AL9902

Document number: DS37878 Rev. 1 - 2

13 of 18

www.diodes.com

July 2015

© Diodes Incorporated

AL9902

General Application

The AL9902 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.

VDD

LDO

OSC

OTP

S

R

250mV

200K

Rsense

7.5V

GND

VDD

LD

PWM

CS

SO

AL9902

Rosc

SW

Rosc

VIN

D1

VIN

Figure 4 Typical Application Circuit

The AL9902 contains a high-voltage LDO (see figure 4), 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 AL9902

’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 to during normal operation the AL9902 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 AL9902 operates and regulates by limiting the peak current of the internal MOSFET; the peak current sense threshold is nominally set at

250mV. The AL9902 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 AL9902

’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 4, the average LED current is not the peak current divided by 2 - 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:





))

I

*

5

.

0

(

I

mV

250

R

RIPPLE

LED

SENSE





.