• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 63 subscribers
  • Views 95 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2623: Selecting initial C3 value to set duty cycle

Matt Lang
Intellectual 540 points
Part Number: LM2623

I am designing with the LM2623A controller.

Vin min = 1.8V

Vin max = 4.5V

Vout = 9.5V

Iout = 0.2 A

In the datasheet, it says selecting C3 is a "trial and error" process. But I was hoping to get some guidance as to what value I should even start with. Example circuit in the datasheet has 4.7pF. 

Following this TI doc
https://www.ti.com/lit/an/snva731/snva731.pdf?ts=1644852689362

It says that the duty cycle is:

D = 1 - ((Vin * eff)/Vout)

In my case D would be 85% with an 80% assumed efficiency. The part does imply a max duty of 90%. So I would be cutting it close.

And because my input voltage changes as the battery discharges, I will need a series resistor to C3, according to this doce
www.ti.com/.../snva050d.pdf

Questions:

1. Does this part have a spice model (TINA, LTSpice) in which I could use to help select C3? Or is that not an accurate option either?

2. What value of C3 should I start with in my case? And what resistor in series should I use?

Any help, tips or tricks are appreciated. 

  • +1
    TI__Expert 5180 points

    Hi Matt,

    1. Sorry that we don't have LM2623 spice model, because it's a relatively old chip. And because its non-linear effect, I think the best method is reserve the C3 and R5 on PCB and test in bench to select the best value.

    2. LM2623 is not a typical PWM converter, but a PFM converter, its control scheme uses a hysteresis window to regulate the output voltage. When the output voltage is below the upper threshold of the window, the LM2623 switches continuously with the duty cycle set by C3. When the output voltage hits the upper threshold of the window, the LM2623 stops switching completely.

    That why the C3 effect is non-linear. For you application, if duty cycle in PWM mode is 85%, then duty cycle will be even higher in PFM mode, maybe 85%-90% is suitable. But as I said before, it is better to test in bench to select the value. Also C3 value is also effected by switching frequency (R3). 

    I think 4.7pF C3 and 100k R3 can be the start value in your case.

    Best Regards,

    Eric Yue