FAN5345S30X 查看數據表（PDF） - Fairchild Semiconductor

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FAN5345S30X

Series Boost LED Driver with Single-Wire Digital Interface

Fairchild Semiconductor 

Circuit Description

Overview

The FAN5345 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 therefore 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.

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.

Driving Eight LEDs in Series

FAN5345S30X 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 in order to maintain stable operation.

Digital Interface

The FAN5345 implements a single-wire digital interface to

program the LED brightness to one of thirty-two (32) levels

spaced in linear steps. With this single-wire solution, the

FAN5345 does not require the system processor to constantly

supply a signal to drive the LEDs.

Digital Dimming Control

The FAN5345 starts driving the LEDs at the maximum

brightness level. After startup, the control logic is ready to

accept programming pulses to decrease the brightness level

by the number of positive edges applied to the EN pin.

Figure 28. Digital Pulse-Dimming Control Diagram shows the

digital pulse dimming control. The dimming control function

has no effect before soft-start finishes. The soft-start takes

about 2ms.

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

FAN5345S20X and FAN5345S30X 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 FAN5345S20X

OVP circuit disables the boost regulator when VOUT exceeds

20.0V and continues to keep the regulator off until VOUT drops

below 19.0V. For FAN5345S30X, the OVP is 30.0V and it

turns back on when VOUT is below 29.0V.

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.

Figure 28. Digital Pulse-Dimming Control Diagram

© 2011 Fairchild Semiconductor Corporation

FAN5345 • Rev. 1.0.0

10

www.fairchildsemi.com

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