PJ2950 查看數據表（PDF） - Promax Johnton

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PJ2950

150mA Low Dropout Voltage Regulators

Promax Johnton 

PJ2950/PJ2951

150mA Low Dropout Voltage Regulators

PROGRAMMING THE OUTPUT VOLTAGE OF PJ2951

External Capacitors

For the stability of the

PJ2950/PJ2951 requires a 1.0 μF or

greater capacitor between output and

ground. Oscillation could occur

without this capacitor. The tantalum

or aluminum electrolytic works fine;

could use the fiLP type but are not

cost effi cient. For the operation of

below -25 ℃ solid tantalum is

recommended since the many

aluminum type have electrolytes the

freeze at about -30℃. The ESR of

about 5 Ω or less and resonant

frequency above 500KHz are most

important parameter in the value of

Capacitor. The Capacitors value can

be increas ed without any limit.

At lower values of output current,

less output capacitance is required

for stability. For the currents below

10 mA the value of capacitor can be

reduce to 0.33μ F and 0.1μF for

1mA. More output capacitance

needed for the 8-pin version at

voltage below 5V since it runs the

error amplifi er at lower gain. At

worst case 3.3μF or greater must be

used for condition of 100mA load at

1.250V output .

The PJ2950 not like other low

dropout regulators will remain stable

and regulation with no load in

addition to the internal voltage

divider. This feature very important

in application like CMOS RAM

keep-alive. The PJ2951 output

voltage setting with minimum load

of 1 μA and external resistors.

If at the input of PJ2950/LP

2951 connected to battery or between

AC filter capacitor and input is 10

inches wire then 1μF tantalum or

aluminum electrolytic capacitor

should be connect ed between input

and ground .

Instability can occur if stray

capacitor to feedback terminal pin 7

of the PJ2951. This could cues more

problem more when using higher

value of external to set the output

voltage . To fix this problem the 100

pF capacitor between output and

feedback and increasing output

capacitance to 3.3μF.

ERROR DETECTION COMPARATOR OUTPUT

The Compactor produces a logic

low output whenever the PJ2951

output falls of regulation by more

than around 5% . This is around 60

mV offs et divided by the 1.250

reference voltage. Thus trip remains

5% below nominal regardl ess of the

programmed output voltage of the

regulator . the figure 1 shows the

timing diagram depicting the

ERROR signal and the regulator

output voltage as the PJ2951 input is

ramped up and down . the ERROR

signal becomes low at around 1.3 V

input , it goes high around 5Volt

input , (input voltage at which Vout

= 4.75). The PJ2951 dropout voltage

depending on the load , the input

voltage trip point around 5 volt will

vary with load current. The output

voltage trip point is around 4.75 volt

it does not change with load.

The error comparator has an open-

collector output which requires an

external pull-up resistor.

Depending on the system

requirem ent the resistor maybe

returned to 5 or 3.3 volt output or

other supply voltage depending to

the system requirements. For

determining the size of the resistor,

note that the output is sinking 400μ

A , this value adds to battery drain in

a low better condition, recommended

values 100K to 1MΩ. If the output

is unused this resistor

is not required.

4-7

2002/01.rev.A

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