Datasheet

LT3509
13
3509fc
APPLICATIONS INFORMATION
Figure 7. BD Tied to Regulated Output
L1
D1
C
OUT
V
OUT
C
BOOST
V
BOOST
–V
SW
V
OUT
MAX V
BOOST
V
IN
 V
OUT
V
OUT
r3V
V
IN
3509 F07
LT3509
GND
V
IN
BD
BOOST
SW
DA
C
IN
Boost Pin Considerations
Figure 7 through Figure 9 show several ways to arrange
the boost circuit. The BOOST pin must be more than 2V
above the SW pin for full effi ciency. For outputs of 3.3V
and higher, the standard circuit Figure 7 is best. For lower
output voltages, the boost diode can be tied to the input
Figure 8. The circuit in Figure 7 is more effi cient because
the boost pin current comes from a lower voltage source.
Finally, as shown in Figure 9, the BD pin can be tied to
another source that is at least 3V. For example, if you are
generating 3.3V and 1.8V, and the 3.3V is on whenever
the 1.8V is on, the 1.8V boost diode can be connected to
the 3.3V output.
In any case, be sure that the maximum voltage at the
BOOST pin is less than 60V and the voltage difference
between the BOOST and SW pins is less than 30V.
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
LV V
MHz
f
OUT F
SW
=+()•
.21
where V
F
is the voltage drop of the catch diode (~0.5V)
and L is in µH.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should
be at least 30% higher. For highest effi ciency, the series
resistance (DCR) should be less than 0.15. Table 2 lists
several vendors and types that are suitable.
The current in the inductor is a triangle wave with an average
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3509 limits its switch current
in order to protect itself and the system from overcurrent
faults. Therefore, the maximum output current that the
LT3509 will deliver depends on the switch current limit,
the inductor value and the input and output voltages.
L1
D1
C
OUT
V
OUT
C
BOOST
V
BOOST
–V
SW
V
IN
MAX V
BOOST
2V
IN
C
IN
V
IN
3509 F08
LT3509
GND
V
IN
BD
BOOST
SW
DA
Figure 8. Supplied from V
IN
L1
D1
C
OUT
V
OUT
C
BOOST
V
BOOST
–V
SW
V
BD
MAX V
BOOST
V
IN
 V
BD
V
BD
r3V
C
IN
V
IN
V
BD
3509 F09
LT3509
GND
V
IN
BD
BOOST
SW
DA
Figure 9. Separate Boost Supply