LT3508
APPLICATIONS INFORMATION
Diode Selection
and higher, the standard circuit (Figure 4a) is best. For
outputsbetween2.8Vand3.3V,useasmallSchottkydiode
(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 efficient 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.
The catch diode (D1 from Figure 1) conducts current only
during switch off time. Average forward current in normal
operation can be calculated from:
IOUT V – V
(
)
IN
OUT
ID(AVG)
=
V
IN
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.
Peakreversevoltageisequaltotheregulatorinputvoltage.
Use a diode with a reverse voltage rating greater than the
inputvoltage.Table4listsseveralSchottkydiodesandtheir
manufacturers.Ifoperatingathighambienttemperatures,
consider using a Schottky with low reverse leakage.
D2
C3
C3
C3
BOOST
LT3508
V
V
OUT
V
SW
IN
IN
Table 4. Schottky Diodes
V
I
V at 1A V at 2A
R
AVE
F
F
GND
PART NUMBER
On Semiconductor
MBR0520L
MBR0540
MBRM120E
MBRM140
Diodes Inc.
B0530W
(V)
(A)
(mV)
(mV)
V
– V V
SW OUT
BOOST
MAX V
V + V
IN OUT
BOOST
20
40
20
40
0.5
0.5
1
(4a)
620
530
550
D2
1
BOOST
LT3508
V
IN
V
OUT
V
SW
30
20
30
40
40
40
0.5
1
IN
B120
500
500
GND
B130
1
B140HB
1
V
– V V
BOOST
SW
BOOST
IN
IN
MAX V
2V
DFLS140
1.1
2
510
(4b)
B240
500
D2
V
> 3V
IN2
BOOST Pin Considerations
BOOST
LT3508
The capacitor and diode tied to the BOOST pin generate
a voltage that is higher than the input voltage. In most
cases, a 0.22ꢀF capacitor and fast switching diode (such
as the CMDSH-3 or MMSD914LT1) will work well. For
applications 1MHz or faster, a 0.1ꢀF capacitor is sufficient.
Use a 0.47ꢀF 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 efficiency. For outputs of 3.3V
V
V
OUT
V
SW
IN
IN
GND
V
– V V
SW IN2
BOOST
3508 F04
MAX V
V + V
BOOST
IN2 IN
MINIMUM VALUE FOR V = 3V
IN2
(4c)
Figure 4. Generating the Boost Voltage
3508fb
13
Linear [ Linear ]
