AL9902

Document number: DS37878 Rev. 1 - 2

14 of 18

www.diodes.com

July 2015

© Diodes Incorporated

AL9902

Setting Operating Frequency

The AL9902 is capable of operating between 25 and 450kHz switching frequency range. The switching frequency is programmed by connecting

25

22

R

µs

In buck mode the duty cycle, D, is

IN

LEDs

V

V

; so when driving small numbers of LEDs from high-input voltages, the duty cycle will be reduced and

value. Reducing the switching frequency will also improve the efficiency.

When operating in buck mode, the designer must keep in mind that the input voltage must be maintained higher than two times the forward

voltage drop across the LEDs. This limitation is related to the output current instability that may develop when the AL9902 operates at a duty cycle

Inductor Selection

The non-isolated buck circuit, Figure 4, is usually selected and has two operation modes: continuous and discontinuous conduction modes. A

buck power stage can be designed to operate in continuous mode for load current above a certain level usually 15% to 30% of full load. Usually

the input voltage range, the output voltage and load current are defined by the power stage specification. This leaves the inductor value as the

only design parameter to maintain continuous conduction mode. The minimum value of inductor to maintain continuous conduction mode can be

determined by the following example.

The required inductor value is determined from the desired peak-to-peak LED ripple current in the inductor; typically around 30% of the nominal

LED current.

L =









OSC

LED

LEDs

IN

f

I

3

.

0

D

V

V









, where D is duty cycle

The next step is determining the total voltage drop across the LED string. For example, when the string consists of 10 High-Brightness LEDs and

Dimming

The LED brightness can be dimmed either linearly (using the LD pin) or via pulse width modulation (using the PWM-D pin); or a combination of

both - depending on the application. Pulling the PWM pin to ground will turn off the AL9902. When disabled, the AL9902

’s quiescent current is

typically 0.5mA. Reducing the LD voltage will reduce the LED current but it will not entirely turn off the external power transistor and hence the

LED current

– this is due to the finite blanking period. Only the PWM pin will turn off the power transistor.

Linear dimming is accomplished by applying a 45 to 250mV analog signal to the LD pin. This overrides the default 250mV threshold level of the

CS pin and reduces the output current. If an input voltage greater than 250mV is applied to the LD then the output current will not change.

The LD pin also provides a simple cost-effective solution to soft start; by connecting a capacitor to the LD pin down to ground at initial power up,

the LD pin will be held low causing the sense threshold to be low. As the capacitor charges up the current sense threshold will increase thereby

causing the average LED current to increase.

PWM dimming is achieved by applying an external PWM signal to the PWM pin. The LED current is proportional to the PWM duty cycle and the

light output can be adjusted between 0 and 100%. The PWM signal enables and disables the AL9902 - modulating the LED current. The ultimate

accuracy of the PWM dimming method is limited only by the minimum gate pulse width, which is a fraction of a percentage of the low frequency

duty cycle. PWM dimming of the LED light can be achieved by turning on and off the converter with low frequency 50Hz to 1000Hz TTL logic level

signal.

With both modes of dimming it is not possible to achieve average brightness levels higher than the one set by the current sense threshold level of

Output Open Circuit Protection

The non-isolated buck LED driver topology provides inherent protection against an open circuit condition in the LED string due to the LEDs being

connected in series with the inductor. Should the LED string become open circuit then no switching occurs and the circuit can be permanently left

in this state with damage to the rest of the circuit.