What is maximum frequency for PWM pin of TL4242?
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What is maximum frequency for PWM pin of TL4242?
Hi Charlie,
Normally, PWM frequency is set much smaller than the turn on/off delay time (please refer to the TL4242 datasheet SLVS641A page 4). If PWM frequency nears this delay time will result the following problems.
o. The output current and duty cycle will miss-matching. E.g. when PWM frequency is 100Hz and duty is 0.8 then the output current is maximum output current * 0.8. However, when PWM frequency increses to 10KHz and duty is also 0.8, the output current will miss-match with duty and become maximum output current *0.89.
o. The non-linear region in the duty cycle versus output curve will increse. E.g. the output current in the duty range between about 0.05 and 0.95 is linear when PWM frequency is 100Hz. But when PWM frequency increse to 10KHz, the linear range will reduce to about 0.15 and 0.85.
I'm working on the detail test result of this non-linear curve.
Regards.
Hughie Lu
6661.20110728_TL4242_PWMFrequencyTest.xls
Hi Charlie,
Please see the attached file for the detail, from the test result we can see.
o. There is no minimum and maximum value for PWM fequency, just depend on the application.
o. When frequency is too low (about or below 60Hz), we will wee the LED blinking.
o. When the frequemcy is too high.
* The non-linear range will increase. E.g. if PWM frequency is 20KHz, LED will remained ON above the 80% duty and remain OFF below 1% duty. The linear region can be used just only between 1-80%.
* The output current and duty cycle will miss-matching. E.g. when the PWM frequency increse to 10KHz and duty cycle is 0.8, the output current will become maximum output current x 0.89 (it should be 0.8).
Regards,
Hughie Lu
Charlie,
It depends on the maximum and minimum duty cycles you would like to output. If you know these values for your application you can determine the maximum PWM frequency for your application.
Un der the following conditions:
Vin = 12V; VLED = 9.9V; Io = 519mA
We were able to determine that the highest PWM frequency possible was around 13Khz. This is based on plotting the output current and inspecting its duty cycle. At frequencies that are too high you will notice that the LED current will have duty cycles that vary from your input duty cycle. In our case we looked for a min. 20% duty and noticed that at frequencies higher than 13Khz the device did not supply accurate current output duty cycles.
The attached diagram shows the current output (channel 4), PWM input voltage (channel 3), and VLED(channel 1).
In general, you should determine the highest frequency at which your minimum and maximum input duty cycles are relatively the same to your output duty cycles.