LM3674MFX-1.2 National Semiconductor, LM3674MFX-1.2 Datasheet - Page 11

• Manufacturer Part Number: LM3674MFX-1.2
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,TSOP,5PIN,PLASTIC
• Manufacturer: National Semiconductor

Operation Description
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
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
to FB the to GND. V
0.5V. The resistor from FB to GND (R2) should be 200 kΩ to
keep the current drawn through this network small but large
enough that it is not susceptible to noise. If R
given the V
feedback network will be 2.5µA. The output voltage formula
is:
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
• V
• I
• R
• R
VOUT (V)
1.875
LOAD
1.0
1.1
1.2
1.5
1.6
1.7
1.8
2.5
2.8
3.3
OUT
DSON,PFET
INDUCTOR
= Output Voltage (V)
FB
is 0.5V, then the current through the resistor
Load current
Drain to source resistance of PFET
switch in the triode region
Inductor resistance
R1 (KΩ)
OUT
200
191
280
357
442
432
464
523
402
464
562
will be adjusted to make FB equal to
TABLE 1. Adjustable LM3674 Configurations for Various V
R2 (KΩ)
200
158
200
178
200
178
178
191
100
100
100
(Continued)
2
is 200KΩ, and
C1 (pF)
8.2
8.2
8.2
6.8
8.2
8.2
6.8
18
18
12
10
OUT
11
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
table.
to guarantee good performance is 1.76µH at I
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:
• V
• R
• R
C2 (pF)
None
None
None
None
None
None
None
None
None
33
33
FB
1
2
= Resistor from V
= Resistor from FB to GND (Ω)
= Feedback Voltage (0.5V typ)
L (µH)
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
2.2
OUT
OUT
to FB (Ω)
CIN (µF)
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
4.7
COUT (µF)
LIM
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(typ) dc
10
10
10
10
10
10
10
10
10
10
10
OUT
"