Datasheet

LT3475/LT3475-1
10
3475fb
current have signifi cantly increased. Exceeding V
IN(MAX)
is safe if the external components have adequate ratings
to handle the peak conditions and if the peak inductor
current does not exceed 3.2A. A saturating inductor may
further reduce performance.
Table 2. Inductors
PART NUMBER
VALUE
(μH)
I
RMS
(A)
DCR
()
HEIGHT
(mm)
Sumida
CR43-3R3 3.3 1.44 0.086 3.5
CR43-4R7 4.7 1.15 0.109 3.5
CDRH4D16-3R3 3.3 1.10 0.063 1.8
CDRH4D28-3R3 3.3 1.57 0.049 3.0
CDRH4D28-4R7 4.7 1.32 0.072 3.0
CDRH6D26-5R0 5.0 2.20 0.032 2.8
CDRH6D26-5R6 5.6 2.0 0.036 2.8
CDRH5D28-100 10 1.30 0.048 3.0
CDRH5D28-150 15 1.10 0.076 3.0
CDRH73-100 10 1.68 0.072 3.4
CDRH73-150 15 1.33 0.130 3.4
CDRH104R-150 15 3.1 0.050 4.0
Coilcraft
DO1606T-332 3.3 1.30 0.100 2.0
DO1606T-472 4.7 1.10 0.120 2.0
DO1608C-332 3.3 2.00 0.080 2.9
DO1608C-472 4.7 1.50 0.090 2.9
MOS6020-332 3.3 1.80 0.046 2.0
MOS6020-472 10 1.50 0.050 2.0
DO3316P-103 10 3.9 0.038 5.2
DO3316P-153 15 3.1 0.046 5.2
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 effi ciency. In addition, low inductance
may result in discontinuous mode operation, which further
reduces maximum load current. For details of maximum
output current and discontinuous mode operation, see Linear
Technology’s Application Note 44. Finally, for duty cycles
greater than 50% (V
OUT
/V
IN
> 0.5), a minimum inductance
is required to avoid sub-harmonic oscillations:
L
MIN
= (V
OUT
+ V
F
)•
800kHz
f
V
SW
20V/DIV
I
L
1A/DIV
V
OUT
500mV/DIV
(AC COUPLED)
3475 F04
Figure 4. Operation above V
IN(MAX)
. Output
Ripple and Peak Inductor Current Increases
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
L = (V
OUT
+ V
F
)•
1.2MHz
f
where V
F
is the voltage drop of the catch diode (~0.4V),
f is the switching frequency and L is in μH. With this value
the maximum load current will be above 1.6A at all duty
cycles. The inductor’s RMS current rating must be greater
than the maximum load current and its saturation current
should be at least 30% higher. For highest effi ciency,
the series resistance (DCR) should be less than 0.15Ω.
Table 2 lists several vendors and types that are suitable.
For robust operation at full load and high input voltages
(V
IN
> 30V), use an inductor with a saturation current
higher than 3.2A.
APPLICATIONS INFORMATION