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LT3475/LT3475-1
APPLICATIONS INFORMATION
Table 4. Schottky Diodes
pin for full ef?ciency. For outputs of 3.3V and higher, the
V R
(V)
I AVE (A)
(A)
V F at 1A
(mV)
V F at 2A
(mV)
standard circuit (Figure 5a) is best. For outputs between
2.8V and 3.3V, use a small Schottky diode (such as the
On Semiconductor
BAT-54). For lower output voltages, the boost diode can be
MBR0540
MBRM120E
MBRM140
40
20
40
0.5
1
1
620
530
550
tied to the input (Figure 5b). The circuit in Figure 5a is more
ef?cient because the BOOST pin current comes from a
lower voltage source. The anode of the boost diode can
Diodes Inc
B120
B130
B140HB
DFLS140
B240
International Recti?er
10BQ030
20
30
40
40
40
30
1
1
1
1.1
2
1
500
500
530
510
420
500
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
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
D2
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
D2
BOOST
LT3475
C3
BOOST
LT3475
C3
V IN
V IN
SW
V OUT
V IN
V IN
SW
V OUT
V IN2 > 3V
GND
V BOOST – V SW ? V OUT
MAX V BOOST ? V IN + V OUT
(5a)
D2
GND
V BOOST – V SW ? V IN
MAX V BOOST ? 2V IN
(5b)
D2
BOOST
LT3475
C3
BOOST
LT3475
C3
V IN
V IN
SW
V OUT
V IN
V IN
SW
V OUT
GND
3475 F05?
GND
V BOOST – V SW ? V IN2
MAX V BOOST ? V IN2 + V IN
MINIMUM VALUE FOR V IN2 = 3V
(5c)
V BOOST – V SW – V Z
MAX V BOOST ? V IN + V OUT – V Z
(5d)
3475 F05?
Figure 5. Generating the Boost Voltage
3475fb
13
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