ACT8712QLCHA-T datasheet(27/36 Pages) ACTIVE-SEMI

27 / 36 page

WLED BIAS DC/DC CONVERTER

Rev0, 25-Feb-08

ACT8712

Innovative Products. Active Solutions.

- 27 -

www.active-semi.com

ActivePMU

TM is a trademark of Active-Semi.

2CTM is a trademark of Philips Electronics.

General Description

REG3 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 REG3

Enable/disable control of REG3 is done through a

combination of the ACT8712’s system control cir-

cuitry, the ON2 pin, and the REG3/ON[ ] bit. REG3

may be enabled when at least one of the following

conditions exists:

1) A valid supply voltage is present at VIN

2) nMSTR is asserted low, or

3) ON1 is asserted high.

When any of these conditions exist, REG3 may be

enabled by either of the following conditions:

1) ON2 is asserted high, or

2) REG3/ON[ ] is set to [1].

When disabled, REG3's quiescent supply current

drops to just 1µA.

As with all non-synchronous step-up DC/DC con-

verters, REG3'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 applications, it is important to consider the

effect that this may have in your application particu-

larly when using fewer than three WLEDs.

Over-Voltage Protection

REG3 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, REG3 will

regulate the top of the LED strong to the OVP

threshold voltage. By default, the ACT8712's OVP

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

grammed to a lower value by writing to the

REG3/VSET[ ] register.

Power-OK Bit

REG3 feature a variety of status bits that can be

read by the system microprocessor. If the voltage at

OV is greater than the OVP threshold, REG3/OK[ ]

will clear to 0.

Compensation and Stability

REG3 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. REG3

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

REG3 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

REG3 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

REG3 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

REG3 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

FUNCTIONAL DESCRIPTION

ACT8712QLCHA-T Datasheet(PDF) 27 Page - Active-Semi, Inc

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