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LM3674MF-1.5 데이터시트(PDF) 11 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
부품명 LM3674MF-1.5
상세설명  2MHz, 600mA Step-Down DC-DC Converter in SOT 23-5
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제조업체  NSC [National Semiconductor (TI)]
홈페이지  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM3674MF-1.5 데이터시트(HTML) 11 Page - National Semiconductor (TI)

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Operation Description (Continued)
will be controlled down to the lowest possible input voltage.
When the device operates near 100% duty cycle, the output
voltage supply ripple is slightly higher, approximately 25mV.
The minimum input voltage needed to support the output
voltage is
I
LOAD
Load current
R
DSON,PFET
Drain to source resistance of PFET
switch in the triode region
R
INDUCTOR
Inductor resistance
Application Information
OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE
(LM3674-ADJ)
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from V
OUT
to FB the to GND. V
OUT will be adjusted to make FB equal to
0.5V. The resistor from FB to GND (R2) should be 200 k
Ω to
keep the current drawn through this network small but large
enough that it is not susceptible to noise. If R
2 is 200K
Ω, and
given the V
FB is 0.5V, then the current through the resistor
feedback network will be 2.5µA. The output voltage formula
is:
V
OUT = Output Voltage (V)
V
FB = Feedback Voltage (0.5V typ)
R
1 = Resistor from VOUT to FB (
Ω)
R
2 = Resistor from FB to GND (
Ω)
For any output voltage greater than or equal to 1.0V a
frequency zero must be added at 45KHz for stability. The
formula is:
For output voltages greater than or equal to 2.5V, a pole
must also be placed at 45KHz as well. If the pole and zero
are at the same frequency the formula for calculation of C2
is:
The formula for location of zero and pole frequency created
by adding C1,C2 are given below. It can be seen that by
adding C1, a zero as well as a higher frequency pole is
introduced.
See the " LM3674-ADJ Configurations for " Various V
OUT"
table.
TABLE 1. Adjustable LM3674 Configurations for Various V
OUT
VOUT (V)
R1 (K
Ω)R2 (KΩ)
C1 (pF)
C2 (pF)
L (µH)
CIN (µF)
COUT (µF)
1.0
200
200
18
None
2.2
4.7
10
1.1
191
158
18
None
2.2
4.7
10
1.2
280
200
12
None
2.2
4.7
10
1.5
357
178
10
None
2.2
4.7
10
1.6
442
200
8.2
None
2.2
4.7
10
1.7
432
178
8.2
None
2.2
4.7
10
1.8
464
178
8.2
None
2.2
4.7
10
1.875
523
191
6.8
None
2.2
4.7
10
2.5
402
100
8.2
None
2.2
4.7
10
2.8
464
100
8.2
33
2.2
4.7
10
3.3
562
100
6.8
33
2.2
4.7
10
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor should not saturate, and the inductor current
ripple should be small enough to achieve the desired output
voltage ripple. Different saturation current rating specifica-
tions are followed by different manufacturers so attention
must be given to details. Saturation current ratings are typi-
cally specified at 25˚C. However, ratings at the maximum
ambient temperature of application should be requested
from the manufacturer. The minimum value of inductance
to guarantee good performance is 1.76µH at I
LIM (typ) dc
current over the ambient temperature range. Shielded
inductors radiate less noise and should be preferred.
There are two methods to choose the inductor saturation
current rating.
Method 1:
The saturation current is greater than the sum of the maxi-
mum load current and the worst case average to peak
inductor current. This can be written as:
www.national.com
11


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