LT3474/LT3474-1
APPLICATIONS INFORMATION
An internal comparator will force the part into shutdown
The switch driver operates either from V or from the
IN
when V falls below 3.5V. If an adjustable UVLO threshold
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.
IN
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:
Open Circuit Protection
2.65V
R2=
TheLT3474hasinternalopencircuitprotection.IftheLEDis
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
VTH – 2.65V
–10.3μA
R1
V
= UVLO Threshold
TH
Example: Switching should not start until the input is
above 8V.
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
= 8V
R1 = 100k
2.65V
8V – 2.65V
R2=
=61.9k
Undervoltage Lockout
–10.3μA
Undervoltagelockout(UVLO)istypicallyusedinsituations
wheretheinputsupplyiscurrentlimited,orhashighsource
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
loadtothesourceandcancausethesourcetocurrentlimit
or latch low under low source voltage conditions. UVLO
prevents the regulator from operating at source voltages
where these problems might occur.
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
V
IN
OUT
10k
2.65V
R1
R2
V
C
SHDN
27V
V
C
10.3μA
C1
GND
100k
3474 F03
3474 F02
Figure 3. Undervoltage Lockout
Figure 2. External Overvoltage Protection
Circuitry for the LT3474-1.
3474fd
9
Linear [ Linear ]
