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Switching Frequency and TPS92662-Q1 Shunt PWM dimming

c tyerman
Prodigy 150 points
Other Parts Discussed in Thread: TPS92662-Q1, TPS92518HV-Q1, TPS92518

Im evaluating the possible use of the TPS92518HV-Q1 in combination with the TPS92662-Q1 and would like some clarification

ive worked through the Typical Application Design Procedure which has the equation (37) D = VLedx / (Vin * n) but the source of "n" isn't specified and gives K when try and copy and paste it what is "n"?

am i correct in assuming when shunt dimming VLEDx is the forward voltage for the led string, and when using TPS92662-Q1 this can vary from the sum of the LEDs to that of a single LED (or that of shunt FETs when all shunted) i am unclear as to the appropriate Voltage to use. using the TPS92662-Q1 leds could be shunted for considerable periods of time  resulting in highly variable voltages

e.g. for 10 LEDs (2.5-3.5 Vf) can swing from 2.5V to 35V  depending on the combination shunted.

I think i should be using the Total Voltage for the string is this correct?

if this is the case it appears that the TPS92518HV-Q1 operates at a variable switching frequency dependent on the led voltage (Datasheet Figure 17. Frequency vs. LED OutputVoltage. Fixed Input Voltage)(limited by minimum on time tLEB = 250 ns)

you recommend in (https://e2e.ti.com/blogs_/b/powerhouse/archive/2016/08/26/led-brightness-adjustment-high-frequency-pwm-dimming) a PWM dimming frequency of at least 2kHz

and the TPS92662-Q1's power supply "is recommended to operate at a switching frequency between 250 times the PWM frequency and 2000 times the PWM frequency." 

if this is correct then the switching frequency of the TPS92518HV-Q1 should be at least 500kHz,

however it appears that the maximum frequency limited by tLEB = 250 ns (equation 10) will drop to lower than 500kHz  when shunt dimmed, by the TPS92662-Q1, to less than the full array of LEDS

e.g 

Vin =40, VLED = 35 (10 LEDs) which gives (40/3.5)/(250*10^-9) = 3.50 MHz

Vin =40, VLED = 3.5 (1 LEDs) which gives (40/3.5)/(250*10^-9) = 350 kHz

is this correct use of that equation

it is not clear what the effect on the TPS92662-Q1 of dropping bellow the recommended switching frequency would be

  • 0
    TI__Mastermind 28655 points
    Hello,

    The 'n' is efficiency, the equation is using 0.9 as an example, which would be 90%.

    You actually want to calculate different operating points. If Vout is close to Vin the switching frequency will be low, it will be highest at Vled = 1/2 Vin. The ripple will remain pretty much constant but there are things that have to be checked such as the minimum on time when the output voltage is low which you've read on.

    Yes, what will happen if the output voltage gets low the minimum on-time will cause the current to creep up. With the TPS92518 you can set an LED_MAXOFF_DAC for shunt fet dimming. This is really set-up for Shunt-FET dimming where the output is bypassed with a MOSFET.

    "Vin =40, VLED = 35 (10 LEDs) which gives (40/3.5)/(250*10^-9) = 3.50 MHz

    Vin =40, VLED = 3.5 (1 LEDs) which gives (40/3.5)/(250*10^-9) = 350 kHz

    is this correct use of that equation" Not necessarily, those two points aren't the same design. If you are calculating the maximum switching frequency where Vout is 1/2 of Vin the switching frequency will be lower when Vout is 3.5 volts. This is where calculating many operating points is useful. You can create this in Mathcad or excel to see what the frequencies will be as well as the on-time and if it's getting too low.

    The PWM dimming frequency doesn't have to be above 2 KHz, it depends on how you plan to use your design. There are applications that require higher PWM dimming so as not to interfere with digital sampled systems such as video cameras. Since the TPS92518 switching frequency varies you won't be able to keep it above 500 KHz under all operating conditions. If you set the TPS92518 to 500 KHz with Vout at 2.5V it will exceed the maximum switching frequency when Vout is 1/2 of Vin.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    Hello,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best Regards,