FP6737 datasheet(8/12 Pages) FITIPOWER

8 / 12 page

FP6737/A-1.6-DEC-2011

FP6737/A

85T

fitipower integrated technology lnc.

Application Information

Inductor Selection

A 10

μ inductor is recommended for most FP6737/A

applications.

Although

small

size

and

high

efficiency are major concerns, the inductor should

have low core losses at 1.2MHz and low DCR

(copper wire resistance).

Capacitor Selection

The small size of ceramic capacitors makes them

ideal for FP6737/A applications.

X5R and X7R

types are recommended because they retain their

capacitance over wider voltage and temperature

ranges better than other types such as Y5V or Z5U.

A 1μF input capacitor and a 0.47μF output capacitor

are sufficient for most FP6737/A applications.

Diode Selection

The Schottky diodes, with their low forward voltage

drop and fast reverse recovery, are the ideal choices

for FP6737/A applications.

The forward voltage

drop of a Schottky diode represents the conduction

losses in the diode while the diode capacitance (CT

or CD) represents the switching losses.

For diode

selection, both forward voltage drop and diode

capacitance need to be considered.

Schottky

diodes with higher current ratings usually have lower

forward voltage drop and larger diode capacitance,

which can cause significant switching losses at the

1MHz switching frequency of the FP6737/A.

Schottky diode rated at 100mA to 400mA is sufficient

for most FP6737/A applications.

LED Current Control

The LED current is controlled by the feedback

resistor (R1 in Figure 20).

The FP6737 feedback

reference is 250mV and the FP6737A feedback

reference is 104mV.

The LED current is VFB/R1.

In order to have accurate LED current, precision

resistors are preferred (1% is recommended).

The

formula and table for R1 selection are shown as

below.

Version

Feedback Reference

Voltage VFB(V)

LED Current

Setting ILED(A)

FP6737

0.25

0.25/R1

FP6737A

0.104

0.104/R1

Table 1. R1 Resistor Value Selection

Over Voltage Protection

The FP6737/A 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 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 exceeds its maximum

rating then damages built-in N-MOS.

In the state,

the OVP protection circuitry will be triggered if

output voltage exceeds 25V (min.).

The FP6737/A

will then stop switching.

The FP6737/A will

automatically recover normal operation until SW is

under 24V (1V hysteresis).

FP6737/A

VIN

SHDN

GND

COUT

0.22uF

Ω

4.7

μH

VIN

CIN

1uF

PGND

OVP

Figure 20. LED Driver with Open-Circuit Protection

Dimming Control

The LED current can be modulated by a DC

voltage, PWM signal or a filtered PWM signal.

(1) Using a PWM Signal

With the PWM signal applied to the

pin, the

FP6737/A is turned on or off by the PWM signal.

The LEDs operate at either zero or full current.

The average LED current increases proportionally

with the duty cycle of the PWM signal.

A 0% duty

cycle will turn off the FP6737/A and corresponds to

zero LED current.

A 100% duty cycle corresponds

to full current.

The typical frequency range of the

PWM signal is 200Hz to 5kHz.

The magnitude of

the PWM signal should be higher than the minimum

voltage.

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