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LM3429: PWM Frequency Selection

KIM Andy
Expert 2530 points
Part Number: LM3429
Other Parts Discussed in Thread: TPS92515, TPS92692, LM3409

LM3429 Test I have a question during the test.

 vin : 12, led : 3.4v x 4 = 13.6, max current 1A

On the DataSheet, I thought that it could be used from 80kHz ~ 119kHz when calculating by the formula (26)

Checked by using actual function generator, there is symptom that current control can not be done according to PWM at 1kHz or more.

When setting the PWM Frequency, please reply if you have any more or need more information.

Test results will be attached.

  • 0
    TI__Mastermind 28655 points
    Hello,

    Are you using this as a buck-boost? There are no test results attached. What is happening when PWM at 1 KHz or more? What is the duty cycle?

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 2530 points

    Hello.

    thank you for your quick reply.

    The customer wants to configure the circuit with buck-boost.

    We will send you a confirmation report on "Current change due to pwm control frequency change".

    The current customer wants to use the frequency of 20kHz.

    The problem is that if you go beyond 1kHz, it will behave abnormally.

    LM3429_PWM_Current__181227.xlsx

  • 0 in reply to KIM Andy
    TI__Mastermind 28655 points
    Hello,

    If they are trying to PWM dim at 20 KHz this is probably the wrong approach. The waveforms attached look like they should for what they are donig. The inductor slew rate won't be fast enough for accurate dimming and may actually miss a portion where the MOSFET is never turned on (two pictures at 20 KHz PWM show this). The method they are using turns on and off the MOSFET allowing the inductor to slew up and down with a delay that is V = L*(di/dt). Also, if the PWM is 20 KHz it will interfere with the switching frequency if it is in the range you mention. One way to achieve 20 KHz PWM dimming would be a two stage solution, a boost into a hysteretic buck converter that can be shunt-FET dimmed (the TPS92515, LM3409 are hysteretic buck controllers). Or, if possible, rearrange the LEDs to have a lower voltage than the input voltage. You may also look at using the TPS92692 with series FET dimming though I would need to find out how that would work at 20 KHz.

    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,