LT3507
APPLICATIONS INFORMATION
Select an inductor greater than L
that keeps the ripple
Output Capacitor Selection
MIN
current below 30% of I
.
LIM
Theoutputcapacitorfilterstheinductorcurrenttogenerate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilize the LT3507’s
control loop. Because the LT3507 operates at a high
frequency, minimal output capacitance is necessary. In
addition, the control loop operates well with or without
the presence of output capacitor series resistance (ESR).
Ceramic capacitors, which achieve very low output ripple
and small circuit size, are therefore an option.
Theinductor’sRMScurrentratingmustbegreaterthanthe
maximum load current and its saturation current should
be greater than I . For highest efficiency, the series
LPK
resistance (DCR) should be less than 0.1Ω. Table 1 lists
several vendors and types that are suitable.
Table 1. Inductors
VALUE
(μH)
I
DCR
(Ω)
HEIGHT
(mm)
SAT
PART NUMBER
Sumida
(A)
You can estimate output ripple with the following
equations:
CDC5D23-2R2
CDRH5D28-2R6
CDRH6D26-5R6
CDH113-100
Coilcraft
2.2
2.6
5.6
10
2.16
2.60
2.00
2.00
0.030
0.013
0.027
0.047
2.5
3.0
2.8
3.7
ΔIL
8 • f •COUT
VRIPPLE
and
=
for ceramic capacitors
VRIPPLE = ΔIL •ESR for electrolytic capacitors
(tantalum and aluminum)
DO1606T-152
LPS6225-222ML
DO1608C-332
MSS6132-472ML
DO1813P-682HC
Cooper
1.5
2.2
3.3
4.7
6.8
2.10
4.00
2.00
2.60
2.20
0.060
0.045
0.080
0.056
0.080
2.0
2.4
2.9
3.2
5.0
whereΔI isthepeak-to-peakripplecurrentintheinductor.
L
The RMS content of this ripple is very low so the RMS
current rating of the output capacitor is usually not of
concern. It can be estimated with the formula:
SD414-2R2
2.2
6.8
10
2.73
2.96
1.90
0.061
0.041
0.111
1.35
3.55
5.0
ΔIL
12
IC(RMS)
=
DRA73-6R8-R
UP1B-100
Another constraint on the output capacitor is that it must
havegreaterenergystoragethantheinductor;ifthestored
energyintheinductortransferstotheoutput, theresulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
Toko
(D62F)847FY-2R4M
(D73LF)817FY-2R2M
2.4
2.2
2.5
2.7
0.037
0.03
2.7
3.0
This analysis is valid for continuous mode operation
(I > I /2). For details of maximum output current in
OUT
LIM
2
⎛
⎞
ILIM
discontinuous mode operation, see Linear Technology’s
COUT >10 •L •
⎜
⎟
V
Application Note AN44. Finally, for duty cycles greater
⎝
⎠
OUT
than 50% (V /V > 0.5), a minimum inductance is
OUT IN
ThelowESRandsmallsizeofceramiccapacitorsmakethem
the preferred type for LT3507 applications. Not all ceramic
capacitorsarethesame,however.Manyofthehighervalue
capacitors use poor dielectrics with high temperature and
voltage coefficients. In particular, Y5V and Z5U types lose
a large fraction of their capacitance with applied voltage
and at temperature extremes. Because loop stability and
requiredtoavoidsubharmonicoscillations.Thisminimum
inductance is:
0.45
fSW
SW1:LMIN = V + V •
(
)
OUT
F
0.9
fSW
SW2, SW3:LMIN = V + V •
(
)
OUT
F
transient response depend on the value of C , this loss
OUT
may be unacceptable. Use X7R and X5R types.
with L
in μH and f in MHz.
SW
MIN
3507fa
12
Linear Systems [ Linear Systems ]
