Datasheet
LT3508
10
3508fd
Table 1. Programming the Switching Frequency
SWITCHING FREQUENCY (MHz)
R
T
(kΩ)
2.5 7.50
2.2 9.76
2 11.5
1.8 14
1.6 16.9
1.4 20.5
1.2 26.1
1 33.2
0.9 38.3
0.8 44.2
0.7 52.3
0.6 61.9
0.5 76.8
0.45 88.7
0.4 100
0.35 115
0.3 140
0.25 169
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
L = V
OUT
+ V
F
(
)
•
1.2µH
f
where V
F
is the voltage drop of the catch diode (~0.4V)
and f is in MHz. 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 0.1Ω.
Table 2 lists several vendors and types that are suitable.
Table 2. Inductor Vendors
VENDOR URL PART SERIES TYPE
Coilcraft www.coilcraft MSS7341 Shielded
Murata www.murata.com LQH55D Open
TDK www.component.tdk.com SLF7045
SLF10145
Shielded
Shielded
Toko www.toko.com DC62CB
D63CB
D75C
D75F
Shielded
Shielded
Shielded
Open
Sumida www.sumida.com CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
Figure 2. Operation Below V
IN(PS)
. V
IN
= 10V, V
OUT
= 1.8V and
f
SW
= 1.5MHz
Figure 3. Operation Above V
IN(PS)
. V
IN
= 18V, V
OUT
= 1.8V
and f
SW
= 1.5MHz. Output Ripple and Peak Inductor Current
Increase
APPLICATIONS INFORMATION
V
OUT
100mV/DIV
(AC)
I
L
500mA/DIV
2µs/DIV
3508 F02
V
OUT
100mV/DIV
(AC)
I
L
500mA/DIV
2µs/DIV
3508 F03
Setting the Switching Frequency
The switching frequency is programmed either by driving
the RT/SYNC pin with a logic level SYNC signal or by tying
a resistor from the RT/SYNC pin to ground. A graph for
selecting the value of R
T
for a given operating frequency
is shown in the Typical Application section. Suggested
programming resistors for various switching frequencies
are shown in Table 1.
Choosing a high switching frequency will allow the smallest
overall solution size. However, at high input voltages the
effi ciency can drop signifi cantly with increasing switching
frequency. The choice of switching frequency will also
impact the input voltage range, inductor and capacitor
selection, and compensation. See the related sections
for details.