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 in-

dependent of the programmed output voltage.

DROPOUT DETECTION COMPARATOR

This comparator produces a logic “LOW” whenever the out-

put falls out of regulation by more than about 5%. This figure

results from the comparator’s built-in offset of 60 mV divided

by the 1.23V reference (refer to block diagrams on page 1).

The 5% low trip level remains constant regardless of the

programmed output voltage. An out-of-regulation condition

can result from low input voltage, current limiting, or thermal

limiting.

Figure 5 gives a timing diagram showing the relationship

between the output voltage, the ERROR output, and input

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

regulator programmed for 5V output. The ERROR signal

becomes low at about 1.3V input. It goes high at about 5V

input, where the output equals 4.75V. Since the dropout

voltage is load dependent, the input voltage trip points will

vary with load current. The 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 output or some other supply voltage. Using

the regulator 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. This

resistor is not required if the output is unused.

When V

IN

≤ 1.3V, the error flag pin becomes a high imped-

ance, allowing the error flag voltage to rise to its pull-up

voltage. Using V

external 5V source) will keep the error flag voltage below

1.2V (typical) in this condition. The user may wish to divide

down the error flag voltage using equal-value resistors

(10 k

Ω suggested) to ensure a low-level logic signal during

any fault condition, while still allowing a valid high logic level

during normal operation.

OUTPUT ISOLATION

The regulator output can be left connected to an active

voltage source (such as a battery) with the regulator input

power shut off, as long as the regulator ground pin is

connected to ground. If the ground pin is left floating,

damage to the regulator can occur if the output is pulled

up by an external voltage source.

REDUCING OUTPUT NOISE

In reference applications it may be advantageous to reduce

the AC noise present on the output. One method is to reduce

regulator bandwidth by increasing output capacitance. This

is relatively inefficient, since large increases in capacitance

are required to get significant improvement.

Noise can be reduced more effectively by a bypass capacitor

placed across R1 (refer to Figure 4). The formula for select-

ing the capacitor to be used is:

This gives a value of about 0.1 µF. When this is used, the

output capacitor must be 6.8 µF (or greater) to maintain

stability. The 0.1 µF capacitor reduces the high frequency

gain of the circuit to unity, lowering the output noise from 260

µV to 80 µV using a 10 Hz to 100 kHz bandwidth. Also, noise

is no longer proportional to the output voltage, so improve-

ments are more pronounced at high output voltages.

AUXILIARY COMPARATOR (LP2953 only)

The LP2953 contains an auxiliary comparator whose invert-

ing input is connected to the 1.23V reference. The auxiliary

comparator has an open-collector output whose electrical

characteristics are similar to the dropout detection compara-

tor. The non-inverting input and output are brought out for

external connections.

01112709

* See Application Hints

** Drive with TTL-low to shut down

FIGURE 4. Adjustable Regulator

01112710

* In shutdown mode, ERROR will go high if it has been pulled up to an

external supply. To avoid this invalid response, pull up to regulator output.

** Exact value depends on dropout voltage. (See Application Hints)

FIGURE 5. ERROR Output Timing

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