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LT3507
12
3507fa
Select an inductor greater than L
MIN
 that keeps the ripple
current below 30% of I
LIM
.
The inductors RMS current rating must be greater than the
maximum load current and its saturation current should
be greater than I
LPK
. For highest ef ciency, the series
resistance (DCR) should be less than 0.1? Table 1 lists
several vendors and types that are suitable.
Table 1. Inductors
PART NUMBER
VALUE
(糎)
I
SAT
(A)
DCR
(?/SPAN>)
HEIGHT
(mm)
Sumida
CDC5D23-2R2
2.2    2.16    0.030    2.5
CDRH5D28-2R6
2.6    2.60    0.013    3.0
CDRH6D26-5R6
5.6    2.00    0.027    2.8
CDH113-100
10    2.00    0.047    3.7
Coilcraft
DO1606T-152
1.5    2.10    0.060    2.0
LPS6225-222ML
2.2    4.00    0.045    2.4
DO1608C-332
3.3    2.00    0.080    2.9
MSS6132-472ML
4.7    2.60    0.056    3.2
DO1813P-682HC
6.8    2.20    0.080    5.0
Cooper
SD414-2R2
2.2    2.73    0.061    1.35
DRA73-6R8-R
6.8    2.96    0.041    3.55
UP1B-100
10    1.90    0.111    5.0
Toko
(D62F)847FY-2R4M    2.4
2.5    0.037    2.7
(D73LF)817FY-2R2M    2.2
2.7    0.03    3.0
This analysis is valid for continuous mode operation
(I
OUT
 > I
LIM
/2). For details of maximum output current in
discontinuous mode operation, see Linear Technologys
Application Note AN44. Finally, for duty cycles greater
than 50% (V
OUT
/V
IN
 > 0.5), a minimum inductance is
required to avoid subharmonic oscillations. This minimum
inductance is:
 
SW1:L
MIN
= V
OUT
+V
F
(
)
"
0.45
f
SW
SW2, SW3:L
MIN
= V
OUT
+V
F
(   )
"
0.9
f
SW
with L
MIN
 in 糎 and f
SW
 in MHz.
Output Capacitor Selection
The output capacitor  lters the inductor current to generate
an output with low voltage ripple. It also stores energy in
order to satisfy transient loads and stabilize the LT3507s
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.
You can estimate output ripple with the following
equations:
V
RIPPLE
=
擨
L
8"f"C
OUT
 for ceramic capacitors
and
V
RIPPLE
=擨
L
"ESR for electrolytic capacitors
(tantalum and aluminum)
where 擨
L
 is the peak-to-peak ripple current in the inductor.
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:
 
 
I
C(RMS)
=
擨
L
12
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor transfers to the output, the resulting
voltage step should be small compared to the regulation
voltage. For a 5% overshoot, this requirement indicates:
   
C
OUT
>10"L"
I
LIM
V
OUT
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
2
The low ESR and small size of ceramic capacitors make them
the preferred type for LT3507 applications. Not all ceramic
capacitors are the same, however. Many of the higher value
capacitors use poor dielectrics with high temperature and
voltage coef cients. In particular, Y5V and Z5U types lose
a large fraction of their capacitance with applied voltage
and at temperature extremes. Because loop stability and
transient response depend on the value of C
OUT
, this loss
may be unacceptable. Use X7R and X5R types.
APPLICATIONS INFORMATION
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