ADP1621ARMZ-R7 datasheet(28/32 Pages) AD

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ADP1621

Rev. A | Page 28 of 32

LED DRIVER APPLICATION CIRCUITS

The ADP1621 can be used as an LED driver. Two LED application

circuits are shown in Figure 41 and Figure 42, where each circuit

is driving 20 white LEDs in series. Each white LED has a typical

current of 150 mA at a typical forward voltage of 4.0 V, with a

maximum voltage of 4.5 V over the temperature range of −40°C

to +125°C.

Two methods for dimming the brightness of the LEDs are

shown in Figure 41 and Figure 42. In Figure 41, a PWM signal

is fed to the SDSN pin to turn the ADP1621 controller on and

off. As a result, the LED current is turned on and off, and the

average LED current is dependent on the PWM duty cycle. The

advantage of this method is that no current flows through the

LEDs during the PWM off cycle. In addition, when the ADP1621 is

on, the forward current through the LEDs is constant, which

guarantees constant color emission across the entire dimming

range. Because the soft start period is fixed at 2048 oscillator

cycles, the PWM frequency range is limited.

As shown in Figure 41, because the natural switching frequency

chosen is 400 kHz, the useful PWM frequency range is 90 Hz to

195 Hz. However, when driving fewer LEDs, the ADP1621 can

be set to run at a faster frequency, increasing the maximum PWM

frequency. The PWM duty cycle can be between 5% and 95%. A

higher PWM duty cycle produces a higher average LED current.

Another method for driving the LEDs is shown in Figure 42,

where the PWM signal is filtered by an RC low-pass filter and is

fed to the FB node. The effective FB voltage at the bottom of the

LED string is modulated in an analog manner by the PWM

duty cycle. Thus, the average current through the LEDs is

modulated accordingly. Unlike the case depicted in Figure 41, a

higher duty cycle produces a lower average LED current using

the filtered PWM scheme in Figure 42. The advantage of this

circuit is that the PWM frequency can be in the range between

90 Hz and 100 kHz, and the duty cycle can be between 5% and

95%. The disadvantage of this method is that the forward

current through the LEDs is directly modified to control the

brightness of the LEDs. Because the wavelength of the light

emitted from an LED is a weak function of its forward current,

perfect color purity across the entire dimming range cannot be

guaranteed.

If PCB space is a constraint, smaller inductors can be selected

for the circuits shown in Figure 41 and Figure 42. For example,

a 4.7 μH inductor can be used, and a 200 kHz switching fre-

quency can be selected. However, with this small inductor, the

system operates in DCM, which is slightly less efficient than

operating in CCM.

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