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FAN5346

Series Boost LED Driver with PWM Dimming Interface

Fairchild Semiconductor 

Circuit Description

Overview

The FAN5346 is an inductive current-mode boost serial

LED driver that achieves LED current regulation by

maintaining 0.250V across the RSET resistor. The current

through the LED string (ILED) is given by:

I LED

=

0.250

RSET

(1)

The voltage VOUT is determined by the sum of the forward

voltages across each LED, plus the voltage across RSET,

which is always 250mV.

Driving Eight LEDs in Series

FAN5346S30X can drive 8 LEDs in series, but the minimum

input voltage (VIN) must be greater than or equal to 2.9V,

while the forward voltage of the white LED should be less

than or equal to 3.2V, and the maximum LED current

cannot exceed 20mA to maintain stable operation.

UVLO and Soft-Start

If EN has been LOW for more than 1ms, the IC may

initiate a “cold start” soft-start cycle when EN rises,

provided VIN is above the UVLO threshold.

PWM Dimming

The FAN5346 uses a PWM signal to directly modulate

output current in the LED string to vary the perceived LED

brightness. When the EN pin is held HIGH, the FB voltage

is 250mV. This voltage is reduced when a PWM signal is

applied to the EN pin, thereby enabling the LEDs to be

dimmed. The FB voltage is given by the equation:

VFB = DutyCycle × 250mV

(2)

where DutyCycle = the duty cycle of the PWM signal

and 250mV is the internal reference voltage.

Figure 28 illustrates how the FAN5346 divides the internal

250mV reference voltage at the duty cycle of the PWM

signal. A low-pass filter filters the PWM signal, which then

is input into the error amplifier as the reference voltage for

the FB pin.

Figure 28. Block Diagram of FB and EN Circuit for

PWM Dimming

Over-Current and Short-Circuit Detection

The boost regulator employs a cycle-by-cycle peak inductor

current limit of 300mA (typical) and 750mA (typical) for

FAN5346S20X and FAN5346S30X, respectively.

Over-Voltage / Open-Circuit Protection

If the LED string is an open circuit, FB remains at 0V and the

output voltage continues to increase in the absence of an over-

voltage protection (OVP) circuit. The FAN5346S20X OVP

circuit disables the boost regulator when VOUT exceeds 20V

and continues to keep the regulator off until VOUT drops below

19V. For FAN5346S30X, the OVP is 30V and it turns back on

when VOUT is below 29V

Thermal Shutdown

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

remains in effect until the die cools to 115°C; at which time, the

circuit is allowed to begin the soft-start sequence.

© 2011 Fairchild Semiconductor Corporation

FAN5346 • Rev. 1.0.1

10

www.fairchildsemi.com

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