Datasheet

2 ms/DIV

MODE/F

1V/Div

DIM

1V/Div

LEDOFF

5V/Div

ILED

100 mA/Ddiv

100

0.3 0.9 1.5 2.1 2.7

DIM (V)

DUTY CYCLE (%)

C5 =

40 PA

2 x f

DIM

x 2.13

LM3431

SNVS547G –NOVEMBER 2007–REVISED MAY 2013

www.ti.com

Dimming

The LM3431 is compatible with both analog and digital LED dimming signals. The MODE/F pin is used to select

analog or digital mode. When MODE/F is pulled above 3.8V, digital mode is enabled and a PWM signal up to 25

kHz can be applied to the DIM pin. In this mode, the LED current regulators will be active when DIM is above 2V

(typical) and inactive when DIM is pulled below 1.1V (typical). Although any pulse width may be used at the DIM

pin, 0.4 µs is the minimum LED on time (in either digital or analog mode). This limits the minimum dimming duty

cycle at high dimming frequencies. For example, at 20 kHz, the dimming duty cycle is limited to 0.8% minimum.

At lower dimming frequencies, the dimming duty cycle can be much lower and the minimum depends on the

application conditions including the FF setting (see Setting FF section). In analog dimming mode, the MODE/F

pin is used to set the PWM dimming frequency, and duty cycle is controlled by varying the analog voltage level at

the DIM pin. To operate in analog mode, connect a capacitor from MODE/F to ground, shown as C5 in the

typical application (without the pull-up resistor installed). The dimming frequency is set according to the following

equation:

(30)

In analog mode, the MODE/F pin will generate a triangle wave with a peak of 2.5V and minimum of 0.37V. The

DIM pin voltage is compared to the MODE/F voltage to create an internal PWM dimming signal whose duty cycle

is proportional to the DIM voltage. When the DIM voltage is above 2.5V, the duty cycle is 100%. Duty cycle will

vary linearly with DIM voltage as shown in Figure 21. Typical analog dimming waveforms are shown below in

Figure 22.

Figure 21. Analog Mode Dimming Duty Cycle vs. DIM voltage

Figure 22. Analog Dimming Mode Waveforms

Product Folder Links: LM3431