FP6745 datasheet(7/11 Pages) FITIPOWER

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FP6745-1.4-AUG-2013

FP6745

85T

fitipower integrated technology lnc.

Applications Information

Operation

The FP6745 is designed in a current mode,

fixed-frequency

pulse-width

modulation

(PWM)

step-up converter to drive up 10 series-connected

WLEDs.

The FP6745 operates well with a variety of

external components.

See the following sections to

optimize

external

components

for

particular

application.

Inductor Selection

For most applications, a 4.7

μH to 15Μh inductor is

recommended for general use.

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:

Where fSW is the 1.3MHz switching frequency.

IL is the inductor ripple current.

Capacitor Selection

The

ceramic

capacitor

ideal

for

FP6745

application.

X5R or X7R types are recommended

because they hold their capacitance over wide

voltage and temperature ranges better 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 use.

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=195mV/RFB

Over Voltage Protection

The FP6745 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 will

monitor the output voltage through SW pin to

protect the converter.

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 42V (typ.).

The FP6745 will then

stop switching and latch off.

The FP6745 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 195mV respectively.

(2) PWM Signal

Changing the LED forward current not only changes

the intensity of the LEDs but also 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 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.

The PWM dimming on time

(TON) must be greater than 1.5μs.

For example,

when dimming frequency is 40KHz, the cycle period

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