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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-
?cient operation.
Open Circuit Protection
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:
V TH – 2.65V
TheLT3474hasinternalopencircuitprotection.IftheLEDis
absent or fails open, the LT3474 clamps the voltage on the
LED pin at 14V. The switching regulator then skips cycles
R2 =
R1
2.65V
– 10.3 μ A
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.
V TH = UVLO Threshold
Example: Switching should not start until the input is
above 8V.
V TH = 8V
R1 = 100k
Undervoltage Lockout
Undervoltage lockout (UVLO) is typically used in situations
where the input supply is current limited, or has high source
R2 =
2.65V
8V – 2.65V
– 10.3 μ A
100k
= 61.9k
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
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.
prevents the regulator from operating at source voltages
where these problems might occur.
LT3474
OUT
10k
V IN
R1
V IN
2.65V
V C
SHDN
27V
V C
C1
R2
10.3μA
GND
100k
3474 F03
3474 F02
Figure 2. External Overvoltage Protection
Figure 3. Undervoltage Lockout
Circuitry for the LT3474-1.
3474fd
9
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