Datasheet

LT3475/LT3475-1
8
3475fb
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 V
IN
> 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 V
IN
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:
R2 =
2.6V
V
TH
–2.6V
R1
–9μA
V
TH
= UVLO Threshold
Example: Switching should not start until the input is
above 8V.
V
TH
= 8V
R1=100k
R2 =
2.6V
8V – 2.6V
100k
–9μA
= 57.6k
Figure 2. Undervoltage Lockout
GND
9μA
2.6V
V
IN
V
C
LT3475
R1
C1
R2
V
IN
SHDN
3475 F02
Table 1. Switching Frequencies
SWITCHING FREQUENCY (MHz) R
T
(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
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 R
T
pin to ground.
A graph for selecting the value of R
T
for a given operating
frequency is shown in the Typical Applications section.
V
C
100k
10k
22V
OUT
3475 F01
Figure 1. External Overvoltage Protection
Circuitry for the LT3475-1
APPLICATIONS INFORMATION