SPX2950 datasheet(9/12 Pages) SIPEX

9 / 12 page

Rev. 3/6/03

SPX2950/SPX2951 150mA Low Dropout Voltage Regulator

PROGRAMMING THE OUTPUT

VOLTAGE OF SPX2951

The SPX2951 may be pin-strapped for 5V using

its internal voltage divider by tying Pin 1 (out-

put) to Pin 2 (sense) and Pin 7 (feedback) to Pin

6 (5V Tap). Also, it may be programmed for any

output voltage between its 1.235V reference

and its 30V maximum rating . As seen in Figure

2, an external pair of resistors is required. Refer

to the below equation for the programming of

the output voltage:

VOUT = VREF × (1 + R1/R2)+ IFBR1

The VREF is 1.235 and IFB is the feedback bias

current, nominally -20nA. The minimum rec-

ommended load current of 1

µA forces an upper

limit of 1.2M

Ω on value of R2. If no load is

presented the IFB produces an error of typically

2% in VOUT which may be eliminated at room

temperature by trimming R1. To improve the

accuracy choose the value of R2 = 100k this

reduces the error by 0.17% and increases the

resistor program current by 12

µA. Since the

SPX2951 typically draws 60

µA at no load with

Pin 2 open-circuited this is a small price to pay.

REDUCING OUTPUT NOISE

It may be an advantage to reduce the AC noise

present at the output. One way is to reduce the

regulator bandwidth by increasing the size of

the output capacitor. This is the only way that

noise can be reduced on the 3 lead SPX2950 but

is relatively inefficient, as increasing the ca-

pacitor from 1

µF to 220µF only decreases the

noise from 430

µV to 160µVRMS for a 100kHz

bandwidth at 5V output. Noise could also be

reduced fourfold by a bypass capacitor across

R1, since it reduces the high frequency gain from

4 to unity. Pick

BYPASS ≅ (1 / 2πR1 × 200 Hz)

or choose 0.01

µF. When doing this, the output

capacitor must be increased to 3.3

µF to main-

tain stability. These changes reduce the output

noise from 430

µV to 100µV

RMS for a 100kHz

bandwidth at 5V output. With the bypass ca-

pacitor added, noise no longer scales with out-

put voltage so that improvements are more dra-

matic at higher output voltages.

100K

OUTPUT

ERROR

______

V OUT

GND

+VIN

1.23V

1.2 to 30V

.01µF

3.3µF

VREF

SHUTDOWN

INPUT

SPX2951

Figure 2. Adjustable Regulator

TYPICAL APPLICATIONS

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