Datasheet

LT3507
12
3507fb
For more information www.linear.com/LT3507
applications inForMation
However, it’s generally better to use an inductor larger
than the minimum value. The minimum inductor has large
ripple currents which increase core losses and require
large output capacitors to keep output voltage ripple low.
Select an inductor greater than L
MIN
that keeps the ripple
current below 30% of I
LIM
.
The inductor’s RMS current rating must be greater than the
maximum load current and its saturation current should
be greater than I
LPK
. For highest efficiency, the series
resistance (DCR) should be less than 0.1Ω. Table 1 lists
several vendors and types that are suitable.
Table 1. Inductors
PART NUMBER
VALUE
(µH)
I
SAT
(A)
DCR
(Ω)
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 Technology’s
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 µH and f
SW
in MHz.
Output Capacitor Selection
The output capacitor filters the inductor current to generate
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.
You can estimate output ripple with the following
equations:
V
RIPPLE
=
Δ
I
L
8 • f • C
OUT
for ceramic capacitors
and
V
RIPPLE
= ΔI
L
• ESR for electrolytic capacitors
(tantalum and aluminum)
where ΔI
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)
=
Δ
I
L
12
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.