LT1943EFE Linear Technology, LT1943EFE Datasheet - Page 12

• Manufacturer Part Number: LT1943EFE
• Description: IC REG SW QUAD TFT LCD 28-TSSOP
• Manufacturer: Linear Technology

Specifications of LT1943EFE

• Applications: Converter, TFT, LCD
• Voltage - Input: 4.5 ~ 22 V
• Number Of Outputs: 4
• Voltage - Output: 1.25 ~ 40 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TSSOP Exposed Pad, 28-eTSSOP, 28-HTSSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

LT1943
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% (V
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:
where f is the switching frequency of the LT1943 and L is
the value of the inductor. The peak inductor and switch
current is
To maintain output regulation, this peak current must be
less than the LT1943’s switch current limit of I
SW1, I
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:
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
12
∆I
I
I
SWPK
OUT(MAX)
L
= (1 – DC)(V
LIM
= I
is at least 2.4A at DC = 0.35 and decreases
LPK
= 3A • (1 – 0.57 • DC) – ∆I
= I
LIM
= I
U
OUT
– ∆I
OUT
+ ∆I
L
+ V
/2
L
F
/2
)/(L • f),
OUT
/V
IN
> 0.5), a minimum
L
/2
LIM
. For
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 I
∆I
Table 1. Inductors.
Part Number
Sumida
CR43-1R4
CR43-2R2
CR43-3R3
CR43-4R7
CDRH3D16-1R5
CDRH3D16-2R2
CDRH3D16-3R3
CDRH4D28-3R3
CDRH4D28-4R7
CDRH4D18-1R0
CDC5D23-2R2
CDRH5D28-2R6
Coilcraft
DO1606T-152
DO1606T-222
DO1606T-332
DO1606T-472
DO1608C-152
DO1608C-222
DO1608C-332
DO1608C-472
MOS6020-222
MOS6020-332
MOS6020-472
D03314-222
1008PS-272
Toko
(D62F)847FY-2R4M
(D73LF)817FY-2R2M
L
/2.
Value (µH)
1.4
2.2
3.3
4.7
1.5
2.2
3.3
3.3
4.7
1.0
2.2
2.6
1.5
2.2
3.3
4.7
1.5
2.2
3.3
4.7
2.2
3.3
4.7
2.2
2.7
2.4
2.2
I
RMS
2.52
1.75
1.44
1.15
1.55
1.20
1.10
1.57
1.32
1.70
2.50
2.60
2.10
1.70
1.30
1.10
2.60
2.30
2.00
1.50
2.15
1.8
1.5
1.6
1.3
2.5
2.7
(A)
DCR (Ω) Height (mm)
0.056
0.071
0.086
0.109
0.040
0.050
0.063
0.049
0.072
0.035
0.013
0.060
0.070
0.100
0.120
0.050
0.070
0.080
0.090
0.035
0.046
0.050
0.200
0.140
0.037
0.03
0.03
OUT
is less than
3.5
3.5
3.5
3.5
1.8
1.8
1.8
3.0
3.0
2.0
2.5
3.0
2.0
2.0
2.0
2.0
2.9
2.9
2.9
2.9
2.0
2.0
2.0
1.4
2.7
2.7
3.0
1943fa