FP6745CS6C datasheet(5/7 Pages) FITIPOWER

5 / 7 page

FP6745C-0.1-SEP-2011

FP6745C

85T

fitipower integrated technology lnc.

Applications Information

Operation

The FP6745C is designed in a current mode,

fixed-frequency

pulse-width

modulation

(PWM)

step-up converter to drive up to 10 series-connected

WLEDs.

The FP6745C operates well with a variety

of external components.

See the following sections

to optimize external components for a particular

application.

Inductor Selection

For

most

applications,

4.7

μH to 15μH is

recommended for general used.

The inductor

parameters, current rating, DCR and physical size,

should be considered.

The DCR of inductor affects

the efficiency of the converter.

The inductor with

lowest DCR is chosen for highest efficiency.

The

saturation current rating of inductor must be greater

than the switch peak current, typically 1.33A.

These factors affect the efficiency, output load

capability, output voltage ripple, and cost.

The inductor selection depends on the switching

frequency and current ripple by the following formula:





















OUT

Where fSW is the 1.3MHz switching frequency.

∆I

L is the inductor ripple current.

Capacitor Selection

The

ceramic

capacitor

ideal

for

FP6745C

application.

X5R or X7R types are recommended

because they hold their capacitance over wide

voltage and temperature ranges than other Y5V or

Z5U types.

The input capacitor can reduced peak

current and noise at power source.

The output

capacitor is typically selected based on the output

voltage ripple requirements.

For most applications,

a 10

μF input capacitors with a 0.47μF output

capacitor are sufficient for general used.

A higher

or lower capacitance may be used depending on the

acceptable noise level.

LED Current Setting

The LED current is specified by resistor from the FB

pin to ground.

In order to have accurate LED

current, precision resistors are preferred (1% is

recommended).

The

LED

current

can

programmed by: ILED=300mV / RFB

Over Voltage Protection

The FP6745C has an internal open-lamp protection

circuit.

In the cases of output open circuit, when

the LEDs are disconnected from the circuit or the

LEDs fail open circuit, the over-voltage function

monitors the output voltage through SW pin to

protect the converter against.

The LED strings

open

will cause N-MOS to switch with a maximum

duty cycle and come out output over-voltage.

This

may cause the SW voltage exceed its maximum

rating then damage built-in N-MOS.

In the state,

the OVP protection circuitry will be triggered if

output voltage exceeds 41V (typ.).

The FP6745C

will then stop switching and latch off.

The

FP6745C

will

automatically

recover

normal

operation until EN input is recycled or the VIN is

re-started.

Dimming Control

There are two different types of dimming control

circuits.

The

LED

current

can

set

modulating the EN pin with a DC voltage or a PWM

signal.

(1) DC Voltage

The dimming control uses a DC voltage ranging

from 0.7V to 1.4V which results in regulating FB

voltage from 0V to 300mV, respectively.

(2) PWM Signal

Changing the LED forward current not only changes

the intensity of the LEDs, but also changes the

color.

Controlling the intensity of the LEDs with a

direct PWM signal allows dimming of the LEDs

without changing the color.

Dimming the LEDs via a PWM signal essentially

involves turning the LED on and off.

The LEDs

operate at either zero or full current.

The

amplitude of the PWM signal should be higher than

the minimum EN dimming voltage (typically 1.4V).

The LED average current increases proportionally

with the duty cycle of the PWM signal.

The

recommended duty cycle range is from 5% to100%.

The color of the LEDs remains unchanged since the

LED current value is either zero or a constant value.

The dimming frequency of the PWM signal can up

to 50kHz and still retain well linearity.

To avoid

audio noise, dimming frequency greater than 20kHz

is recommended.

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