SPX2954 データシートの表示（PDF） - Unspecified

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SPX2954

250mA Low Dropout Voltage Regulator

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SPX2954

Applications Information

External Capacitors

The stability of the SPX2954 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 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 increased 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 SPX2954 unlike other low dropout regulators will remain

stable and in regulation with no load in addition to the

internal voltage divider. This feature is especially important

in applications like CMOS RAM keep-alive. When setting

the output voltage of the SPX2954 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

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 3.3µF.

Error Detection Comparator Output

The comparator produces a logic low output whenever

the SPX2954 output falls out of regulation by more than

around 5%. This is around 60mV offset divided by the

1.235 reference voltage. This trip level remains 5% below

normal regardless of the programmed output voltage 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 VOUT = 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 output, which

requires an external pull-up resistor. Depending on the

system requirements the resistor 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 1MΩ. If the output is

unused this resistor is not required.

4.75V

OUTPUT

VOLTAGE

_______

ERROR*

+5.0V

+ INPUT

VOLTAGE 1.3V

* See Application Info.

+

+

Figure 1. Error Output Timing

Programming the Output Voltage of SPX2954

The SPX2954 may be pin-strapped for 5V using its internal

voltage divider by tying Pin 1 (output) 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:

VOUT = VREF x (1 + R1/R2) + IFB R1

The VREF is 1.235 and IFB is the feedback bias current,

nominally -20nA. The minimum recommended load current

of 1µA forces an upper limit of 1.2MΩ 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 12µA. Since the SPX2954

typically draws 60µA at no load with Pin 2 open-circuited,

this is a small price to pay.

REV1A

9/15

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