LT1943 Ver la hoja de datos (PDF) - Linear Technology
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LT1943 Datasheet PDF : 20 Pages
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LT1943
U
OPERATIO
maximum load current. For details of maximum output
current and discontinuous mode operation, see Linear
Technology’s Application Note AN44. Finally, for duty
cycles greater than 50% (VOUT/VIN > 0.5), a minimum
inductance is required to avoid subharmonic oscillations.
See AN19.
The current in the inductor is a triangle wave with an
average value equal to the load current. The peak switch
current is equal to the output current plus half the peak-to-
peak inductor ripple current. The LT1943 limits its switch
current in order to protect itself and the system from
overload faults. Therefore, the maximum output current
that the LT1943 will deliver depends on the switch current
limit, the inductor value, and the input and output voltages.
When the switch is off, the potential across the inductor is
the output voltage plus the catch diode drop. This gives the
peak-to-peak ripple current in the inductor:
∆IL = (1 – DC)(VOUT + VF)/(L • f),
where f is the switching frequency of the LT1943 and L is
the value of the inductor. The peak inductor and switch
current is
ISWPK = ILPK = IOUT + ∆IL/2
To maintain output regulation, this peak current must be
less than the LT1943’s switch current limit of ILIM. For
SW1, ILIM is at least 2.4A at DC = 0.35 and decreases
linearly to 1.6A at DC = 0.8, as shown in the Typical
Performance Characteristics section. The maximum out-
put current is a function of the chosen inductor value:
IOUT(MAX) = ILIM – ∆IL/2
= 3A • (1 – 0.57 • DC) – ∆IL/2
Choosing an inductor value so that the ripple current is
small will allow a maximum output current near the switch
current limit.
One approach to choosing the inductor is to start with the
simple rule given above, look at the available inductors,
and choose one to meet cost or space goals. Then use
these equations to check that the LT1943 will be able to
deliver the required output current. Note again that these
equations assume that the inductor current is continuous.
Discontinuous operation occurs when IOUT is less than
∆IL/2.
Table 1. Inductors.
Part Number
Sumida
Value (µH) IRMS (A) DCR (Ω) Height (mm)
CR43-1R4
1.4
2.52 0.056
3.5
CR43-2R2
2.2
1.75 0.071
3.5
CR43-3R3
3.3
1.44 0.086
3.5
CR43-4R7
4.7
1.15 0.109
3.5
CDRH3D16-1R5
1.5
1.55 0.040
1.8
CDRH3D16-2R2
2.2
1.20 0.050
1.8
CDRH3D16-3R3
3.3
1.10 0.063
1.8
CDRH4D28-3R3
3.3
1.57 0.049
3.0
CDRH4D28-4R7
4.7
1.32 0.072
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
DO1606T-332
3.3
1.30 0.100
2.0
DO1606T-472
4.7
1.10 0.120
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
D03314-222
2.2
1.6
0.200
1.4
1008PS-272
2.7
1.3
0.140
2.7
Toko
(D62F)847FY-2R4M
2.4
2.5
0.037
2.7
(D73LF)817FY-2R2M 2.2
2.7
0.03
3.0
1943fa
12
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