The main output requires less capacitance at lighter load cur-

rents. This capacitor can be reduced to 0.68

μF for currents

below 10 mA or 0.22

μF for currents below 1 mA.

Programming the main output for voltages below 5V requires

more output capacitance for stability. For the worst-case con-

dition of 1.23V output and 250 mA of load current, a 6.8

μF

(or larger) capacitor should be used.

A 1

μF capacitor should be placed from the input pin to ground

if there is more than 10 inches of wire between the input and

the AC filter capacitor or if a battery input is used.

Stray capacitance to the Feedback terminal can cause insta-

bility. This problem is most likely to appear when using high

value external resistors to set the output voltage. Adding a

100 pF capacitor between the Output and Feedback pins and

increasing the output capacitance to 6.8

μF (or greater) will

cure the problem.

MINIMUM LOAD ON MAIN OUTPUT

When setting the main output voltage using an external re-

sistive divider, a minimum current of 10

μA is recommended

through the resistors to provide a minimum load.

It should be noted that a minimum load current is specified in

several of the electrical characteristic test conditions, so the

specified value must be used to obtain test limit correlation.

PROGRAMMING THE MAIN OUTPUT VOLTAGE

The main output may be pin-strapped for 5V operation using

its internal resistive divider by tying the Output and Sense pins

together and also tying the Feedback and 5V Tap pins to-

gether.

Alternatively, it may be programmed for any voltage between

the 1.23V reference and the 29V maximum rating using an

external pair of resistors (see Figure 3 ). The complete equa-

tion for the output voltage is:

where V

pin bias current (−20 nA typical). The minimum recommended

load current of 1

μA sets an upper limit of 1.2 MΩ on the value

of R2 in cases where the regulator must work with no load

(see MINIMUM LOAD).

If I

produce a small error in V

reduce this error to 0.16% (typical) while increasing the re-

sistor program current to 12

μA. Since the typical quiescent

current is 130

μA, this added current is negligible.

1133911

*See Application Hints

**Drive with high to shut down

FIGURE 3. Adjustable Regulator

DROPOUT VOLTAGE

The dropout voltage of the regulator is defined as the mini-

mum input-to-output voltage differential required for the out-

put voltage to stay within 100 mV of the output voltage

measured with a 1V differential. The dropout voltage is inde-

pendent of the programmed output voltage.

DROPOUT DETECTION COMPARATOR

This comparator produces a logic “LOW” whenever the main

output falls out of regulation by more than about 5%. This fig-

ure results from the comparator's built-in offset of 60 mV

divided by the 1.23V reference (refer to block diagram). The

5% low trip level remains constant regardless of the pro-

grammed output voltage. An out-of-regulation condition can

result from low input voltage, current limiting, or thermal lim-

iting.

Figure 4 gives a timing diagram showing the relationship be-

tween the main output voltage, the ERROR output, and input

voltage as the input voltage is ramped up and down to a reg-

ulator whose main output is programmed for 5V. The ER-

ROR signal becomes low at about 1.3V input. It goes high at

about 5V input, where the main output equals 4.75V. Since

the dropout voltage is load dependent, the input voltage trip

points will vary with load current. The main output voltage

trip point does not vary.

The comparator has an open-collector output which requires

an external pull-up resistor. This resistor may be connected

to the regulator main output or some other supply voltage.

Using the main output prevents an invalid “HIGH” on the com-

parator output which occurs if it is pulled up to an external

voltage while the regulator input voltage is reduced below

1.3V. In selecting a value for the pull-up resistor, note that

while the output can sink 400

μA, this current adds to battery

drain. Suggested values range from 100 k

Ω to 1 MΩ. The

resistor is not required if the output is unused.

13

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11339 Version 8 Revision 4

Print Date/Time: 2009/12/07 17:53:15