SPX2954AT5-3.3 datasheet(8/7 Pages) SIPEX

Part #	SPX2954AT5-3.3

Description	250 mA Low Drop Out Voltage Regulator

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Manufacturer	SIPEX [Sipex Corporation]

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8 / 7 page

Rev. 3/6/03

SP2954 250mA Low Dropout Voltage Regulator

8

EXTERNAL CAPACITORS

The stability of the SPX2954 requires a 1

µF or

greater capacitor between output and ground.

Oscillation could occur without this capacitor.

Most types of tantalum or aluminum electro-

lytic works fine here. For operations below

-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 0.33

µF 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 3.3

µF or greater must be used for the condition of

150mA load at 1.23V output.

The SPX2954 unlike other low dropout regula-

tors will remain stable and in regulation with no

load in addition to the internal voltage divider.

This feature is especially important in applica-

tions like CMOS RAM keep-alive. When set-

ting the output voltage of the SPX2954 version

with external resistors, a minimum load of 1

µA

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 1

µF tantalum

or aluminum electrolytic capacitor should be

placed from the input to the ground.

Instability can occur if there is stray capacitance

to the SPX2954 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

3.3

µF.

ERROR DETECTION COMPARATOR

OUTPUT

The comparator produces a logic low output

whenever the SPX2954 output falls out of regu-

lation by more than around 5%. This is around

60 mV 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 SPX2954 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 V

OUT

= 4.75). Since the

SPX2954’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

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

battery condition. Suggested values range from

100k to 1M

Ω. If the output is unused this resis-

tor is not required.

PROGRAMMING

THE

OUTPUT

VOLTAGE OF SPX2954

The SPX2954 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 using an external

pair of resistors. Refer to the below equation for

the programming of the output voltage:

V

OUT = VREF x (1 + R1/R2)+ IFBR1

The V

REF is 1.235 and IFB is the feedback bias

current, nominally -20nA. The minimum

recommended load current of 1

µA forces an

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