LT1941 Ver la hoja de datos (PDF) - Linear Technology
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LT1941 Datasheet PDF : 24 Pages
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LT1941
APPLICATIONS INFORMATION
STEP-DOWN CONSIDERATIONS
FB Resistor Network
The output voltage is programmed with a resistor divider
(refer to the Block Diagram) between the output and the
FB pin. Choose the resistors according to
R1 = R2(VOUT/628mV – 1)
R2 should be 10k or less to avoid bias current errors.
Input Voltage Range
The minimum operating voltage is determined either by the
LT1941’s undervoltage lockout of ~3.3V or by its maximum
duty cycle. The duty cycle is the fraction of time that the
internal switch is on and is determined by the input and
output voltages:
DC = (VOUT + VF)/(VIN – VSW + VF)
where VF is the forward voltage drop of the catch diode
(~0.4V) and VSW is the voltage drop of the internal switch
(~0.3V at maximum load). This leads to a minimum input
voltage of:
VIN(MIN) = (VOUT + VF)/DCMAX – VF + VSW
with DCMAX = 0.78.
The maximum operating voltage is determined by the
absolute maximum ratings of the VIN and BOOST pins
and by the minimum duty cycle DCMIN = 0.15:
VIN(MAX) = (VOUT + VF)/DCMIN – VF + VSW
This limits the maximum input voltage to ~14V with
VOUT = 1.8V and ~19V with VOUT = 2.5. Note that this is
a restriction on the operating input voltage; the circuit
will tolerate input voltage transients up to the Absolute
Maximum Rating.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is
L = (VOUT + VF)/1.6 for SW1
L = (VOUT + VF)/1.1 for SW2
where VF is the voltage drop of the catch diode (~0.4V) and
L is in μH. With this value the maximum load current will
be 2.1A for SW1 and 1.4A for SW2, independent of input
voltage. 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 efficiency, the
series resistance (DCR) should be less than 0.1Ω. Table 1
lists several vendors and types that are suitable.
Table 1. Inductors
PART NUMBER
VALUE
ISAT
(μH)
(A)
DCR HEIGHT
(Ω)
(mm)
Sumida
CR43-1R4
1.4
2.52
0.056
3.5
CR43-2R2
2.2
1.75
0.071
3.5
CDRH3D16-1R5
1.5
1.55
0.040
1.8
CDRH4D28-3R3
3.3
1.57
0.049
3.0
CDRH4D18-1R0
1.0
1.70
0.035
2.0
CDC5D23-2R2
2.2
2.50
0.03
2.5
CDRH5D28-2R6
2.6
2.60
0.013
3.0
Coilcraft
DO1606T-152
1.5
2.10
0.060
2.0
DO1606T-222
2.2
1.70
0.070
2.0
DO1608C-152
1.5
2.60
0.050
2.9
DO1608C-222
2.2
2.30
0.070
2.9
DO1608C-332
3.3
2.00
0.080
2.9
DO1608C-472
4.7
1.50
0.090
2.9
MOS6020-222
2.2
2.15
0.035
2.0
MOS6020-332
3.3
1.8
0.046
2.0
MOS6020-472
4.7
1.5
0.050
2.0
DO3314-222
2.2
1.6
0.200
1.4
Toko
(D62F)847FY-2R4M
2.4
2.5
0.037
2.7
(D73LF)817FY-2R2M
2.2
2.7
0.03
3.0
The optimum inductor for a given application may differ
from the one indicated by this simple design guide. A
larger value inductor provides a slightly higher maximum
load current and will reduce the output voltage ripple. If
1941fb
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