Datasheet

LT3430/LT3430-1
10
34301fa
inductor value to achieve a desirable output ripple volt-
age level. If output ripple voltage is of less importance,
the subsequent suggestions in Peak Inductor and Fault
Current and EMI will additionally help in the selection of
the inductor value.
Peak-to-peak output ripple voltage is the sum of a triwave
(created by peak-to-peak ripple current (I
LP-P
) times ESR)
and a square wave (created by parasitic inductance (ESL)
and ripple current slew rate). Capacitive reactance is as-
sumed to be small compared to ESR or ESL.
V I ESR ESL
dI
dt
RIPPLE LP P
=
()()
+
()
-
where:
ESR = equivalent series resistance of the output capaci-
tor
ESL = equivalent series inductance of the output capaci-
tor
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 = 22µH, ESR =
0.080Ω and ESL = 10nH, output ripple voltage can be
approximated as follows:
IA
dI
dt
VA
mV
RIPPLE
P-P
P-P
=
()
−
()
()
()()
=
==
=
()()
+
()()
()
=+=
−
−
−
540 5
40 22 10 200 10
099
40
22 10
10 1 8
0 99 0 08 10 10 10 1 8
0 079 0 018 97
63
6
6
96
••
.
•
•.
.. • .
..
To reduce output ripple voltage further requires an increase
in the inductor value with the trade-off being a physically
larger inductor with the possibility of increased component
height and cost.
Ceramic Output Capacitor
An alternative way to further reduce output ripple voltage
is to reduce the ESR of the output capacitor by using a
ceramic capacitor. Although this reduction of ESR removes
a useful zero in the overall loop response, this zero can
be replaced by inserting a resistor (R
C
) in series with the
V
C
pin and the compensation capacitor C
C
. (See Ceramic
Capacitors in Applications Information.)
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.
If maximum load current is 1A, for instance, a 1A induc-
tor may not survive a continuous 4A overload condition.
Dead shorts will actually be more gentle on the inductor
because the LT3430/LT3430-1 have frequency and current
limit foldback.
APPLICATIONS INFORMATION
Table 2
VENDOR/
PART NO.
VALUE
(µH)
I
DC
(Amps)
DCR
(Ohms)
HEIGHT
(mm)
Sumida
CDRH104R-150 15 3.6 0.050 4
CDRH104R-220 22 2.9 0.073 4
CDRH104R-330 33 2.3 0.093 4
CDRH124-220 22 2.9 0.066 4.5
CDRH124-330 33 2.7 0.097 4.5
CDRH127-330 33 3.0 0.065 8
CDRH127-470 47 2.5 0.100 8
CEI122-220 22 2.3 0.085 8
Coiltronics
UP3B-330 33 3 0.069 6.8
UP3B-470 47 2.4 0.108 6.8
UP4B-680 68 4.3 0.120 7.9
Coilcraft
DO3316P-153 15 3 0.046 5.2
DO5022p-683 68 3.5 0.130 7.1