Application Information

DC/DC CONVERTORS

Adjustable Buck - Output Voltage Selection

The buck converter output voltage of the adjustable version

device can be set via the selection of the external feedback

resistor network forming the output feedback between the

output voltage side of the Inductor and the FB pin and the FB

Pin and GND.

20152958

Adjustable Buck Converter Components

V

0.5V. The resistor from FB to ground (R

around 200k

Ω to keep the current drawn through the resistor

network well below the 16µA quiescent current level (PFM

mode) but large enough that it is not susceptible to noise. If

R2 is 200k

Ω and with V

resistor feedback network will be 2.5µA.

The formula for output voltage selection is

V

V

FB

- feedback voltage (0.5V)

R

1FB

- feedback resistor from V

R

2FB

- feedback resistor from FB to GND

For any out voltage greater than or equal to 1.0V a zero

should be added around 45 kHz by the addition of a capaci-

tor C1. The formula for the calculation of C1 is:

For recommended component values see Table 1

TABLE 1. Buck Component Configurations for Various Output Voltage Values

V

RFB1 (k

Ω)

RFB2 (k

Ω)

C1 (pF)

C2 (pF)

L (µH)

C

1.0

200

200

18

none

2.2

10

1.2

280

200

12

none

2.2

10

1.4

360

200

10

none

2.2

10

1.5

360

180

10

none

2.2

10

1.6

442

200

8.2

none

2.2

10

1.85

540

200

6.8

none

2.2

10

2.5

402

100

8.2

none

2.2

10

2.8

464

100

8.2

33

2.2

10

3.3

562

100

6.8

33

2.2

10

Buck Inductor Selection

There are two main considerations when choosing an induc-

tor; the inductor should not saturate, and the inductor current

ripple is small enough to achieve the desired output voltage

ripple. Different saturation current rating specs are followed

by different manufacturers so attention must be given to

details. Saturation current ratings are typically specified at

25˚C so ratings at max ambient temperature of application

should be requested from manufacturer.

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

•

I

•

I

•

V

•

L : min inductor value including worst case tolerances

(30% drop can be considered for method 1)

•

f : minimum switching frequency (1.6Mhz)

•

V

Method 2:

A more conservative and recommended approach is to

choose an inductor that has saturation current rating greater

than the max current limit of 1220mA.

A 2.2µH inductor with a saturation current rating of at least

1250mA is recommended for most applications.The induc-

tor’s resistance should be less than 0.3

Ω for good efficiency.

For low-cost applications, an unshielded bobbin inductor

could be considered. For noise critical applications, a toroi-

dal or shielded-bobbin inductor should be used. A good

practice is to lay out the board with overlapping footprints of

both types for design flexibility. This allows substitution of a

low-noise shielded inductor, in the event that noise from

low-cost bobbin models is unacceptable.

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