lt3980 Linear Technology Corporation, lt3980 Datasheet - Page 10

• Manufacturer Part Number: lt3980
• Description: 58v, 2a, 2.4mhz Step-down Switching Regulator With 85?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3980
max load), f
t
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:
where V
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
and decreases with higher inductance and faster switch-
ing frequency. A reasonable starting point for selecting
the ripple current is:
where I
guarantee suffi cient output current, peak inductor current
must be lower than the LT3980’s switch current limit (I
The peak inductor current is:
10
ON(MIN)
ΔI
I
V
L(PEAK)
IN MIN
L
(
= 0.4(I
OUT(MAX)
IN(MIN)
is the minimum switch on time (~200ns). Note that
)
= I
=
SW
OUT(MAX)
1–
OUT(MAX)
is the minimum input voltage, and t
is the switching frequency (set by R
V
f
is the maximum output load current. To
SW OFF MIN
OUT
t
L
+
)
increases with higher V
+ ΔI
V
(
D
L
/2
)
–
V
D
+
V
SW
IN
OFF(MIN)
or V
T
), and
LIM
OUT
).
where I
the maximum output load current, and ΔI
ripple current. The LT3980’s switch current limit (I
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:
Be sure to pick an inductor ripple current that provides
suffi cient maximum output current (I
The largest inductor ripple current occurs at the highest
V
specifi ed maximum, the inductor value should be chosen
according to the following equation:
where V
V
voltage, f
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 effi ciency 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
Murata
TDK
Toko
Sumida
NEC
Vishay
IN
IN(MAX)
I
L
OUT(MAX)
. To guarantee that the ripple current stays below the
=
⎛
⎜
⎝
L(PEAK)
V
D
is the maximum input voltage, V
OUT
f
SW
SW L
is the voltage drop of the catch diode (~0.4V),
URL
www.murata.com
www.component.tdk.com
www.toko.com
www.sumida.com
www.nec-tokin.com
www.vishay.com
= I
Δ
is the switching frequency (set by RT), and
+
I
is the peak inductor current, I
V
LIM
D
⎞
⎟
⎠
– ΔI
⎛
⎜
⎜
⎝
1–
L
/2
V
V
OUT
IN MAX
(
+
V
PART SERIES
LQH55D
SLF10145
D75C
D75F
CDRH74
CR75
CDRH8D43
MPLC073
MPBI0755
IHLP2525CE01
)
D
⎞
⎟
⎟
⎠ ⎠
OUT(MAX)
OUT
L
is the inductor
is the output
OUT(MAX)
).
TYPE
Open
Shielded
Shielded
Open
Shielded
Open
Shielded
Shielded
Shielded
Shielded
LIM
3980f
) is
is