I am designing a LED driver circuit using LT3475EFE-1#PBF IC for controlling two LEDS. I have a signal from an I/O pin for LED ON-OFF control which is tied to PWM input (as recommended). Typical circuits tie an NFET to the cathode of LED and the gate is controlled by PWM input.

Can I connect my control signal to PWM pin and perform LED lighting without using a NFET? I don't want to achieve LED dimming control, just switching action is enough.

Any help would be appreciated as I need to design the circuit in mass scale.

LED control circuit


1 Answer 1


According to the LT3475 data sheet: -

For PWM dimming ratios of 20 to 1 or less, the NFET can be omitted

This means use it in its normal mode of operation but if you require a wider control over the dimming then use a FET. Normal mode: -

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  • \$\begingroup\$ What does "ratio of 20 : 1" mean in terms of pwm? \$\endgroup\$
    – Passerby
    Apr 15, 2014 at 15:07
  • \$\begingroup\$ @passerby I think its residual cap voltage keeping the led slightly powered ie you can't dim it below one twentieth of full brightness. \$\endgroup\$
    – Andy aka
    Apr 15, 2014 at 16:24
  • \$\begingroup\$ hi Andy...you are correct...The FET can be avoided in normal mode of operation.But i need to control LED through ON-OFF signal input to PWM pin without a NFET. I don't require dimming of LEDs. \$\endgroup\$
    – user40383
    Apr 16, 2014 at 7:19
  • \$\begingroup\$ @user40383 I'm no expert on this chip, but if it is residual charge keeping the LED glowing then maybe a forward-biased diode in series with the LED might persuade it to glow less when the PWM pin is deactivated? \$\endgroup\$
    – Andy aka
    Apr 16, 2014 at 7:23

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