LT3475IFE-PBF データシートの表示（PDF） - Linear Technology

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LT3475IFE-PBF

Dual Step-Down 1.5A LED Driver

Linear Technology 

LT3475/LT3475-1

APPLICATIONS INFORMATION

Open Circuit Protection

The LT3475 has internal open-circuit protection. If the LED

is absent or is open circuit, the LT3475 clamps the voltage

on the LED pin at 14V. The switching regulator then oper-

ates at a very low frequency to limit the input current. The

LT3475-1 has no internal open circuit protection. With the

LT3475-1, be careful not to violate the ABSMAX voltage of

th BOOST pin; if VIN > 25V, external open circuit protection

circuitry (as shown in Figure 1) may be necessary.The

output voltage during an open LED condition is shown in

the Typical Performance Characteristics section.

Undervoltage Lockout

Undervoltage lockout (UVLO) is typically used in situations

where the input supply is current limited, or has high source

resistance. A switching regulator draws constant power

from the source, so the source current increases as the

source voltage drops. This looks like a negative resistance

load to the source and can cause the source to current limit

or latch low under low source voltage conditions. UVLO

prevents the regulator from operating at source voltages

where these problems might occur.

An internal comparator will force the part into shut-

down when VIN falls below 3.7V. If an adjustable UVLO

threshold is required, the SHDN pin can be used. The

threshold voltage of the SHDN pin comparator is 2.6V. An

internal resistor pulls 9μA to ground from the SHDN pin

at the UVLO threshold.

Choose resistors according to the following formula:

2.6V

R2 =

VT H

– 2.6V

R1

– 9μA

VTH = UVLO Threshold

Example: Switching should not start until the input is

above 8V.

VTH = 8V

R1=100k

R2 =

2.6V

8V – 2.6V

100k

–

9μA

=

57.6k

8

OUT

10k

22V

VC

100k

3475 F01

Figure 1. External Overvoltage Protection

Circuitry for the LT3475-1

VIN

VIN

R1

SHDN

C1 R2

2.6V

9μA

GND

LT3475

VC

3475 F02

Figure 2. Undervoltage Lockout

Keep the connections from the resistors to the SHDN pin

short and make sure the coupling to the SW and BOOST

pins is minimized. If high resistance values are used, the

SHDN pin should be bypassed with a 1nF capacitor to

prevent coupling problems from switching nodes.

Setting the Switching Frequency

The LT3475 uses a constant frequency architecture that

can be programmed over a 200kHz to 2MHz range with a

single external timing resistor from the RT pin to ground.

A graph for selecting the value of RT for a given operating

frequency is shown in the Typical Applications section.

Table 1. Switching Frequencies

SWITCHING FREQUENCY (MHz)

RT (kΩ)

2

4.32

1.5

6.81

1.2

9.09

1

11.8

0.8

16.9

0.6

24.3

0.4

40.2

0.3

57.6

0.2

100

3475fb

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