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

LOAD

Load current

DSON,PFET

Drain to source resistance of PFET

switch in the triode region

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

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

Ω, and

given the V

feedback network will be 2.5µA. The output voltage formula

is:

•

V

•

V

•

R

Ω)

•

R

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

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

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:

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