FAN7340 データシートの表示（PDF） - Fairchild Semiconductor

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FAN7340

LED Backlight Driving Boost Switch

Fairchild Semiconductor 

Functional Description

The FAN7340 operates as a constant-current source for

driving high-current LEDs. It uses Current-Mode control

with programmable slope compensation to prevent sub-

harmonic oscillation.

The IC provides protections such as open-LED

protection, over-voltage protection, and over-current

protection for improved system reliability. The IC

internally generates a FAULT OUT signal with a delay in

case an abnormal LED string condition occurs. PWM

dimming and analog dimming functions can be

implemented independently. Internal soft-start prevents

inrush current flowing into output capacitor at startup.

Circuit operation is explained in the following sections.

VCC Under-Voltage Lockout (UVLO)

An internal regulator provides the regulated 5V used to

power the IC. The Under-Voltage Lockout (UVLO) turns

off the IC in the event of the voltage dropping below the

specific threshold level. The UVLO circuit inhibits

powering the IC until a voltage reference is

established ,up to predetermined threshold level.

Enable

Applying voltage higher than 1.22V (typical) to the ENA

pin enables the IC. Applying voltage lower than 1.15V

(typical) to the ENA pin disables the IC. If ENA pin

voltage is higher than 1.22V (typical) and VCC is higher

than 9.0V (typical.), the IC starts to supply 5V reference

voltage from VCC.

Oscillator (Boost Switching Frequency)

Boost switching frequency is programmed by the value

of the resistor connected from the RT pin to ground. RT

pin voltage is set to 2V. The current through the RT pin

resistor determines boost switching frequency according

to formula:

f

.

kHz

(1)

Soft-Start Function at Startup

During initial startup, the switching device can be

damaged due to the over-current coming from the input

line by the negative control. This can result in the initial

overshoot of the LED current. Therefore, during initial

startup, the soft-start control gradually increases the duty

cycle so that the output voltage can rise smoothly to

control inrush current and overshoot.

FAN7340 adapts the soft-start function in the boost

converter stage. During soft-start period, boost switch

turn-on duty is limited by clamped CMP voltage. The

soft-start period is dependent on boost switching

frequency, which is decided by the RT resistor (Equation

1). Soft-start period is set to be cumulative time when the

BDIM (PWM dimming) signal is HIGH:

T 600 / f sec

(2)

Figure 26. Soft-Start Waveforms

LED Current Setting

During the boost converter operating periods, the output

LED current can be set by equation:

I

ADIM V

R

60mΩ

(3)

where ADIM(V) is

ADIM pin applied voltage and, RSENSE is the sensing

resistor value. An additional 60mΩ comes from an

internal wire bonding resistor. To calculate LED

current precisely, consider the wire bonding resistor.

Analog Dimming and PWM Dimming

Analog dimming is achieved by varying the voltage level

at the ADIM pin. This can be implemented either with a

potentiometer from the VREF pin or from an external

voltage source and a resistor divider circuit. The ADIM

voltage level is adjusted to be the same as the feedback

level (VSENSE). A VADIM range from 0.3V to 3V is

recommended.

PWM dimming (BDIM) helps achieve a fast PWM

dimming response in spite of the shortcomings of the

boost converter. The PWM dimming signal controls three

nodes in the IC; gate signal to the switching FET, gate

signal to the dimming FET, and output connection of the

trans-conductance amplifier. When the PWM dimming

signal is HIGH, the gates of the switching FET and

dimming FET are enabled. At the same time, the output

of the transconductance ap-amp is connected to the

compensation network. This allows the boost converter

to operate normally.

Dynamic Contrast Ratio

The Dynamic Contrast Ratio (DCR) means the maximum

contrast ratio achievable by adjusting the amount of light

(dimming) of the screen instantaneously using the

backlight during the extremely short period of time.

FAN7340 can normally drive the LED backlight under

0.1% dimming duty cycle at 200Hz dimming frequency.

Even operating at 5µs-dimming FET turn-on time and

extremely low dimming duty, FAN7340 can operate

LEDs with normal peak current level.

© 2012 Fairchild Semiconductor Corporation

FAN7340 • 1.0.0

11

www.fairchildsemi.com

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