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LM3409 Enable pin confusion

MagicMike
Prodigy 50 points

Hello,

I am quite confused about the Enable pin and the power modes. I find the datasheet unclear about the enable pin, also my measurements conflict with the datasheet. The datasheet says:

But what happens in between 0.5 and 1.74V?

I see a falling and rising threshold voltage, but what is the hysteresis for? I measure a rising threshold of 1.3V and a falling threshold of 1.1V. That is 200mV hysteresis.

The datasheet says that at a logic low voltage(below 0.5V) the VCC regulator remains active. When i ramp up the enable voltage i only measure VCC when the driver is switching(above 1.3V). When i ramp it down again the VCC is gone as soon as the driver stops switching.

Can someone explain me?

Kind regards,
Michiel Cnossen

  • 0
    Prodigy 50 points
    Someting went wrong with the datasheet screenshots i added to the post.. They are gone. I will try to correct this.
  • 0
    TI__Mastermind 25500 points
    The 0.5V/1.74V levels are specified to make sure the threshold is exceeded over all conditions (Vin, temp, etc...) and is heavily guardbanded. The reality of it is that it is a threshold near 1.3V with some hysteresis to prevent a noisy signal from false triggering enable. The hysteresis is also not a critical spec, it's just there for some noise rejection so it is a spec that can vary widely. +/- 50% is not out of the question as you have seen.
    In any case, that section (7.3.6) is not very clear without also reading the low power shutdown section of the datasheet and perhaps Vcc should not be in the parantheses. All the circuitry remains active for a fast turn on, but only as long as the Vcc capacitor remains charged above its UVLO threshold, then it will enter low power shutdown. This is fine for most PWM dimming applications, but the UVLO pin can be used for PWM dimming as well and the Vcc will remain up.
  • 0 in reply to Clinton Jensen
    Prodigy 50 points

    I understand. Thank you for explaining.