Datasheet

LT1766/LT1766-5
12
1766fc
If maximum load current is 0.5A, for instance, a 0.5A
inductor may not survive a continuous 2A overload con-
dition. Dead shorts will actually be more gentle on the
inductor because the LT1766 has frequency and current
limit foldback.
Peak switch and inductor current can be signifi cantly higher
than output current, especially with smaller inductors
and lighter loads, so don’t omit this step. Powdered iron
cores are forgiving because they saturate softly, whereas
ferrite cores saturate abruptly. Other core materials fall
somewhere in between. The following formula assumes
continuous mode of operation, but errs only slightly on
where:
ESR = equivalent series resistance of the output
capacitor
ESL = equivalent series inductance of the output
capacitor
dI/dt = slew rate of inductor ripple current = V
IN
/L
Peak-to-peak ripple current (I
LP-P
) through the inductor
and into the output capacitor is typically chosen to be
between 20% and 40% of the maximum load current. It
is approximated by:
I
VVV
VfL
LP P
OUT IN OUT
IN
-
=
()( )
()()()
–
Example: with V
IN
= 40V, V
OUT
= 5V, L = 47μH, ESR = 0.1Ω
and ESL = 10nH, output ripple voltage can be approximated
as follows:
IA
dI
dt
VA
mV
RIPPLE
P-P
P-P
=
()
−
()
()
()()
=
==
=
()()
+
()()
()
=+=
−
−
−
540 5
40 47 10 200 10
0 465
40
47 10
10 0 85
0 465 0 1 10 10 10 0 85
0 0465 0 0085 55
63
6
6
96
••
.
•
•.
..• .
..
To reduce output ripple voltage further requires an increase
in the inductor value or a reduction in the capacitor ESR.
The latter can effect loop stability since the ESR forms
a useful zero in the overall loop response. Typically the
inductor value is adjusted with the trade-off being a
physically larger inductor with the possibility of increased
component height and cost. Choosing a smaller inductor
with lighter loads may result in discontinuous operation
but the LT1766 is designed to work well in both continuous
or discontinuous mode.
Peak Inductor Current and Fault Current
To ensure that the inductor will not saturate, the peak
inductor current should be calculated knowing the
maximum load current. An appropriate inductor should
then be chosen. In addition, a decision should be made
whether or not the inductor must withstand continuous
fault conditions.
APPLICATIONS INFORMATION
Table 2
VENDOR/
PART NO.
VALUE
(μH)
I
DC
(AMPS)
DCR
(OHMS)
HEIGHT
(mm)
Coiltronics
CTX15-1P 15 1.4 0.087 4.2
CTX15-1 15 1.1 0.08 4.2
CTX33-2P 33 1.3 0.126 6
CTX33-2 33 1.4 0.106 6
UP2-330 33 2.4 0.099 5.9
UP2-470 47 1.9 0.146 5.9
UP2-680 68 1.7 0.19 5.9
UP2-101 100 1.4 0.277 5.9
Sumida
CDRH6D28-150M 15 1.4 0.076 3
CDRH6D38-150M 15 1.6 0.062 4
CDRH6D28-330M 33 0.97 0.122 3
CDRH104R-330M 33 2.1 0.069 3.8
CDRH125-330M 33 2.1 0.044 6
CDRH104R-470M 47 2.1 0.095 3.8
CDRH125-470M 47 1.8 0.058 6
CDRH6D38-680M 68 0.75 0.173 4
CDRH104R-680M 68 1.5 0.158 3.8
CDRH125-680M 68 1.5 0.093 6
CDRH104R-101M 100 1.35 0.225 3.8
CDRH125-101M 100 1.3 0.120 6
Coilcraft
DT3316P-153 15 1.8 0.06 5
DT3316P-333 33 1.3 0.09 5
DT3316P-473 47 1 0.11 5