ACT8828 datasheet(35/49 Pages) ACTIVE-SEMI

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STEP-UP DC/DC CONVERTER

ACT8828

Rev 3, 24-Sep-09

Active-Semi

Innovative Power

- 35 -

www.active-semi.com

ActivePMU

TM and ActivePathTM are trademarks of Active-Semi.

2CTM is a trademark of Philips Electronics.

FUNCTIONAL DESCRIPTION

General Description

REG4 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 REG4

REG1 is typically enabled and disabled using the

ACT8828's

closed-loop

enable/disable

control

scheme, including the nPBIN input. Refer to the

System Startup and Shutdown section for more

information about this function.

Each regulator is enabled when the following

conditions are met:

1) nPBIN is pushed low via 100kΩ resistance,

2) ON4 is asserted high to enable REG4,

3) REG4/ON4[ ] is set to 1 when ON4 is high

When none of these conditions are true, REG4 is

disabled, and each regulator’s quiescent supply

current drops to less than 1μA.

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

converters, REG4’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

particularly when using fewer than three WLEDs.

Over-Voltage Protection

REG4 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, REG4 will

regulate the top of the LED strong to the OVP

threshold voltage. By default, the ACT8828’s OVP

threshold is set at 28.5V, although it may be

programmed to a lower value by writing to the

REG4/VSET[ ] register.

Power-OK Bit

REG4 features a Power-OK status bit that can be

read by the system microprocessor via the I2C

interface. If the voltage at OV is greater than the

OVP threshold, REG4/OK[ ] will clear to 0.

Compensation and Stability

REG4 utilizes current-mode control and an internal

compensation

network

optimize

transient

performance, ease compensation, and improve

stability over a wide range of operating conditions.

REG4 is a flexible regulator, and its external

components can be chosen to achieve the smallest

possible footprint or to achieve the highest possible

efficiency.

Inductor Selection

REG4

was

designed

provide

excellent

performance 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 possible efficiency.

Optimizing for Smallest Footprint

REG4 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

REG4 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) operation and

achieves very small inductor ripple current.

Output Capacitor Selection

REG4 was designed to operate with output

capacitors 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,

needed.

Ceramic

capacitors

are

recommended for most applications.

Rectifier Selection

REG4 requires a Schottky diode to rectify the

inductor 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.

ACT8828 Datasheet(PDF) 35 Page - Active-Semi, Inc

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