lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 10

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1941
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
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:
where V
(~0.4V) and V
(~0.3V at maximum load). This leads to a minimum input
voltage of:
with DC
The maximum operating voltage is determined by the
absolute maximum ratings of the V
by the minimum duty cycle DC
This limits the maximum input voltage to ~14V with V
= 1.8V and ~19V with V
restriction on the operating input voltage; the circuit will
tolerate input voltage transients up to the Absolute Maxi-
mum Rating.
Inductor Selection and Maximum Output Current
A good first choice for the inductor value is
10
R1 = R2(V
DC = (V
V
V
L = (V
L = (V
IN(MIN)
IN(MAX)
MAX
F
OUT
OUT
is the forward voltage drop of the catch diode
OUT
= (V
= (V
= 0.78.
+ V
+ V
OUT
SW
+ V
OUT
OUT
F
F
/628mV – 1)
is the voltage drop of the internal switch
)/1.6 for SW1
)/1.1 for SW2
F
)/(V
U
+ V
+ V
IN
F
F
)/DC
)/DC
– V
U
OUT
SW
MAX
MIN
= 2.5. Note that this is a
MIN
+ V
– V
– V
IN
W
= 0.15:
F
)
F
F
and BOOST pins and
+ V
+ V
SW
SW
U
OUT
where V
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
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
DO3314-222
1008PS-272
Toko
(D62F)847FY-2R4M
(D73LF)817FY-2R2M
F
is the voltage drop of the catch diode (~0.4V) and
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
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
I
(A)
1.8
1.5
1.6
1.3
2.5
2.7
SAT
0.140
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
0.037
DCR
0.03
0.03
(Ω)
HEIGHT
(mm)
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
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