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LM3409 slow response on chaning Iadj

Jeroen Wilbers
Prodigy 20 points
Other Parts Discussed in Thread: LM3409

LS, I am using the LM3409 with a current setpoint. For my application I need to control the current through the LED (2A, 2V)

how ever I get a delay when switching to low (~50us)  and a delay when swiching to high (~120us).

I am afraid the on delay is due to the fact the output voltage drops below 1.24V

However the 'off' delay is a mistery to me.

In the figure:

Blue is my setpoint on I adj.

Yeallow the voltage across the LED

Purple the output current (measured across a 0.12ohm resistor)

Can you help me with what is going on here?

(PS I need current control and a common Cathode LED)

  • 0
    TI__Mastermind 25500 points
    I could take a look, but I don't see a figure attached. Could you attach it along with a schematic and operating conditions (input voltage, LED high and low current levels, LED voltage at each current level)?
    Also yes, there is an extra delay when the output voltage drops below 1.24V because the part is operating in maximum OFF time mode. There is a way to avoid this in the datasheet under the PWM dimming section by OR-ing in a higher voltage. You can also run the off timer from the input voltage if you don't mind the switching frequency change associated with that.