Datasheet

LT3430/LT3430-1
11
34301fa
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
the high side for discontinuous mode, so it can be used
for all conditions.
II
I
I
VVV
VfL
PEAK OUT
LP P
OUT
OUT IN OUT
IN
=+ =+
()( )
()( )()()
-
22
–
EMI
Decide if the design can tolerate an “open” core geometry
like a rod or barrel, which have high magnetic fi eld radiation,
or whether it needs a closed core like a toroid to prevent
EMI problems. This is a tough decision because the rods
or barrels are temptingly cheap and small and there are
no helpful guidelines to calculate when the magnetic fi eld
radiation will be a problem.
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 fi nal choice. They have ex-
perience with a wide range of inductor types and can tell
you about the latest developments in low profi le, 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 LT3430/LT3430-1 is 3A. Unlike most current mode
converters, the LT3430/LT3430-1 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 compensation required to prevent subharmonic
oscillations in current mode converters. (For detailed
analysis, see Application Note 19.)
The LT3430/LT3430-1 are 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 infi nitely large inductor, but with fi nite
inductor size, maximum load current is reduced by one-
half peak-to-peak inductor current (I
LP-P
). The following
formula assumes continuous mode operation, implying
that the term on the right is less than one-half of IP.
I
OUT(MAX)
=
Continuous Mode
I–
I
2
=I
P
LP-P
P
−
+
()
−
()
()()
VVVVV
LfV
OUT F IN OUT F
–
2
IIN
()
For V
OUT
= 5V, V
IN
= 12V, V
F(D1)
= 0.52V, f = 200kHz and
L = 15µH:
I
A
OUT MAX
()
−
=−
+
()
−
()
()()
()
=− =
3
5 0 52 12 5 0 52
2 15 10 200 10 12
30525
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 10 200 10 24
3 0 71 2 29
63
To calculate actual peak switch current with a given set
of conditions, use:
II
I
VVVVV
LfV
SW PEAK
OUT
P
OUT
OUT F IN OUT F
IN
()
=+
=+
+−
()
()()( )
I
2
L-P
() –
2
APPLICATIONS INFORMATION
*US Patent # 6,498,466