RT8487

Copyright © 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS8487-00

March

2015

www.richtek.com

9

Application Information

RT8487 is a boundary mode, high efficiency constant

current controller with internal high side driver, which

can be used in buck and buck-boost configuration, to

provide a constant output current to the (LED) load. It

contains special circuitry for achieving high power

factor and low input current THD, while minimizing

external

component

count.

The

small

SOT23-6

package keeps application footprint small, and makes

RT8487 a cost effective solution for off-line LED

drivers.

The RT8487 can achieve high accuracy LED output

current via the average current feedback loop control.

The internal sense voltage (250mV typ.) is used to set

the average output current.

The average current is

set by the external resistor, RS.

The sense voltage is

also used for over current protection(OCP) function.

The typical OCP threshold is about seven times of the

sense voltage threshold.

Under Voltage Lockout (UVLO)

The RT8487 includes a UVLO function with 10.8V

hysteresis.

For system start up, the VIN must rise over 18V (typ.)

to turn on the GATE terminal.

The GATE terminal will turn off if VIN falls below 7.2V

(typ.)

Setting Average Output Current

The output current that flows through the LED string is

set by an external resistor, RS, which is connected

between

the GND and SENSE pins. The relationship between





OUT

S

250

I

=

mA

R

Start-Up Resistor

The start-up resistor should be chosen to set the start

up current exceeds certain minimum value. Otherwise,

the RT8487 may latch off and the system will never

start. The start-up current equals





2 90V / R1 +R2



(for 110VAC regions), and equals





2 180V / R1 +R2



(for 220VAC regions). The typical required minimum

start-up current is 100

A. The typical total start up

resistance (R1 + R2) is around 1M Ohm for universal

inputs.

Input Diode Bridge Rectifier Selection

The current rating of the input bridge rectifier is

LED current. The voltage rating of the input bridge

calculated as below :





BR

AC(MAX)

V

= 1.2

2

V



and the parameter 1.2 is used for safety margin.

For this example :



 



BR

AC(MAX)

V

= 1.2

2

V

= 1.2

2

264 = 448V









this case, a 600V, 0.5A bridge rectifier can be chosen.

Input Capacitor Selection

For High Power Factor application, the input Capacitor

line voltage sine-wave.

should be large enough to handle the input voltage.



 



IN

AC(MAX)

V

1.2

2

V

= 1.2

2

264 = 448V









