LT3508
APPLICATIONS INFORMATION
Table 1. Programming the Switching Frequency
V
OUT
100mV/DIV
(AC)
SWITCHING FREQUENCY (MHz)
R (kΩ)
T
2.5
2.2
2
7.50
9.76
11.5
14
I
L
500mA/DIV
1.8
1.6
1.4
1.2
1
3508 F02
2ꢀs/DIV
16.9
20.5
26.1
33.2
38.3
44.2
52.3
61.9
76.8
88.7
100
Figure 2. Operation Below VIN(PS). VIN = 10V, VOUT = 1.8V and
fSW = 1.5MHz
0.9
0.8
0.7
0.6
0.5
0.45
0.4
0.35
0.3
0.25
V
OUT
100mV/DIV
(AC)
I
L
500mA/DIV
3508 F03
2ꢀs/DIV
115
Figure 3. Operation Above VIN(PS). VIN = 18V, VOUT = 1.8V
and fSW = 1.5MHz. Output Ripple and Peak Inductor Current
Increase
140
169
Inductor Selection and Maximum Output Current
Setting the Switching Frequency
A good first choice for the inductor value is:
The switching frequency is programmed either by driving
the R /SYNC pin with a logic level SYNC signal or by tying
1.2MHz
T
L = V
+ V •
(
)
OUT
F
a resistor from the R /SYNC pin to ground. A graph for
T
f
selecting the value of R for a given operating frequency
T
whereV isthevoltagedropofthecatchdiode(~0.4V)andL
F
is shown in the Typical Application section. Suggested
programming resistors for various switching frequencies
are shown in Table 1.
isinμH.Theinductor’sRMScurrentratingmustbegreater
than the maximum load current and its saturation current
should be at least 30% higher. For highest efficiency, the
series resistance (DCR) should be less than 0.1Ω. Table 2
lists several vendors and types that are suitable.
Choosingahighswitchingfrequencywillallowthesmallest
overall solution size. However, at high input voltages the
efficiency can drop significantly 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.
Table 2. Inductor Vendors
VENDOR
Coilcraft
Murata
TDK
URL
PART SERIES
MSS7341
LQH55D
SLF7045
SLF10145
TYPE
Shielded
Open
Shielded
Shielded
www.coilcraft
www.murata.com
www.component.tdk.com
Toko
www.toko.com
DC62CB
D63CB
D75C
Shielded
Shielded
Shielded
Open
D75F
Sumida
www.sumida.com
CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Shielded
Open
3508fb
10
Linear [ Linear ]
