SPX2950ACN-3.3(2003) Просмотр технического описания (PDF) - Signal Processing Technologies

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SPX2950ACN-3.3
(Rev.:2003)

150mA Low Dropout Voltage Regulator

Signal Processing Technologies 

APPLICATION HINTS

EXTERNAL CAPACITORS

The stability of the SPX2950/SPX2951 requires

a 1µF or greater capacitor between output and

ground. Oscillation could occur without this

capacitor. Most types of tantalum or aluminum

electrolytic works fine here. For operations be-

low -25°C solid tantalum is recommended since

the many aluminum types have electrolytes that

freeze at about -30°C. The ESR of about 5Ω or

less and resonant frequency above 500kHz are

the most important parameters in the value of

the capacitor. The capacitors value may be in-

creased without limit.

At lower values of output current, less output

capacitance is required for stability. For the

currents below 10mA the value of the capacitor

can be reduced to 0.33µF and 0.1µF for 1mA.

More output capacitance is needed for the 8-pin

version at voltages below 5V since it runs the

error amplifier at lower gain. At worst case 3.3µF

or greater must be used for the condition of

150mA load at 1.23V output.

The SPX2950/51 unlike other low dropout regu-

lators will remain stable and in regulation with

no load in addition to the internal voltage di-

vider. This feature is especially important in

applications like CMOS RAM keep-alive. When

setting the output voltage of the SPX2950/51

version with external resistors, a minimum load

of 1µA is recommended

If there is more than 10 inches of wire between

the input and the AC filter capacitor or if a

battery is used as the input then a 1µF tantalum

or aluminum electrolytic capacitor should be

placed from the input to the ground.

Instability can occur if there is stray capacitance

to the SPX2951 feedback terminal (pin 7). This

could cause more problems when using a higher

value of external resistors to set the output

voltage. This problem can be fixed by adding a

100pF capacitor between output and feedback

and increasing the output capacitor to at least

3.3µF.

4.75V

OUTPUT

VOLTAGE

_______

ERROR*

+5.0V

+ INPUT

VOLTAGE 1.3V

* See Application Info.

Fig. 1 ERROR Output Timing

+

+

ERROR DETECTION COMPARATOR

OUTPUT

The comparator produces a logic low output

whenever the SPX2951 output falls out of regu-

lation by more than around 5%. This is around

60mV offset divided by the 1.235 reference

voltage. This trip level remains 5% below nor-

mal regardless of the programmed output volt-

age of the regulator.

Figure 1 shows the timing diagram depicting the

ERROR signal and the regulator output voltage

as the SPX2951 input is ramped up and down.

The ERROR signal becomes low at around 1.3V

input, and goes high around 5V input (input

voltage at which VOUT = 4.75). Since the

SPX2951’s dropout voltage is load dependent,

the input voltage trip point (around 5V) will

vary with the load current. The output voltage

trip point (approx. 4.75V) does not vary with

load.

The error comparator has an open-collector out-

put, which requires an external pull-up resistor.

Depending on the system requirements the re-

sistor may be returned to 5V output or other

supply voltage. In determining the value of this

resistor, note that the output is rated to sink

400µA, this value adds to battery drain in a low

battery condition. Suggested values range from

100K to 1MΩ. If the output is unused this

resistor is not required.

Rev. 3/6/03

SPX2950/SPX2951 150mA Low Dropout Voltage Regulator

8

©Copyright 2003 Sipex Corporation

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