LT3434 LINER [Linear Technology], LT3434 Datasheet - Page 13

• Manufacturer Part Number: LT3434
• Description: High Voltage 3A, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
To calculate actual peak switch current in continuous
mode with a given set of conditions, use:
If a small inductor is chosen which results in discontinous
mode operation over the entire load range, the maximum
load current is equal to:
CHOOSING THE INDUCTOR
For most applications the output inductor will fall in the
range of 15µH to 100µH. Lower values are chosen to
reduce physical size of the inductor. Higher values allow
more output current because they reduce peak current
seen by the LT3434 switch, which has a minimum 3A limit.
Higher values also reduce output ripple voltage and reduce
core loss.
When choosing an inductor you might have to consider
maximum load current, core and copper losses, allow-
able component height, output voltage ripple, EMI, fault
current in the inductor, saturation and of course cost.
The following procedure is suggested as a way of han-
dling these somewhat complicated and conflicting
requirements.
1. Choose a value in microhenries such that the maximum
load current plus half of the inductor ripple current is
less than the minimum peak switch current (I
Choosing a small inductor with lighter loads may result
in discontinuous mode of operation, but the LT3434 is
designed to work well in either mode.
Assume that the average inductor current is equal to
load current and decide whether or not the inductor
I
I
SW PK
OUT MAX
(
(
)
=
)
I
=
OUT
2
(
I
V
+
PK
OUT
U
V
2
OUT
2
)(
( )( )( )
2
f L V
( )( )( )
V
U
(
L f V
IN
V
IN
–
–
V
IN
OUT
V
IN
OUT
W
)
)
U
PK
).
Table 3. Inductor Selection Criteria
VENDOR/
PART NO.
Sumida
CDRH104R-150
CDRH104R-220
CDRH104R-330
CDRH124-220
CDRH124-330
CDRH127-330
CDRH127-470
CEI122-220
Coiltronics
UP3B-330
UP3B-470
UP4B-680
Coilcraft
DO3316P-153
DO5022p-683
2. Calculate peak inductor current at full load current to
must withstand continuous fault conditions. If maxi-
mum load current is 1A, for instance, a 1A inductor may
not survive a continuous 4A overload condition.
For applications with a duty cycle above 50%, the
inductor value should be chosen to obtain an inductor
ripple current of less than 40% of the peak switch
current.
ensure that the inductor will not saturate. Peak current
can be significantly 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 opera-
tion, but it errs only slightly on the high side for discon-
tinuous mode, so it can be used for all conditions.
VALUE
(
µ
15
22
33
22
33
33
47
22
33
47
68
15
68
H)
(Amps)
3.6
2.9
2.3
2.9
2.7
3.0
2.5
2.3
2.4
4.3
3.5
I
DC
3
3
(Ohms)
0.050
0.073
0.093
0.066
0.097
0.065
0.100
0.085
0.069
0.108
0.120
0.046
0.130
DCR
LT3434
HEIGHT
13
(mm)
4.5
4.5
6.8
6.8
7.9
5.2
7.1
4
4
4
8
8
3
3434f