ZXLD1100 데이터 시트보기 (PDF) - Diodes Incorporated.

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ZXLD1100

ADJUSTABLE LED DRIVER WITH INTERNAL SWITCH AND OPEN CIRCUIT PROTECTION IN SC70-6

Diodes Incorporated. 

ZXLD1100

APPLICATIONS

Programming the maximum LED current

The maximum LED current is programmed by adding a

single resistor in series with the LED chain. The current

is determined by the resistor value and feedback

voltage and is given by:

ILED = VFB/R1

where VFB=100mV

The table below gives recommended resistor values

for required LED currents:

Dimming Control using a DC voltage

For applications where the EN pin is not available a DC

voltage can be used to control dimming. By adding

resistors R2 and R3 and applying a DC voltage, the LED

current can be adjusted from 100% to 0%. As the DC

voltage increases, the voltage drop across R2 increases

and the voltage drop across R1 decreases, thus

reducing the current through the LEDs. Selection of R2

and R3 should ensure that the current from the DC

voltage is much less than the LED current and much

larger than the feedback current. The component

values in the diagram below represent 0% to 100%

dimming control from a 0 to 2V DC voltage.

LED Current

10mA

15mA

20mA

30mA

R1 Value

10⍀

6.8⍀

5⍀

3.3⍀

Dimming Control via a PWM signal on the EN pin

A Pulse Width Modulated (PWM) signal can be applied

to the EN pin in order to adjust the output current to a

value below the maximum LED current. Two modes of

adjustment are possible as described below.

True Analog Dimming - Filtered ‘DC’ mode

If a PWM signal of 10kHz or higher is applied to the EN

pin, the device will remain active when the EN pin is

low. However, the input to the internal low pass filter

will be switched alternately from VREF to ground, with a

duty cycle (D) corresponding to that of the PWM signal.

This will present a filtered dc voltage equal to the duty

cycle multiplied by VREF to the control loop and will

produce a dc output current lower than the maximum

set value. This current is given by:

Dimming Control using a logic signal

For applications where the LED current needs to be

adjusted in discrete steps a logic signal can be applied

as shown in the diagram below. When Q1 os ‘off’, R1

sets the minimum LED current. When Q1 is ‘on’, R2 sets

the LED current that will be added to the minimum LED

current. The formula for selecting values for R1 and R2

are given below:

IOUTdc = 0.1D/R1

This mode of adjustment minimizes flicker in the light

output and system noise.

Pulsed Dimming - Gated Mode

If a lower frequency of 1kHz or less is applied to the EN

pin, the device will be gated ‘on’ and ‘off’ at a duty cycle

(D) corresponding to that of the input signal. The

average output current is then given by:

IOUTavg » 0.1D/R1

This mode may be preferred over dc current control if

the purest white output is required. However, note the

120␮s nominal turn-off delay of the device, when using

the device in this mode.

MOSFET ‘off’

I LED( MIN )

V FB

RLED

MOSFET ‘on’

I LED( MAX )

V FB

RLED

I LED( MIN )

where VFB = 100mV

ISSUE 4 - JULY 2004

SEMICONDUCTORS

8

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