AL9901

Document number: DS37713 Rev. 1 - 2

11 of 20

www.diodes.com

March 2015

© Diodes Incorporated

AL9901

Applications Information

The AL9901 is capable of operating in isolated or non-isolated topologies. It can also be made to operate in continuous as well as discontinuous

conduction mode.

Figure 1 Functional Block Diagram

The AL9901 contains a high voltage LDO (see figure 1) the output of the LDO provides a power rail to the internal circuitry including the gate

In a non-isolated Buck LED driver, when the gate pin goes high, the internal power MOSFET (Q1) is turned on causing current to flow through the

MOSFET Q1 is turned off. The energy stored in the inductor causes the current to continue to flow through the LEDs via diode D1.

The AL9901’s LDO provides all power to the rest of the IC including Gate drive, and this removes the need for large, high-power start-up resistors.

This means that during normal operation the AL9901 requires around 0.5mA from the high voltage power rail. The LDO can also be used to

supply up to 1mA to external circuits.

The AL9901 operates and regulates by limiting the peak current of the internal MOSFET; the peak current sense threshold is nominally set at

250mV. The AL9901 is capable of operating in a fixed frequency (PWM) mode and also variable frequency (fixed off-time) mode to regulate the

LED current.

The same basic operation is true for isolated topologies; however in these the energy stored in the transformer delivers energy to LEDs during the

off-cycle of the internal MOSFET.

The on-resistance of the AL9901’s internal power MOSFET means that it can drive up to 2A.

Design Parameters

Setting the LED Current

In the non-isolated buck converter topology, figure 1, the average LED current is not the peak current divided by two - however, there is a certain

error due to the difference between the peak and the average current in the inductor. The following equation accounts for this error:

)

*

5

.

0

(

250mV

R

SENSE

RIPPLE

LED

I

I

+

=