Datasheet

11
LT3431
sn3431 3431fs
APPLICATIO S I FOR ATIO
WUUU
Additional Considerations
After making an initial choice, consider additional factors
such as core losses and second sourcing, etc. Use the
experts in Linear Technology’s Applications department if
you feel uncertain about the final choice. They have
experience with a wide range of inductor types and can tell
you about the latest developments in low profile, surface
mounting, etc.
Maximum Output Load Current
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
P
). The current rating
for the LT3431 is 3A. Unlike most current mode convert-
ers, the LT3431 maximum switch current limit does not
fall off at high duty cycles. Most current mode converters
suffer a drop off of peak switch current for duty cycles
above 50%. This is due to the effects of slope compensa-
tion required to prevent subharmonic oscillations in cur-
rent mode converters. (For detailed analysis, see Applica-
tion Note 19.)
The LT3431 is able to maintain peak switch current limit
over the full duty cycle range by using patented circuitry to
cancel the effects of slope compensation on peak switch
current without affecting the frequency compensation it
provides.
Maximum load current would be equal to maximum
switch current for an infinitely large inductor , but with
finite inductor size, maximum load current is reduced by
one-half peak-to-peak inductor current (I
LP-P
). The follow-
ing formula assumes continuous mode operation, imply-
ing that the term on the right is less than one-half of I
P
.
I
OUT(MAX)
=
Continuous Mode
I–
I
2
=I
P
LP-P
P
−
+
()
−
()
()()( )
VVVVV
LfV
OUT F IN OUT F
IN
–
2
For V
OUT
= 5V, V
IN
= 12V, V
F(D1)
= 0.52V, f = 500kHz
and L = 10µH:
I
A
OUT MAX
()
−
=−
+
()
−
()
()()
()
=− =
3
5 0 52 12 5 0 52
2 15 10 500 10 12
30327
63
.–.
••
..
Note that there is less load current available at the higher
input voltage because inductor ripple current increases. At
V
IN
= 24V, duty cycle is 23% and for the same set of
conditions:
I
A
OUT MAX()
.–.
••
..
=−
+
()
−
()
()()
()
=− =
−
3
5 0 52 24 5 0 52
2 15 10 500 10 24
3 0 43 2 57
63
Table 2
VENDOR/ VALUE I
DC
DCR HEIGHT
PART NUMBER (
µ H) (Amps) (Ohms) (mm)MAX
Sumida
CDRH8D28-4R7 4.7 3.4 0.019 3
CDRH8D28-7R3 7.3 2.8 0.030 3
CDRH8D43-100 10 4 0.029 4.5
CDRH8D43-150 15 2.9 0.042 4.5
CEI122-100 10 3.4 0.029 3
CEI122(H)-150 15 3.6 0.071 3
CDRH104R-150 15 3.6 0.037 4
CDRH104R-220 22 2.9 0.054 4
CDRH124-330 33 2.9 0.066 4.5
Coiltronics
UP2B-6R8 6.8 3.6 0.020 6
UP2B-100 10 3.3 0.027 6
UP3B-220 22 3.7 0.049 6.8
UP3B-330 33 3.0 0.069 6.8
Coilcraft
DO1813P-472 4.7 2.6 0.054 5
DS3316P-472 4.7 3.2 0.054 5.08
DS3316P-682 6.8 2.8 0.075 5.08
DO3316P-103 10 3.8 0.038 5.21
DO3316P-153 15 3.0 0.046 5.21
there are no helpful guidelines to calculate when the
magnetic field radiation will be a problem.