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LM3423-Q1: about min PWM H period

Kaji@PAN
Mastermind 7795 points
Part Number: LM3423-Q1

Hi,

I am considering the following contents, is it possible?
The dimming method is only PWM dimming and analog dimming is not performed.
The width of the minimum PWM H period is 1.5us.
In my opinion, it can be set up to the operating frequency of 2 MHz, so it seems that it can be realized if securing at least 3 pulses (= 1.5 us) of switching pulse.
Although it is contents not described in the data sheet, it must be guaranteed over the entire temperature range.

Best Regards,

  • 0
    TI__Mastermind 25500 points

    Hello,

    There is a minimum pulse width "tPULSE" described in the PWM dimming section of the datasheet. It should be accurate if you use the worst case values for the variables in the equation. So it really depends on those parameters. Keep in mind that if you want to switch at 2MHz you will need to be careful to select a FET that will not current limit VCC. The current draw from VCC is Qg*Fsw with Qg being the total gate charge of the FET. Depending on your input/output voltages the AC losses could also get very high at 2MHz.

    Regards,

    Clint

  • 0 in reply to Clinton Jensen
    Mastermind 7795 points

    Thank you for answering.
    Calculated for tPULSE.
    (specification)
    VIN: 8V - 16V
    VOUT: 10V - 16V
    ILED: 700 mA
    Frequency: 2 MHz
    Attach the calculation result.
    It seems that tPULSE can be cleared, but since the device Ton-min is about 200 ns, this constraint can not be cleared in the case of the step-down condition. Is my calculation correct?
    Also, are there any ideas to clear the specifications?

    Best,Regards,

    tPULSE_20170309.xlsx

  • 0 in reply to Kaji@PAN
    TI__Mastermind 25500 points

    Hello,

    For those conditions you are correct, the minimum duty cycle is limited by the minimum on time. The only way to adjust for that is to lower the switching frequency, increase the min Vout, or decrease max Vin until it is no longer violating that condition.

    Regards,

    Clint