LT3430 Linear Technology, LT3430 Datasheet - Page 11

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
tors 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 mate-
rials 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.
EMI
Decide if the design can tolerate an “open” core geometry
like a rod or barrel, which have high magnetic field
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 field 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 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
for the LT3430 is 3A. Unlike most current mode convert-
ers, the LT3430 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.)
I
PEAK
I
OUT
I
LP P
U
2
-
U
I
OUT
V
W
OUT
P
). The current rating
2
V
V
IN
IN
–
f L
U
V
OUT
The LT3430 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
ing formula assumes continuous mode operation, imply-
ing that the term on the right is less than one-half of I
For V
and L = 15 H:
Note that there is less load current available at the higher
input voltage because inductor ripple current increases. At
V
conditions:
culate actual peak switch current with a given set of
conditions, use:
IN
I
Continuous Mode
I –
I
I
I
OUT(MAX)
P
OUT MAX
OUT MAX
SW PEAK
= 24V, duty cycle is 23% and for the same set of
OUT
(
I
LP-P
2
= 5V, V
)
=
= I
3 0 5 2 5
3 0 71 2 29
I
P
3
I
3
OUT
OUT
IN
= 12V, V
2 15 10
2 15 10
V
.
.
5 0 52 24 5 0 52
OUT
5 0 52 12 5 0 52
I
(
L -P
V
P
•
2
•
OUT
.
.
.
.
F(D1)
V V
2
A
F
6
6
L f V
A
V V
2
200 10
F
= 0.52V, f = 200kHz
200 10
L f V
IN
)
IN
•
•
IN
V
– .
– .
LP-P
OUT
IN
3
3
V
OUT
). The follow-
12
24
LT3430
–
V
–
F
sn3430 3430is
To cal-
V
11
F
P
.