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PJ2950

150mA Low Dropout Voltage Regulators

Promax Johnton 

PJ2950/PJ2951

150mA Low Dropout Voltage Regulators

The PJ2951 output voltage is

programmable for any value from it

 reference voltage of 1.250 volt

and it maximum rating of 30 volt .

For example for 5 volt needs to pin-

strapped and using the internal

voltage divider by tying pin 1 to 2

and pin 7 to pin 6. Refer to Figure

2, there are two ext ernal resistor

required for this programming .

REDUCING OUTPUT NOISE

It could be an advantage to reduce

the AC noise present at the output

One way is to reduce the regulator

bandwidth by increasing the value of

the output capacitor . this is the only

method that noise could reduce on

the PJ2950 but is relatively

inefficient , as increasing capacitor

from 1μF to 220μF only decreas es

the noise from 430μV to 160 μV

Refer to the below equation for the

programming the output voltage:

Volt = Vref x (1+R1\R2)+IFBR1

The Vref is 1.250 and IFB is the

feedback bias current , nominally -20

nA. The minimum recommended

load current of 1μA forces an upper

limit of 1.2MΩ on value of R2 , If

no load presented the IFB produces

error of typically 2% in Vout which

rms . for a 100KHz bandwidth at 5

volt output .

Noise could be also reduce by

fourfold by a bypass capacitor

across R1 since it reduces the high

frequency gain from 4 to unity . pick

CBYPASS = 1 / 2πR1 x 200Hz or

choose 0.01 μF . When doing this,

the output capacitor must be

increas ed to 3.3 μF to maintain

maybe eliminated at room

temperature by trimming R1 . To

improve the accuracy chooses the

value of R2 =100K and this

reduces the error by 0.17% and

increas es the resistor program

current by 12 μ A . The PJ2951

typically draw 60 μA at no load

with pin 2 open-circuit,and this

value does not look much .

stability . These changes the output

noise from 430μV rms . for a 100

kHz bandwidth 5 & 3.3 volt output.

With the bypass capacitor added,

noise no longer scales with output

voltage so that improvements are

more dramatic higher output

voltage .

FIGURE 1.ERROR OUTPUT TIMING

FIGURE 2.ADJUSTABLE REGULATOR

PJ2951

5-7

2002/01.rev.A

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