LT3980IMSE-TRPBF データシートの表示（PDF） - Linear Technology
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LT3980IMSE-TRPBF
Linear Technology
LT3980IMSE-TRPBF Datasheet PDF : 24 Pages
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LT3980
Applications Information
drop (~0.5V), VSW is the internal switch drop (~0.5V at
max load), fSW is the switching frequency (set by RT),
and tON(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:
VIN(MIN)
=
1–
VOUT + VD
fSW tOFF(MIN)
–
VD
+
VSW
where VIN(MIN) is the minimum input voltage, and tOFF(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 ΔIL increases with higher VIN or VOUT
and decreases with higher inductance and faster switching
frequency. A reasonable starting point for selecting the
ripple current is:
ΔIL = 0.4(IOUT(MAX))
where IOUT(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 (ILIM).
The peak inductor current is:
IL(PEAK) = IOUT(MAX) + ΔIL/2
where IL(PEAK) is the peak inductor current, IOUT(MAX) is
the maximum output load current, and ΔIL is the inductor
ripple current. The LT3980’s switch current limit (ILIM) 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:
IOUT(MAX) = ILIM – ΔIL/2
Be sure to pick an inductor ripple current that provides
sufficient maximum output current (IOUT(MAX)).
The largest inductor ripple current occurs at the highest
VIN. To guarantee that the ripple current stays below the
specified maximum, the inductor value should be chosen
according to the following equation:
L
=
VOUT + VD
fSW∆IL
1–
VOUT + VD
VIN(MAX)
where VD is the voltage drop of the catch diode (~0.4V),
VIN(MAX) is the maximum input voltage, VOUT is the output
voltage, fSW 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
Murata www.murata.com
TDK
www.component.tdk.com
Toko
www.toko.com
Sumida www.sumida.com
NEC
Vishay
www.nec-tokin.com
www.vishay.com
PART SERIES
LQH55D
SLF10145
D75C
D75F
CDRH74
CR75
CDRH8D43
MPLC073
MPBI0755
IHLP2525CE01
TYPE
Open
Shielded
Shielded
Open
Shielded
Open
Shielded
Shielded
Shielded
Shielded
3980fa
10
For more information www.linear.com/LT3980
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