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The LM3404 is a "Constant Current Buck Regulator for Driving High Power LEDs." One of the features the datasheet boasts is "LED dimming using pulse width modulation (PWM)." My question is, using this chip (and its associated support ciruitry), which of the following accurately reflects the nature of the current delivered to the LEDs?

  • the LEDs are actually switched on and off (chopped current) at the configured constant current in relation to the PWM input signal
  • the LED current is proportionately (i.e. in accordance with PWM duty cycle) reduced from the nominal fully bright constant current, but still driven by constant current (i.e. unmodulated).
  • Something else, if so please explain
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    \$\begingroup\$ Suggestion when asking such questions, a link to the datasheet would be appreciated. \$\endgroup\$ – DoxyLover Jun 20 '17 at 21:17
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The datasheet (http://www.ti.com/lit/ds/symlink/lm3404hv.pdf) describes the DIM pin as follows:

Input for PWM dimming. Connect a logic-level PWM signal to this pin to enable and disable the power MOSFET and reduce the average light output of the LED array

Therefore, the LEDs are dimmed by turning on and off along with the PWM input. There is no analog dimming.

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  • \$\begingroup\$ Ya seems right, that's how I read it too. Just wanted another opinion before drawing that conclusion. \$\endgroup\$ – vicatcu Jun 21 '17 at 20:35

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