LT3474/LT3474-1
APPLICATIONS INFORMATION
Inductor Selection and Maximum Output Current
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A
larger value inductor provides a higher maximum load
current, and reduces the output voltage ripple. If your
load is lower than the maximum load current, then you
can relax the value of the inductor and operate with higher
ripple current. This allows you to use a physically smaller
inductor, or one with a lower DCR resulting in higher
efficiency. Be aware that if the inductance differs from
the simple rule above, then the maximum load current
will depend on input voltage. In addition, low inductance
mayresultindiscontinuousmodeoperation,whichfurther
reduces maximum load current. For details of maximum
output current and discontinuous mode operation, see
Linear Technology’s Application Note 44. Finally, for duty
A good first choice for the inductor value is
900kHz
L = (VOUT + V )•
F
f
where V is the voltage drop of the catch diode (~0.4V), f
F
is the switching frequency and L is in μH. With this value
the maximum load current will be 1.1A, independent of
input voltage. The inductor’s RMS current rating must be
greater than the maximum load current and its saturation
currentshouldbeatleast30%higher.Forhighestefficiency,
the series resistance (DCR) should be less than 0.2Ω.
Table 2 lists several vendors and types that are suitable.
For robust operation at full load and high input voltages
(V > 30V), use an inductor with a saturation current
IN
cycles greater than 50% (V /V > 0.5), a minimum
OUT IN
higher than 2.5A.
inductanceisrequiredtoavoidsub-harmonicoscillations.
See Application Note 19.
Table 2. Inductors
VALUE
(μH)
I
DCR
(Ω)
HEIGHT
(mm)
Thecurrentintheinductorisatrianglewavewithanaverage
value equal to the load current. The peak switch current
is equal to the output current plus half the peak-to-peak
inductor ripple current. The LT3474 limits its switch cur-
rentinordertoprotectitselfandthesystemfromoverload
faults. Therefore, the maximum output current that the
LT3474 will deliver depends on the switch current limit,
the inductor value, and the input and output voltages.
RMS
PART NUMBER
Sumida
(A)
CR43-3R3
3.3
4.7
3.3
3.3
4.7
10
1.44
1.15
1.1
0.086
0.109
0.063
0.049
0.072
0.048
0.076
0.072
0.13
3.5
3.5
1.8
3
CR43-4R7
CDRH4D16-3R3
CDRH4D28-3R3
CDRH4D28-4R7
CDRH5D28-100
CDRH5D28-150
CDRH73-100
CDRH73-150
Coilcraft
1.57
1.32
1.3
3
3
When the switch is off, the potential across the inductor
is the output voltage plus the catch diode drop. This gives
the peak-to-peak ripple current in the inductor
15
1.1
3
10
1.68
1.33
3.4
3.4
15
1–DC V + V
(
)(
)
OUT
F
ΔIL =
DO1606T-332
DO1606T-472
DO1608C-332
DO1608C-472
MOS6020-332
MOS6020-472
3.3
4.7
3.3
4.7
3.3
10
1.3
1.1
2
0.1
0.12
0.08
0.09
0.046
0.05
2
2
L • f
(
)
2.9
2.9
2
where f is the switching frequency of the LT3474 and L
is the value of the inductor. The peak inductor and switch
current is
1.5
1.8
1.5
2
ΔIL
2
ISW PK =IL PK =IOUT
+
(
)
(
)
3474fd
11
Linear [ Linear ]
