AL9910/AL9910A/AL9910-5

UNIVERSAL HIGH VOLTAGE HIGH BRIGHTNESS LED DRIVER

AL9910/AL9910A/AL9910-5

Document number: DS35103 Rev. 7 - 2

9 of 16

www.diodes.com

February 2012

© Diodes Incorporated

Inductor Selection

The non-isolated buck circuit, Figure 1, is usually selected and it 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 each diode has a forward voltage drop of 3.0V at its nominal current; the total LED voltage

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_D pin to ground will turn off the AL9910. When

disabled, the AL9910’s quiescent current is typically 0.5mA (0.65 for AL9910A). 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_D 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_D pin. The LED current is proportional to the

PWM duty cycle and the light output can be adjusted between zero and 100%.. The PWM signal enables and disables the

AL9910 - 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

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.