LP2954, LP2954A

SNVS096D – JUNE 1999 – REVISED MARCH 2013

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recommended load current of 1

μA sets an upper limit of 1.2 MΩ on the value of R2 in cases where the regulator

eliminated at room temperature by trimming R1. For better accuracy, choosing R2 = 100 k

Ω will reduce this error

to 0.17% while increasing the resistor program current to 12

μA. Since the typical quiescent current is 120 μA,

this added current is negligible.

* See Application Hints

** Drive with TTL-low to shut down

Figure 24. Adjustable Regulator

DROPOUT DETECTION COMPARATOR

This comparator produces a logic “LOW” whenever the output 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. 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 25 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 comparator 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 kΩ to 1 MΩ. This resistor is not

required if the output is unused.

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.

12

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