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TPS92518: PWMx frequency and switching frequency Fsw

KANJI SHIBATANI
Mastermind 6205 points
Part Number: TPS92518

 

Hello

My customer started design using TPS92518.

Customer use Vin=24V or Vin=48V as following.

In order to select same inductor= 82uH ( also same LEDX_TOFF_DAC =21 ), Fsw should be changed.

 

(a)VIN=24V,VLED=10.5V   548KHz

(b)VIN=24V,VLED=12V     542KHz

(c)VIN=48V,VLED=12.3V   895KHz

(d)VIN=48V,VLED=39V     386KHz

 

Q1

I think Fsw = 384KHz ~ 895KHz is essentially no problem, but how do you think?

However, in order to have PWM dimming =0.1% duty, PWM dimming input frequency  in case(c): 895kHz  should be higher than (a)(b) and (d)?

For example, PWM dimming frequency should be 2KHz in case(c), while ohter case may be 1KHz from data sheet 8.3.8.4.1 .

How do you think?

Also, what value " n " is recommended  PWMx frequency versus  Fsw;siwithing frequency  = 1: n?

 

8.3.8.4.1 PWM:

A linearity limitation occurs at very small PWM duty cycles; the PWM dimming on-time becomes short enough that it contains only one

or a few switching cycles. If the PWMx pulse falls during an off-time (see Figure 28), the pulse length is not able to change because the switch is already off.

 

Q2

Equation (5) on data sheet can be used to estimate output accuacy of average LED current. ( excluding Rsense and L(uH) )

Anyway, there are three +/- on equation (5). ... please kindly refer attached.

Max: (+), (+), (-) Min: (-),(-), (+)

Is it correct? Please confirm.

Best Regards

  • 0
    TI__Mastermind 28655 points
    Hello Kanji,

    1) Yes that switching frequency range is fine. You can PWM dim at 1 KHz or 2KHz it just depends on how accurate you want the dimming to be. If the LED current ripple is fairly low it's not as critical. If you want it to be more accurate at 0.1% a lower PWM frequency will be better. "Also, what value " n " is recommended PWMx frequency versus Fsw;siwithing frequency = 1: n?" This depends on what is acceptable operating for you. Sometimes trying different PWM frequencies and dutycycles and observing is the best was to see if it is acceptable.

    2) That looks correct.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points

    Dear Irwin-san

    Thank you very much.