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LM3404 - the calculation formula for Rz and the tolerance of swithcing frequnecy

Hirotaka Matsumoto
Guru 27555 points
Other Parts Discussed in Thread: LM3404, LM3404HV

Hi all

Would you mind if we ask LM3404?

<Question1>
On the data sheet P12, Rz(for Overvoltage and Overcurrent Comparator) resistance is recommended that is should be maximum 1kohm.
So, could you let us know the calculation formula for Rz?
Rz=(Vin-Vz)/Iz or Rz=Vz/Iz ??
Is our recognition correct?

<Question2>
We recognize that LM3404 and LM3404HV devices can be programmed for switching frequencies ranging from the 10’s of kHz to over 1 MHz.
How much is the tolerance of  swithcing frequnecy?

We need your help.

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Mastermind 25500 points

    Hello Hirotaka,

    To answer your questions:

    1. The circuit will try to regulate CS to around 200mV so you would use Rz = 200mV/Iz.

    2. The switching frequency variation depends on a lot of factors since the converter is hysteretic. The ON time will remain fairly constant given a constant input voltage, but there will be some variation as shown in the electrical characteristics table. But the OFF time could vary with variations in inductance and LED forward voltage. So the overall variation will depend on the particular inductor and LED stack used for an application.

    Thanks,

    Clint