Datasheet

LT3508
13
3508fd
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:
I
D(AVG)
=
I
OUT
V
IN
–V
OUT
(
)
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 cur-
rent 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
V
R
(V)
I
AVE
(A)
V
F
at 1A
(mV)
V
F
at 2A
(mV)
On Semiconductor
MBR0520L 20 0.5
MBR0540 40 0.5 620
MBRM120E 20 1 530
MBRM140 40 1 550
Diodes Inc.
B0530W 30 0.5
B120 20 1 500
B130 30 1 500
B140HB 40 1
DFLS140 40 1.1 510
B240 40 2 500
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 ap-
plications 1MHz or faster, a 0.1µF capacitor is suffi cient.
Use a 0.47µF capacitor or greater for applications 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
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
(V
AUX
) 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.
APPLICATIONS INFORMATION
Figure 4. Generating the Boost Voltage
V
IN
BOOST
GND
SW
V
IN
LT3508
(4a)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
OUT
MAX V
BOOST
≅ V
IN
+ V
OUT
V
IN
BOOST
GND
SW
V
IN
LT3508
(4b)
D2
V
OUT
C3
V
BOOST
– V
SW
≅ V
IN
MAX V
BOOST
≅ 2V
IN
D2
V
IN
BOOST
GND
SW
V
IN
LT3508
(4c)
3508 F04
V
OUT
V
BOOST
– V
SW
≅ V
AUX
MAX V
BOOST
≅ V
AUX
+ V
IN
MINIMUM VALUE FOR
V
AUX = 3V
V
AUX
> 3V
C3