LT3474/LT3474-1
APPLICATIONS INFORMATION
The switch driver operates either from V
IN
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 V
IN
>
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.
OUT
10k
27V
V
C
C1
R2
10.3μA
GND
An internal comparator will force the part into shutdown
when V
IN
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
V
TH
– 2.65V
– 10.3μA
R1
V
TH
= UVLO Threshold
Example: Switching should not start until the input is
above 8V.
V
TH
= 8V
R1 = 100k
R2
=
2.65V
=
61.9k
8V – 2.65V
– 10.3μA
100k
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.
LT3474
V
IN
R1
SHDN
V
IN
2.65V
V
C
100k
3474 F03
3474 F02
Figure 2. External Overvoltage Protection
Circuitry for the LT3474-1.
Figure 3. Undervoltage Lockout
3474fd
9
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