Datasheet

LT3475/LT3475-1
13
3475fb
Table 4. Schottky Diodes
V
R
(V)
I
AVE
(A)
(A)
V
F
at 1A
(mV)
V
F
at 2A
(mV)
On Semiconductor
MBR0540 40 0.5 620
MBRM120E 20 1 530
MBRM140 40 1 550
Diodes Inc
B120 20 1 500
B130 30 1 500
B140HB 40 1 530
DFLS140 40 1.1 510
B240 40 2 500
International Rectifi er
10BQ030 30 1 420
BOOST Pin Considerations
The capacitor and diode tied to the BOOST pin gener-
ate a voltage that is higher than the input voltage. In
most cases, a 0.22μF capacitor and fast switching diode
(such as the CMDSH-3 or MMSD914LT1) will work well.
Figure 5 shows three ways to arrange the boost circuit.
The BOOST pin must be more than 2.5V above the SW
pin for full effi ciency. For outputs of 3.3V and higher, the
standard circuit (Figure 5a) is best. For outputs between
2.8V and 3.3V, use a small Schottky diode (such as the
BAT-54). For lower output voltages, the boost diode can be
tied to the input (Figure 5b). The circuit in Figure 5a is more
effi cient because the BOOST pin current comes from a
lower voltage source. The anode of the boost diode can
be tied to another source that is at least 3V. For example, if
you are generating a 3.3V output, and the 3.3V output is on
whenever the LED is on, the BOOST pin can be
connected to the 3.3V output. For LT3475-1 applications
with higher output voltages, an additional Zener diode
may be necessary (Figure 5d) to maintain pin voltage
below the absolute maximum. In any case, be sure that
the maximum voltage at the BOOST pin is both less than
60V and the voltage difference between the BOOST and
SW pins is less than 30V.
The minimum operating voltage of an LT3475 application
is limited by the undervoltage lockout (~3.7V) and by the
maximum duty cycle. The boost circuit also limits the
minimum input voltage for proper start up. If the input
voltage ramps slowly, or the LT3475 turns on when the
output is already in regulation, the boost capacitor may
not be fully charged. Because the boost capacitor charges
Figure 5. Generating the Boost Voltage
V
IN
BOOST
GND
SW
V
IN
LT3475
(5a)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT3475
(5b)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
D2
V
IN
BOOST
GND
SW
V
IN
LT3475
(5c)
3475 F05
V
OUT
V
BOOST
– V
SW
≅ V
IN2
MAX V
BOOST
≅ V
IN2
+ V
IN
MINIMUM VALUE FOR V
IN2
=
3V
V
IN2
> 3V
C3
D2
V
IN
BOOST
GND
SW
V
IN
LT3475
(5d)
3475 F05
V
OUT
V
BOOST
– V
SW
– V
Z
MAX V
BOOST
≅ V
IN
+ V
OUT
– V
Z
C3
APPLICATIONS INFORMATION