參數(shù)資料
型號(hào)： LTC1938IDD#TRPBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 4 A SWITCHING REGULATOR, 3300 kHz SWITCHING FREQ-MAX, PDSO10
封裝： 3 X 3 MM, LEAD FREE, PLASTIC, MO-229WEED-2, DFN-10
文件頁(yè)數(shù)： 3/24頁(yè)
文件大?。?/td> 272K
代理商： LTC1938IDD#TRPBF
LT1938
11
1938f
frequency will be necessary to achieve safe operation at
high input voltages.
If the output is in regulation and no short-circuit or start-up
events are expected, then input voltage transients of up to
25V are acceptable regardless of the switching frequency.
In this mode, the LT1938 may enter pulse skipping opera-
tion where some switching pulses are skipped to maintain
output regulation. In this mode the output voltage ripple
and inductor current ripple will be higher than in normal
operation.
The minimum input voltage is determined by either the
LT1938’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 OFF MIN
DSW
()
=
+
1–
–
where VIN(MIN) is the minimum input voltage, and tOFF(MIN)
is the minimum switch off time (150ns). 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 switch-
ing 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 sufcient output current, peak inductor current
must be lower than the LT1938’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 LT1938’s switch current limit (ILIM) is
at least 3.5A at low duty cycles and decreases linearly to
2.5A at DC = 0.8. The maximum output current is a func-
tion of the inductor ripple current:
IOUT(MAX) = ILIM – ΔIL/2
Be sure to pick an inductor ripple current that provides
sufcient maximum output current (IOUT(MAX)).
The largest inductor ripple current occurs at the highest
VIN. To guarantee that the ripple current stays below the
specied maximum, the inductor value should be chosen
according to the following equation:
L
VV
fI
VV
V
OUT
D
L
OUT
D
IN MAX
=
+
+
()
Δ
1–
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 current rating must be greater than the
maximum load current and its saturation current should be
about 30% higher. For robust operation in fault conditions
(start-up or short circuit) and high input voltage (>30V),
the saturation current should be above 3A. To keep the
efciency high, the series resistance (DCR) should be less
than 0.1
Ω, 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.componenttdk.com
SLF7045
SLF10145
Shielded
Toko
www.toko.com
D62CB
D63CB
D75C
D75F
Shielded
Open
Sumida
www.sumida.com
CR54
CDRH74
CDRH6D38
CR75
Open
Shielded
Open
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