© 2010 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FAN5776

• Rev. 1.1.0

11

Circuit Description

Overview

The FAN5776 is a 1.8 MHz synchronous step-up DC-DC

converter with integrated constant-current high-side LED

drivers capable of driving one to five LED strings up to 5 x

25 mA LED current.

The device starts when at least one LED string is utilized and

the appropriate EN pin is enabled. The device is disabled in

30 ms by setting both EN pins LOW.

LED string voltage, and it is sampled at every falling LED PWM

cycle. For 100% duty cycle, the LED-pin voltage is sampled

The LED strings can be disabled by connecting them to VOUT

or shorting them to GND. They can also be left disconnected. If

the LED string is temporarily disabled or shorted, the device

must be re-enabled to enable the string again.

The LED drivers work independently and allow multiple LED

voltages, such that many types of LEDs can be driven at the

same time and some strings can be used to drive a single LED

voltage is defined by the highest LED voltage and the LED

driver dropout voltage is increased to provide the LED string a

specific voltage. If the voltage difference between the LED

strings is large, the system efficiency may decrease.

LED Current

ISET and GND pins. The same current is applied to across all

In general, the LED string current can be calculated as follows:

500

(1)

Startup

The three different startup functions depend on the system

configuration:

1.

All LED strings are utilized: Setting one or both EN pins

FAN5776 starts to step up or down to the appropriate

regulated voltage.

2.

rises to 7.5 V while the shorted LED string is disabled and

the device starts to step up or down to regulated voltage.

3.

the device starts to step down to regulated voltage.

These functions work for each group independently. If all five

1) and goes to the highest voltage required by LED1-3. Then

regulates to highest voltage required by LED1-5.

the device stays disabled and a new enable cycle is required.

PWM Dimming

A LED PWM signal of 100 Hz to 800 Hz can be applied to

EN13 and EN45 pins to control LED1-3 and LED4-5 light

intensity. The LED current is a linear function of the LED

PWM duty cycle from 100% down to 0.4%. The FAN5776

can be started by a PWM signal with a low duty cycle to

enable smooth startup. EN13 and EN45 pins can be

operated either synchronously or asynchronously, which

makes it possible to use the device to backlight two

separate displays at the same time.

Under-Voltage Lockout (UVLO)

The Under-Voltage Lockout circuitry turns off all MOSFETs and

the device remains in a very low quiescent current state until

Short-Circuit Protection (SCP)

The LED driver output current is limited to 0.5 µA or less when

a LED number pin voltage is below 1.0 V. This limit shall be

applied within one LED PWM cycle, or 10 ms, whichever

elapses first.

Over-Voltage Protection (OVP)

When the regulator is active, it monitors the VOUT pin. If the

the capacitor at VOUT discharges below 8.5 V.

LED-Open Detection

are disabled. If all LED pin

greater than 9.0 V, device is disabled and a new startup cycle

is required.

Over-Current Protection (OCP)

The PWM converter is protected against overload through

cycle-by cycle current limit using a fixed internal limit.

Thermal Shutdown

When the die temperature exceeds 150°C, reset occurs and

remains in effect until the die cools to 125°C; at which time,

the circuit enters the normal soft-start sequence.

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