Datasheet

I
RIPPLE
I
OUTMAX
+I
SAT
>
Fz =
1
(2 * S * R1 * C1)
Fp =
1
2 * S * (R1 R2) * (C1+C2)
C
2
=
1
2 x S x R
2
x 45 kHz
C
1
=
1
2 x S x R
1
x 45 kHz
V
OUT
=
R
1
R
2
V
FB
*
+ 1
( )
LM3674
SNVS405F DECEMBER 2005REVISED MAY 2013
www.ti.com
(4)
V
OUT
= Output Voltage (V)
V
FB
= Feedback Voltage (0.5V typ)
R
1
= Resistor from V
OUT
to FB ()
R
2
= Resistor from FB to GND ()
For any output voltage greater than or equal to 1.0V a frequency zero must be added at 45KHz for stability. The
formula is:
(5)
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:
(6)
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.
(7)
See the LM3674-ADJ Configurations for Various V
OUT
table. Table 1
Table 1. Adjustable LM3674 Configurations for Various V
OUT
VOUT (V) R1 (K) R2 (K) 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 inductor: 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 specifications are followed by different manufacturers so attention must be given to details. Saturation
current ratings are typically 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 ensure 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 maximum load current and the worst case average to peak
inductor current. This can be written as:
(8)
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