ACT8740 datasheet(24/40 Pages) ACTIVE-SEMI

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WLED BIAS DC/DC CONVERTER

ACT8740

Rev PrB, 25-Feb-08

Innovative Products. Active Solutions.

- 24 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

General Description

REG2 is a highly efficient step-up DC/DC converter

that employs a fixed frequency, current-mode,

PWM architecture. This regulator is optimized for

white-LED bias applications consisting of up to

seven white-LEDs.

Enabling and Disabling REG2

Enable/disable control of REG2 is achieved through

the ACT8740’s I2C serial interface. Enable REG2 by

setting REG2/ON[ ] to [1], disable REG2 by clearing

REG2/ON[ ] to [0]. When disabled, REG2’s quies-

cent supply current drops to just 1µA. As with all

non-synchronous

step-up

DC/DC

converters,

REG2’s application circuit produces a DC current

path between the input and the output in shutdown

mode. Although the forward drop of the WLEDs

makes this leakage current very small in most appli-

cations, it is important to consider the effect that this

may have in your application particularly when us-

ing fewer than three WLEDs.

Over-Voltage Protection

REG2 features internal over-voltage protection

(OVP) circuitry which protects the system from LED

open-circuit fault conditions. If the voltage at OV

ever reaches the over-voltage threshold, REG2 will

regulate the top of the LED strong to the OVP

threshold voltage. By default, the ACT8740’s OVP

threshold is set at 28.5V, although it may be pro-

grammed to a lower value by writing to the

REG2/VSET[ ] register.

Power-OK Bit

REG2 features a Power-OK status bit that can be

read by the system microprocessor via the I2C inter-

face. If the voltage at OV is greater than the OVP

threshold, REG2/OK[ ] will clear to 0.

Compensation and Stability

REG2 utilizes current-mode control and an internal

compensation network to optimize transient per-

formance, ease compensation, and improve stabil-

ity over a wide range of operating conditions. REG2

is a flexible regulator, and its external components

can be chosen to achieve the smallest possible

footprint or to achieve the highest possible effi-

ciency.

Inductor Selection

REG2 was designed to provide excellent perform-

ance across a wide range of applications, but was

optimized for operation with inductors in the 10µH

to 22µH range, although larger inductor values of

up to 68µH can be used to achieve the highest pos-

sible efficiency.

Optimizing for Smallest Footprint

REG2 is capable of operating with very low inductor

values in order to achieve the smallest possible

footprint. When solution size is of primary concern,

best results are achieved when using an inductor

that ensures discontinuous conduction mode (DCM)

operation over the full load current range.

Optimizing for Highest Efficiency

REG2 achieves excellent efficiency in applications

that demand the longest possible battery life. When

efficiency is the primary design consideration, best

results are achieved when using an inductor that

results in continuous conduction mode (CCM) op-

eration and achieves very small inductor ripple cur-

rent.

Output Capacitor Selection

REG2 was designed to operate with output capaci-

tors ranging from 0.47µF to 10µF, providing design

flexibility. A 1µF output capacitor is suitable for most

applications, although larger output capacitors may

be used to minimize output voltage ripple, if

needed. Ceramic capacitors are recommended for

most applications.

Rectifier Selection

REG2 requires a Schottky diode to rectify the induc-

tor current. Select a low forward voltage drop

Schottky diode with a forward current (IF) rating that

is sufficient to support the maximum switch current

and a sufficient peak repetitive reverse voltage

(VRRM) to support the output voltage.

Setting the LED Bias Current

The LED bias current is set by a resistor con-

nected from FB2 and ground, and the regulator is

satisfied when the LED current is sufficient to gen-

erate 250mV across this resistor. Once the bias

current is programmed, the LED current can be

FUNCTIONAL DESCRIPTION

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