Rev. 10/25/00

SPX2700/01/02

APPLICATION HINTS

EXTERNAL CAPACITORS

The stability of the SPX2700/01/02 requires a 10

µF 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 500 kHz are the most important

parameters in the value of the capacitor. The capacitor 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 22

µF and 0.15µF 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 22

µF or

greater must be used for the condition of 750mA load at 1.23V

output.

The SPX2700/01/02, 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

SPX2700/01/02 version with external resistors, a minimum load of

5mA 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

µA tantalum or aluminum electrolytic capacitor

should be placed from the input to the ground.

ERROR DETECTION COMPARATOR OUTPUT

The Comparator produces a logic low output whenever the

SPX2700/01/02 output falls out of regulation by more than around

5%. This is around 75mV 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 SPX2700/01/02 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 SPX2700/01/02’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 250

µA, 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 resistor is not required.

PROGRAMMING

THE

OUTPUT

VOLTAGE

OF

SPX2700/01/02

The SPX2700/01/02 may be programmed for any output voltage

between its 1.235V reference and its 30V maximum rating . As seen

in Figure 2, an external pair of resistors is required. Refer to the

below equation for the programming of the output voltage:

nA. The minimum recommended load current of 1

µA forces an

upper limit of 1.2 M

increases the resistor program current by 12

µA. Since the

SPX2700/01/02 typically draws 100

µA at no load with

SHUTDOWN open-circuited this is a small price to pay

REDUCING OUTPUT NOISE

It may be an advantage to reduce the AC noise present at the output.

One way is to reduce the regulator bandwidth by increasing the size

of the output capacitor. This is the only way that noise can be

reduced on the 3 lead SPX2700/01/02 but is relatively inefficient, as

increasing the capacitor from 1

µF to 220µF only decreases the noise

from 430

since it reduces the high frequency gain from 4 to unity. Pick

or choose 0.01

µF. When doing this, the output capacitor must be

increased to 3.3

µF to maintain stability. These changes reduce the

output noise from 430

5V output. With the bypass capacitor added, noise no longer scales

with output voltage so that improvements are more dramatic at higher

output voltages.

+

+

+

+

4.75V

OUTPUT

VOLTAGE

ERROR*

_______

INPUT

VOLTAGE

1.3V

5.0V

Figure 1. ERROR Output Timing

_______

* See Application Info.

SPX2703

Fig.2 Adjustable Voltage Regulator

V

IN

V

OUT

GND

ADJ

SHUTDOWN