to modify the LM5145EVM for your application to eliminate PCB layout issue, then tune COMP network based on actual inductor and output cap used.

B R

Andy

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LM5145: LM5145 frequency drops