AL3050

Document number: DS38578 Rev. 2 - 2

8 of 16

www.diodes.com

July 2016

© Diodes Incorporated

AL3050

Open LED Protection

Application Information

1. General Operation

The AL3050 is a high efficiency boost converter with integrated power diode in a small package size. The device is ideal for driving white LED in

series. The serial LED connection provides even illumination by sourcing the same output current through all LEDs, eliminating the need for

expensive factory calibration. The device integrates a 30V/0.7A low side switch MOSFET and a 30V power diode, and operates in pulse width

modulation (PWM) with 750 kHz fixed switching frequency. For operation see the block diagram. The duty cycle of the converter is set by the error

amplifier output and the current signal applied to the PWM control comparator. The control architecture is based on traditional current-mode

control; therefore, slope compensation is added to the current signal to allow stable operation for duty cycles larger than 50%. The feedback loop

regulates the FB pin to a low reference voltage (200mV typical), reducing the power dissipation in the current sense resistor.

2. Soft Start-up

Soft-start circuitry is integrated into the IC to avoid a high inrush current during start-up. After the device is enabled, the voltage at FB pin ramps up

slowly to the reference voltage in 32 steps with each step taking 341

μs. This ensures that the output voltage rises slowly to reduce the input

current. Additionally, during the start up process, the current limit of the switch is set to half of the normal current limit spec. During this period, the

input current is kept below 360mA (typical).

3. Open LED Protection

Open LED protection circuitry prevents IC damage as the result of white LED disconnection. The AL3050 monitors the voltages at the VOUT pin

and FB pin. The circuitry turns off the switch MOSFET and shuts down the IC completely if both of the following two conditions are met: 1) the

VOUT voltage reaches OVP threshold (28.2V typical), 2) FB voltage is lower than half of its regulation voltage. This means the LED string is open

or the FB pin is short to ground. During open LED condition, the cycle by cycle switch current limit is set to be 0.3A typically to protect external

devices. As a result, the output voltage falls to the level of the input supply. The device remains in shutdown mode until it is enabled by pulling

down the CTRL pin logic low for at least 3.5ms and then pulling it high.

4. Shutdown

The AL3050 enters shutdown mode when the CTRL voltage is logic low for more than 3.5ms. During shutdown, the input supply current for the

device is less than 1μA (max). Although the internal MOSFET does not switch in shutdown mode, there is still a DC current path between the input

and the LEDs through the inductor and the power diode. The minimum forward voltage of the LED array must exceed the maximum input voltage

to ensure that the LEDs remain off in shutdown. In the typical application with two or more LEDs, the forward voltage is large enough to reverse

bias the diode and keep leakage current low.

5. Current Setting

The FB voltage is regulated by a low 0.2V reference voltage. The LED current is programmed externally using a current-sense resistor in series

with the LED string. The LED current can be calculated by Equation 1:

SET

FB

LED

R

V

I



(1)

Time 10

s /div

200mV/div

10V/div

200mA/div

20V/div