APW7008_12 datasheet(13/22 Pages) ANPEC

13 / 22 page

Rev. A.4 - Jun., 2009

APW7008

www.anpec.com.tw

Function Description

Soft-Start

The APW7008 provides the soft-start function to limit the

inrush current during start-up. When the input voltage is

supplied to the device and exceeds the UVLO voltage,

the output capacitor is charged directly from input with a

limited current source. Approximate 100

µs after the out-

put voltage approaches the input voltage, the device starts

to provide the programmed LED current and determines

which of 1x and 1.5x, or 2x mode is required. When the

programmed LED current can be reached with 1x mode,

the soft-start is completed and the device operates in 1x

mode. When the programmed LED current cannot be

reached, the charge pump goes into 1.5x mode. If the

1.5x mode charge pump cannot suffice for the LED cur-

rent need, the charge pump will switch to 2x mode.

Mode Transition

The APW7008 operates in 1x, 1.5x, and 2x charge pump

modes and automatically switches the charge pump

modes depend on the input voltage to maintain the re-

quired power for high power efficiency. If the APW7008

operates in 1x mode, the VOUT is pulled up to the VIN.

When the VIN decreases, the VILED will decease to main-

tain the regulated LED current. Until the VILED is below

100mV, the device will switch to 1.5x mode. In 1.5x mode,

the VILED is regulated to 0.18V, and the output voltage is

VF+0.18V. If the VIN continues to decrease until the VILED

is below 100mV again, the device will switch to 2x mode.

When the VIN rises and reaches by approximately VOUT-

300mV, the APW7008 switches back to 1.5x mode. If

the VIN continues to rise and reach by approximately

VOUT+300mV, the APW7008 switches back to 1x mode.

The 2x charge pump is enough to suffice the White LED

for a Li-ion battery application. The APW7008 ensures

that in the 1x mode for as long as possible to increase

the efficiency and extend the operating range by using

the 2x mode. The transition voltages from 1x to 1.5x and

1.5x to 2x are given by:

Control Logic Pins

The APW 7008 provides three logic input pins to en-

able or disable a combination of LEDs. Table1 shows

the truth table of the logic pins. If the LED channels are

not used, connecting the ILED pins to VOUT to turn off

the respective LED channels.

Control Logic Pin

LED Status

CTRL2

CTRL1

CTRL0

LED4

LED3

LED2

LED1

OFF

Table 1. The Truth Table of Control Logic Pins

LED Current Setting

Connect a resistor from ISET pin to GND to set the LED

current. The ISET voltage is typically 1.2V, the LED cur-

rent is typically 400 times the current through the ISET

resistor. The LED current is given by:

where VF is the forward voltage of LED

IOUT is the output current

ROUT1X is the output impedance in 1x mode = 1.6

Ω

ROUT1.5X is the output impedance in 1.5x mode = 7

Ω

The APW 7008 provides up 30mA of LED current per

LED and the device has a max current matching of

±5%

between any two LED currents and a max current accu-

racy of

±8%. If high accuracy is required, using a 1%

precision surface mount resistor for the need.

ILED (mA)

RSET (k

Ω)

16.5

VTRANS1X = VF + 0.1V + (IOUT x ROUT1X)

VTRANS1.5X = [VF + 0.1V + (IOUT x ROUT1.5X)] / 1.5

Table 2. RSET Value Selection

LED

SET

400

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