# Can LED Driver chips drive the LED with continuous ON DIMMING?

I am assigned with the design of a LED driver with 1 A current and 5 V system power. The customer wants to control all the LEDs with simple switches, nothing digital is involved. So I am thinking of using this LED driver:

http://www.monolithicpower.com/DesktopModules/DocumentManage/API/Document/getDocument?id=66

I thought that if I apply 5V system supply to EN/DIM pin via a switch I can get a constant continuous current(1A). And when the switch is OFF, LED will turn off.

These are diagrams in the datasheet also:

And here is the description for EN/DIM pin:

On/Off Control Input and Dimming Command Input. A voltage greater than 0.7V will turn on
the chip. When the EN/DIM pin voltage (with respect to INGND) rises from 0.7V to 1.4V, the
LED current will change from 0% to 100% of the maximum LED current. To use PWM
dimming, apply a 100Hz to 1kHz square wave signal with amplitude greater than 1.4V to this
pin.


So I am a little confused. I thought Analog dimming was the thing I was up to. However, in pin description, it says apply 100Hz to 1KHz square signal. So question is simple as this. Do I get constant Current for over half hour if I apply constant 5V to EN/DIM pin, or it does not work that way?

Please look at page 8 of the datasheet under "Dimming Control". You can do either PWM or analog dimming. For analog, you vary the EN/DIM pin between 0.7V (0%) and 1.4V (100%). For PWM, you use a 1HKz signal with on > 1.4V and the duty cycle controls the brightness.

You can do it either way.

And to answer your question, putting 5V into EN should give you 1A to the LEDs continuously.

You are correct that if you apply a constant 5 V to the EN/DIM pin, the LED current will be 1 A. This doesn't not use the dimming feature (analog or PWM)*.

Dimming refers to reducing the brightness of the LEDs, and there are two ways to accomplish that:

1. Analog: Dimming occurs when the input voltage is between 0.7 V (LED current near 0) and 1.4 V (LED current is 1 A), so that the output current varies roughly linearly with input voltage over that range. If the voltage is greater than 1.4 V, the LED current will still be 1 A, and no dimming occurs.
2. PWM: Dimming occurs by rapidly switching the LEDs on and off, so that their effective brightness corresponds to the percentage of the time that they are ON (duty cycle). Here the On signal is any voltage applied to EN/DIM that is greater than 1.4 V.

* technically you're using PWM with a 100% duty cycle, meaning always ON. (the analog dimming feature only applies when the input voltage is on the range of 0.7 V to 1.4 V).