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LM3429-Q1/LM3423-Q1

kura
Genius 4840 points
Other Parts Discussed in Thread: LM3423-Q1, LM3429-Q1, LM3429, LM3421

Our customer is checking LM3429-Q1 or LM3423-Q1 datasheet.

They would like to use these part for following application.

<Application>

Vin:6V-18V

Vout:24.75V-33.75V

ILED:1.5A

nLED:9 series

Vf:3.25V(min)-3.75V(max)

PWM Dimming 5000:1

SW frequency:2MHz

 

Question1:

Could you please let us know whether LM3429-Q1 or LM3423-Q1 suit this application?

Question2:

Could you please let us know Maximum Junction Temperature of LM3429-Q1

Only "internally Limited" description is found Datasheet P4.

www.ti.com/.../snvs616h.pdf

Question3:

Could you please let us know Minimum Sw frequency of  LM3429-Q1.

Question4:

PWM dimming minimum and maximum frequency of LM3429-Q1.

regards,

  • 0
    TI__Mastermind 25500 points

    1: Either should be able to provide the boost function just fine, but there are some things to consider. First is the FET selection. 2MHz is very fast (and not the most efficient) switching frequency. You will need to choose a FET with very low total gate charge to run that fast to keep the VCC current below its current limit. The VCC current required is Qg*Fsw. Also, to get 5000:1 dimming will depend on the dimming frequency, and it will likely require the disconnect FET as used with the LM3421/23. It is unlikely the LM3429 can reach this spec unless you externally drive a disconnect FET.

    2: The max junction temperature is the thermal shutdown limit which is 165C with 25C of hysteresis as specified in the electrical characteristics.

    3: There really is no minimum switching frequency, but a practical minimum to keep the circuit size reasonable would probably be around 200kHz.

    4: The maximum switching frequency is specified as 2MHz with the considerations mentioned in #1. The PWM frequency range can be from about 100Hz to up to around 20kHz if a disconnect FET is used. But the lower the frequency you use the better contrast ratio you can achieve. If you need 5000:1 you will not get it dimming in the kHz range most likely. 120Hz to 250Hz is pretty common for applications that require high contrast ratio.

  • 0 in reply to Clinton Jensen
    Genius 4840 points
    Thank you for your reply.
    To achieve 5000:1 ,we will recommend LM3421/3423 using with disconnect FET.(PWM:120Hz to 250Hz)
    We have additional question.
    Could you please let us know LM3421/3423 PWM minimum on time (@PWM:120Hz to 250Hz).
    regards,
  • 0 in reply to kura
    TI__Mastermind 25500 points
    The minimum on time is application dependent. There is an equation in the PWM Dimming section in Detailed Description in the datasheet you can refer to.