lt3508euf-trpbf Linear Technology Corporation, lt3508euf-trpbf Datasheet - Page 13

• Manufacturer Part Number: lt3508euf-trpbf
• Description: Dual Monolithic 1.4a Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

Diode Selection
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
The only reason to consider a diode with a larger current
rating than necessary for nominal operation is for the
worst-case condition of shorted output. The diode current
will then increase to the typical peak switch current.
Peak reverse voltage is equal to the regulator input voltage.
Use a diode with a reverse voltage rating greater than the
input voltage. Table 4 lists several Schottky diodes and their
manufacturers. If operating at high ambient temperatures,
consider using a Schottky with low reverse leakage.
Table 4. Schottky Diodes
PART NUMBER
On Semiconductor
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
B120
B130
B140HB
DFLS140
B240
BOOST Pin Considerations
The capacitor and diode tied to the BOOST pin generate
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. For
applications 1MHz or faster, a 0.1μF capacitor is suffi cient.
Use a 0.47μF capacitor or greater for applicaitons running
below 500kHz. Figure 4 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
APPLICATIONS INFORMATION
I
D AVG
(
)
=
I
OUT
(
V
IN
V
IN
–
V
(V)
V
20
40
20
40
30
20
30
40
40
40
OUT
R
)
I
(A)
0.5
0.5
0.5
1.1
AVE
1
1
1
1
1
2
V
F
(mV)
620
530
550
500
500
510
at 1A
V
F
(mV)
500
at 2A
and higher, the standard circuit (Figure 4a) 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 4b). The circuit in
Figure 4a is more effi cient because the boost pin current
comes from a lower voltage source. Finally, the anode of
the boost diode can be tied to another source that is at
least 3V (Figure 4c). For example, if you are generating
a 3.3V output, and the 3.3V output is on whenever the
particular channel is on, the anode of the BOOST diode
can be connected to the 3.3V output. 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.
V
IN2
> 3V
V
V
V
IN
IN
IN
Figure 4. Generating the Boost Voltage
V
MAX V
V
MAX V
V
MAX V
MINIMUM VALUE FOR V
BOOST
BOOST
BOOST
V
BOOST
V
BOOST
V
BOOST
D2
D2
IN
– V
IN
– V
IN
– V
SW
SW
SW
BOOST
LT3508
BOOST
BOOST
LT3508
LT3508
GND
GND
GND
2V
V
V
V
IN2
V
IN
V
IN
IN
OUT
IN2
+ V
+ V
SW
SW
SW
OUT
(4b)
(4a)
(4c)
IN
IN2 =
3V
C3
C3
C3
D2
LT3508
3508 F04
V
V
OUT
OUT
V
13
OUT
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