LT3476 Просмотр технического описания (PDF) - Linear Technology

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LT3476

High Current Quad Output LED Driver

Linear Technology 

LT3476

Applications Information

Layout Hints

The high speed operation of the LT3476 demands careful

attention to board layout. Several items are worthy of note.

The exposed pad of the package is the only GND terminal

of the IC and is also important to thermal management

for the IC, so it is crucial to achieve a good electrical and

thermal contact between the exposed pad and the ground

plane of the board. Also, the Schottky rectifier and the

capacitor between GND at the cathode of the Schottky

are in the high frequency switching path where current

flow is discontinuous. These elements should be placed

so as to minimize the path between SW and the GND of

the IC. To reduce EMI, it is important to minimize the area

of the SW trace. Use a GND plane under SW to minimize

interplane coupling to sensitive signals. To obtain good

current regulation accuracy and eliminate sources of

channel-to-channel coupling, the CAP and LED inputs of

each channel of the LT3476 should be run as separate lines

back to the terminals of the appropriate sense resistor.

Since there is a small DC input bias current (~50µA) to

the LED and CAP inputs, resistance in series with these

inputs should be minimized, otherwise there will be an

offset. Finally, the bypass capacitor on the VIN supply to

the LT3476 should be placed as close as possible to the

VIN terminal of the device.

Open-Circuit Protection/Overvoltage Lockout

The LT3476 has independent internal overvoltage/open-

circuit protection (OVP) for all four converters, sensed

through their respective CAP inputs. The purpose of the

OVP feature is to protect the main switch of the device

from damage. In the boost configuration, if the LEDs are

disconnected from the circuit or fail open, the converter

output voltage at CAP is clamped at the OVP voltage of

35V (typ). Figure 1 shows the transient response of the

step-up converter application with LED1 disconnected.

With LED1 disconnected, the converter switches at cur-

rent limit as the output ramps up to OVP. Upon reaching

the OVP clamp voltage, the converter will switch with a

reduced current limit to regulate the converter output

voltage at the OVP clamp. In the buck mode application

shown in the Block Diagram, should the external supply

for CAP exceed the OVP clamp, then switching will be

inhibited for the converter. In order for the overvoltage

protection feature to adequately protect the switch, it is

important that the CAP input sample a voltage at or near

the highest voltage reached by the SW node. As a result,

this OVP function will not provide adequate protection

from open load events in isolated power configurations

such as the 1:1 flyback, since input and output voltage

magnitudes must be summed to obtain the voltage seen

by the switch.

35V

V(CAP)

20V

LED

DISCONNECT

HERE

I(SW)

1A/DIV

0A

20µs/DIV

3476 F01

Figure 1. LED Disconnect Transient

Setting the Switching Frequency

The switching frequency of the LT3476 is set by an exter-

nal resistor connected between the RT pin and GND. Do

not leave this pin open. Also, do not load this pin with a

capacitor. A resistor must always be connected for proper

operation. See Table 1 below or see the Oscillator Frequency

vs RT graph in the Typical Performance Characteristics for

resistor values and corresponding switching frequencies.

Table 1. Switching Frequency vs RT

SWITCHING FREQUENCY (kHz)

200

400

1000

1200

2000

RT (kΩ)

140

61.9

21

16.2

8.25

In general, a lower switching frequency should be used

where either very high or very low switch duty cycle opera-

tion is required, or higher efficiency is desired. Selection

of a higher switching frequency will allow use of smaller

value external components and yield a smaller solution

size and profile. Also for high frequency PWM dimming,

a higher switching frequency (shorter switching period)

will give better dimming control since for turning on the

3476fb

7

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