C1

RFB1

RFB2

COUT

10 µF

2.2 µH

L1

SW

FB

LP8720

VOUT

LP8720

SNVS575B – JULY 2008 – REVISED MAY 2013

www.ti.com

Buck Converter (continued)

Unless otherwise noted, VVBATT = VVINB = VVIN1 = VVIN2 = 3.6V, GND = GNDB = 0V, CVBATT = CVIN1 = CVIN2 = 2.2 µF, CVINB = 10

µF. Typical values and limits appearing in normal type apply for TJ=25°C. Limits appearing in boldface type apply over the

entire junction temperature range for operation, TJ= -40 to +125°C

(1)(2)

Limit

Parameter

Test Conditions

Typ

Units

Min

Max

fOSC

Internal Oscillator Frequency

PWM Mode

1.9

2.1

2

MHz

1.7

2.3

Efficiency

IOUT = 5 mA, PFM-mode

88%

VOUT = 1.2V

(3)

IOUT = 200 mA, PWM-mode

90%

VOUT = 1.2V

(3)

TSTUP

Startup Time

IOUT = 0, VOUT = 1.2V

(3)

140

µs

Table 11. Buck Output Voltage Programming in Register 0x06 and 0x07

BUCK1_Vx

VOUT

BUCK1_Vx

VOUT

5h’00

External resistor divider

5h’10

1.55

5h’01

0.80

5h’11

1.60

5h’02

0.85

5h’12

1.65

5h’03

0.90

5h’13

1.70

5h’04

0.95

5h’14

1.75

5h’05

1.00

5h’15

1.80

5h’06

1.05

5h’16

1.85

5h’07

1.10

5h’17

1.90

5h’08

1.15

5h’18

1.95

5h’09

1.20

5h’19

2.00

5h’0A

1.25

5h’1A

2.05

5h’0B

1.30

5h’1B

2.10

5h’0C

1.35

5h’1C

2.15

5h’0D

1.40

5h’1D

2.20

5h’0E

1.45

5h’1E

2.25

5h’0F

1.50

5h’1F

2.30

Output Voltage Selection Using External Resistor Divider

Buck1 output voltage can be programmed via the selection of the external feedback resistor network.

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

around 200 k

Ω to keep the current drawn through the resistor network to a minimum but large enough that it is

not susceptible to noise. With RFB2= 200 kΩ and VFB at 0.5V, the current through the resistor feedback network

will be 2.5 µA.

12

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