C1 =

1

(2 x S x R1FB x 45 x 10

3)

VOUT = VFB x

R1FB

R2FB

1+

©

§

C1

R1FB

R2FB

COUT

10 PF

2.2 PH

L1

SW

FB

LP3905

VOUT

C2

LP3905

www.ti.com

SNVS374D – JUNE 2006 – REVISED MAY 2013

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.

Figure 10. Adjustable Buck Converter Components

VOUT will be adjusted to make the voltage at FB equal to 0.5V. The resistor from FB to ground (RFB2) should be

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 VFB at 0.5V, the

current through the resistor feedback network will be 2.5µA.

The formula for output voltage selection is:

(3)

VOUT - output voltage (Volts)

VFB

- feedback voltage (0.5V)

R1FB - feedback resistor from VOUT to FB

R2FB - 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

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

(4)

For recommended component values see Table 1

Table 1. Buck Component Configurations for Various Output Voltage Values

VOUT (V)

RFB1 (k

Ω)

RFB2 (k

Ω)

C1 (pF)

C2 (pF)

L (µH)

COUT (µF)

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

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