Datasheet

LT3980
10
3980fa
For more information www.linear.com/LT3980
drop (~0.5V), V
SW
is the internal switch drop (~0.5V at
max load), f
SW
is the switching frequency (set by R
T
),
and t
ON(MIN)
is the minimum switch on time (~200ns).
Note that a higher switching frequency will depress the
maximum operating input voltage. Conversely, a lower
switching frequency will be necessary to achieve safe
operation at high input voltages.
Input voltages up to 58V are acceptable regardless of the
switching frequency. In this mode, the LT3980 may enter
pulse-skipping operation where some switching pulses
are skipped to maintain safe inductor current.
The minimum input voltage is determined by either the
LT3980’s minimum operating voltage of ~3.6V or by its
maximum duty cycle (see equation in previous section).
The minimum input voltage due to duty cycle is:
V
VV
ft
VV
IN MIN
OUT D
SW
OFFMIN
DS
W
()
()
=
+
+
1–
–
where V
IN(MIN)
is the minimum input voltage, and t
OFF(MIN)
is the minimum switch off time (200ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The
ripple current ΔI
L
increases with higher V
IN
or V
OUT
and decreases with higher inductance and faster switching
frequency. A reasonable starting point for selecting the
ripple current is:
ΔI
L
= 0.4(I
OUT(MAX)
)
where I
OUT(MAX)
is the maximum output load current. To
guarantee sufficient output current, peak inductor current
must be lower than the LT3980’s switch current limit (I
LIM
).
The peak inductor current is:
I
L(PEAK)
= I
OUT(MAX)
+ ΔI
L
/2
where I
L(PEAK)
is the peak inductor current, I
OUT(MAX)
is
the maximum output load current, and ΔI
L
is the inductor
ripple current. The LT3980’s switch current limit (I
LIM
) is
4A at low duty cycles and decreases linearly to 3A at DC
= 0.8. The maximum output current is a function of the
inductor ripple current:
I
OUT(MAX)
= I
LIM
– ΔI
L
/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (I
OUT(MAX)
).
The largest inductor ripple current occurs at the highest
V
IN
. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
L
VV
fI
VV
V
OUT D
SW L
OUT D
IN MAX
=
+






+





∆
1–
()



where V
D
is the voltage drop of the catch diode (~0.4V),
V
IN(MAX)
is the maximum input voltage, V
OUT
is the output
voltage, f
SW
is the switching frequency (set by RT), and
L is in the inductor value.
The inductor’s RMS and saturation current rating must
be greater than the maximum load current. For robust
operation in fault conditions (start-up or short circuit) and
high input voltage (>40V), the saturation current should
be above 3.5A. To keep the efficiency high, the series
resistance (DCR) should be less than 0.1Ω, and the core
material should be intended for high frequency applications.
Table 1 lists several vendors and suitable types.
Table 1. Inductor Vendors
VENDOR URL PART SERIES TYPE
Murata www.murata.com LQH55D Open
TDK www.component.tdk.com SLF10145 Shielded
Toko www.toko.com D75C
D75F
Shielded
Open
Sumida www.sumida.com CDRH74
CR75
CDRH8D43
Shielded
Open
Shielded
NEC www
.nec-tokin.com MPLC073
MPBI0755
Shielded
Shielded
Vishay www
.vishay.com IHLP2525CE01
Shielded
applicaTions inFormaTion