LP2950

http://onsemi.com

9

Figure 16. Adjustable Regulator

Error

Output

Shutdown

Input

Vin

Vout

1.23 to 30 V

3.3

µF

0.01

µF

NC

NC

R2

R1

100 k

5

3

Error

SD

Gnd

FB

47

6

VO T

SNS

2

Vout

Vin

8

1

The complete equation for the output voltage is:

where Vref is the nominal 1.235 V reference voltage and IFB

is the feedback pin bias current, nominally – 20 nA. The

minimum recommended load current of 1.0

µA forces an

upper limit of 1.2 M

Ω on the value of R2, if the regulator

must work with no load. IFB will produce a 2% typical error

in Vout which may be eliminated at room temperature by

adjusting R1. For better accuracy, choosing R2 = 100 k

reduces this error to 0.17% while increasing the resistor

program current to 12

µA. Since the LP2951 typically draws

75

µA at no load with Pin 2 open circuited, the extra 12 µA

of current drawn is often a worthwhile tradeoff for

eliminating the need to set output voltage in test.

Output Noise

In many applications it is desirable to reduce the noise

present at the output. Reducing the regulator bandwidth by

increasing the size of the output capacitor is the only method

for reducing noise on the 3 lead LP2950. However,

increasing the capacitor from 1.0

µF to 220 µF only

decreases the noise from 430

µV to 160 µVrms for a 100 kHz

bandwidth at the 5.0 V output.

Noise can be reduced fourfold by a bypass capacitor

across R1, since it reduces the high frequency gain from 4

to unity. Pick

C

Bypass [

1

2

pR1 x 200 Hz

or about 0.01

µF. When doing this, the output capacitor must

be increased to 3.3

µF to maintain stability. These changes

reduce the output noise from 430

µV to 126 µVrms for a

100 kHz bandwidth at 5.0 V output. With bypass

capacitor added, noise no longer scales with output voltage

so that improvements are more dramatic at higher output

voltages.

Figure 17. 1.0 A Regulator with 1.2 V Dropout

0.01

µF

10 k

MTB23P06E

1.0

µF

Unregulated

Input

Error

Output

Shutdown

Input

Vout

5.0 V

±1.0%

0 to 1.0 A

220

µF

2.0 k

5

3

Error

SD

Gnd

FB

47

6

VO T

SNS

2

Vout

Vin

8

1

0.002

µF

1.0 M

LP2951CN