Circuit description

AN2983

18/32

Doc ID 15679 Rev 1

And then:

Equation 49

On the demonstration board several measures have been taken with Vin = 48 V, and

Rb = 1 k

Ω.

Figure 13

is the plot of the LED current as a function of the voltage on the LED string (VLed)

with Ra (actually R17 + R20) as a parameter.

Figure 13.

I_Led/VLed

2.6

Shutdown/dimming

The D_Dim input (pin 1 of the J1 connector) has a dual function.

●

Module enable

●

Digital (PWM) dimming

Forcing it low, or leaving it floating, enables the controller's normal activity (current sink),

while pulling it high (> 2.7 V) causes the module to shut down. Therefore, this input can be

used to switch the load on and off.

Figure 14

and Figure 15 highlight the turn-on and turn-off with an input voltage (Vin) of 48 V

and an equivalent output voltage (VLed) of 20 V. In these conditions, the rise and fall times

of the load current are lower than 10 µS.

Applying a square wave to this pin forces the module to 'work and stop' at the input signal

frequency; in this way the average load current can be modified by simply changing the duty

cycle of the PWM control.

Supposing a dimming frequency of 200 Hz is used, and taking into account the 10 µS of

rise/fall time of the LED current, we can estimate the minimum dimming as

(20 uS)*(200 Hz) = 0.4%.

Figure 16

, 17, 18, 19, and 20 show the LED current for different dimming factors between

1% and 99%.

(

)

10

*

985

.

0

10

*

68

.

1

Rb

Ra

6

4

=

=

⎟

⎠

⎞

⎜

⎝

⎛

−

−