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TPS92691: How to select appropiate values for Rls2, Rls1 and diode zener in a Buck Converter topolgy using TPS92691

Aaron_Rangel
Intellectual 440 points
Part Number: TPS92691

Hello,

I have designed a Buck connverter with the TPS92691 driver.

Vin = +18V to +26Vdc, Vnom = +24Vdc

Vo = 5Vdc (that is the Vf tipycal led value used as a load)

Io = +4.5Adc.

My doubt is about the correct values for Rls2, Rls1 and the zener diode used in the output stage of the converter. (Schematic R1, D1 and R7).

The datasheet only describes that this values can modify the time raise and fall for when a P-FET is used, which is my case. The schematic is as it is described in page 18.

The zener diode is used in the examples described in the datasheet.

I know that this is a bit dependant of the mosfet selected, but what is exactly the goal of this network to desing the adequate values? What's the correct way to design this network?

Another thing that I want to know is if that can produce a change in the output current when pwm dimming frequency is changed.

I mean, if and improper design of this network can produce this issue.

As far as I know changes in pwm dimming frequency should not change the output current value, only the duty cycle would produce it.

Regards,

  • 0
    TI__Mastermind 18890 points

    Hi Aaron,

    Q3, R1 and R7 does the level shift of the TPS92691 output of DDRV for PWM the FET Q1.  The main intent is for the level shift is to get the right voltage across the Gate and Source of Q1 while D1 is used to clamp the Gate-Source voltage from going above the damaging thresthold.

    So R1 and R7 are the pull up or down of the gate voltage.  The smaller values these are the faster you can switch the FET but there is also power static loss involved with lower R1 and R7 values.  The 1k and 2k values are chosen to compromise the losses.  As you mentioned the R1 and R7 along with the gate charge required for Q1 determine the switching speed.

    The 12KHz you are PWM'ing along wiht narrow duty cycle can be an issue here since it's really fast PWM'ing.  I've asked a question in your other post about you  application here and whether you can go lower on PWM frequency.  For LED and headlight application, the PWM frequency is usually from 120 Hz to 400 Hz.

    You can look at the differential voltage of the Gate and Source of Q1 along with the PWM'ing current on the LED with a current probe to determine whether your circuit is fast enough to drive the FET. 

    Thanks Tuan