LM5072: soft start capacitor's value

Polly Chung1

Part Number: LM5072

Dear team,

My customer use our LM5072 as their PD solution. They found that the LED will flicker when LM5072 power on. So they would like to change Css from 1uF to 2.2uF? Is there any risk for this change? Thank you.

Regards,

Polly Chung

over 5 years ago

0 Thomas Amlee over 5 years ago

TI__Genius 9230 points

Hi Polly,

I will get back to you tomorrow.

- Tom

0 Polly Chung1 over 5 years ago in reply to Thomas Amlee

TI__Intellectual 2110 points

Hi Tom,

Any feedback on this question? Thank you.

Regards,

Polly Chung

0 Thomas Amlee over 5 years ago in reply to Polly Chung1

TI__Genius 9230 points

Hi Polly,

I am assuming that the LED you are referring to is the one connected to nPGOOD & CPGOOD like the example circuit in Figure 22.

Is it flickering after startup, when the PWM controller is enabled or during startup?

Could you send a schematic of the design & a waveform of the failure?

Thanks,
Tom

0 Polly Chung1 over 5 years ago in reply to Thomas Amlee

TI__Intellectual 2110 points

Dear Tom,

I will ask customer to provide schematic later. Before providing schematic, could you please let us know if they is any risk that customer change Css from 1uF to 2.2uF? Thank you.

Regards,

Polly Chung

0 Thomas Amlee over 5 years ago in reply to Polly Chung1

TI__Genius 9230 points

Hi Polly,

Increasing the Css cap will increase startup time, delaying the amount of time it takes for the converter to reach steady state & provide the intended output voltage.

Thanks,
Tom

0 Thomas Amlee over 5 years ago in reply to Thomas Amlee

TI__Genius 9230 points

Polly,

There is no risk to the PD system if you increase the CSS cap. It only affects the startup time of the PWM controller

See block diagram for the PWM controller within the LM5072, (datasheet, page 10). Since we are using a larger capacitor at the SS pin & the 10uA current source is constant, it will take longer to charge this capacitor. As this capacitor charges, the voltage limit will gradually be reduced on the COMP pin (eventually being pulled to 5V) & the PWM controller will gradually begin switching, starting the converter.

For more info on this, please see the "Soft Start" section on page 22 of the datasheet.

- Tom

0 Polly Chung1 over 5 years ago in reply to Thomas Amlee

TI__Intellectual 2110 points

Dear Tom,

Thanks for your details explanation.

Regards,
Polly Chung

Power management

Power management forum

LM5072: soft start capacitor's value