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LT3474/LT3474-1

3474fd

APPLICATIONS INFORMATION

The switch driver operates either from VIN or from the

BOOST pin. An external capacitor and internal Schottky

diode are used to generate a voltage at the BOOST pin

that is higher than the input supply. This allows the driver

to saturate the internal bipolar NPN power switch for ef-

ﬁcient operation.

Open Circuit Protection

The LT3474 has internal open circuit protection. If the LED is

absent or fails open, the LT3474 clamps the voltage on the

LED pin at 14V. The switching regulator then skips cycles

to limit the input current. The LT3474-1 has no internal

open circuit protection. With the LT3474-1, be careful not

to violate the ABSMAX voltage of the BOOST pin; if VIN >

25V, external open circuit protection circuitry (as shown in

Figure 2) 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 shutdown

when VIN falls below 3.5V. If an adjustable UVLO threshold

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

voltage of the SHDN pin comparator is 2.65V. A internal

resistor pulls 10.3μA to ground from the SHDN pin at the

UVLO threshold.

Choose resistors according to the following formula:

R2 =

2.65V

TH – 2.65V

– 10.3μA

VTH = UVLO Threshold

Example: Switching should not start until the input is

above 8V.

VTH = 8V

R1 = 100k

R2 =

2.65V

8V – 2.65V

100k

– 10.3μA

= 61.9k

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.

GND

10.3μA

2.65V

VIN

LT3474

VIN

SHDN

3474 F03

Figure 3. Undervoltage Lockout

Figure 2. External Overvoltage Protection

Circuitry for the LT3474-1.

100k

10k

27V

OUT

3474 F02

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