SPX2920S-5.0 Просмотр технического описания (PDF) - Signal Processing Technologies

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SPX2920S-5.0

400mA Low Drop Out Voltage Regulator with Shutdown

Signal Processing Technologies 

APPLICATION HINTS

EXTERNAL CAPACITORS

The stability of the SPX2920 requires a 2.2µF 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 of below -25°C solid

tantalum is recommended since the many aluminum types have

electrolytes the 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 capacitor

value can 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 0.5µF and 0.15µF for 1mA. More

output capacitance needed for the 8-pin version at voltages below

5V since it runs the error amplifier at lower gain. At worst case

5µF or greater must be used for the condition of 250mA load at

1.23V output.

The SPX2920, 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

application like CMOS RAM keep-alive. When setting the output

voltage of the SPX2920, a minimum load of 10mA 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 0.1µF

tantalum or aluminum electrolytic capacitor should be placed

from the input to the ground.

Instability can occur if there is stray capacitance to the SPX2920

feedback terminal (pin 7). This could cause more problems when

using a higher value of external resistors to set the output voltage.

SPX2920

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 SPX2920

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 SPX2920

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 SPX2920’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 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.

PROGRAMMING THE OUTPUT VOLTAGE OF

SPX2920

The SPX2920 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).

4.75V

OUTPU T

VOLTAGE

_______

ERROR*

IN PU T

VOLTAGE

+5 . 0 V

+1 . 3 V

+

+

* See A pplication Info. _ _ __ _ __

Figure 1. ER R O R O utput Tim ing

+VIN

8

+VIN

1

VOUT

100k

Shutdown

3

Input

OFF

ON

SPX2920

FB

7

SD GND ERROR

5

4

1.2V TO 26V

+

R1

0.01uF

10uF

VREF

R2

ERROR

OUTPUT

Rev. 10/30/00

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