Datasheet

LT3434
13
3434fb
APPLICATIO S I FOR ATIO
WUUU
To calculate actual peak switch current in continuous
mode with a given set of conditions, use:
II
VVV
LfV
SW PK OUT
OUT IN OUT
IN
()
–
=+
()
()()( )
2
If a small inductor is chosen which results in discontinous
mode operation over the entire load range, the maximum
load current is equal to:
I
IfLV
VVV
OUT MAX
PK IN
OUT IN OUT
()
–
=
()( )( )
()( )
2
2
2
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
PK
).
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
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.
2. Calculate peak inductor current at full load current to
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.
Table 3. Inductor Selection Criteria
VENDOR/ VALUE I
DC
DCR HEIGHT
PART NO. (
µ H) (Amps) (Ohms) (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 3
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