LP2952-N, LP2952A, LP2953, LP2953A

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SNVS095F – MAY 2004 – REVISED MARCH 2015

Application Information (continued)

* See Power Supply Recommendations

** Drive with TTL-low to shutdown

Figure 31. Adjustable Regulator

8.1.4 Dropout Voltage

The dropout voltage of the regulator is defined as the minimum input-to-output voltage differential required for the

output voltage to stay within 100 mV of the output voltage measured with a 1-V differential. The dropout voltage

is independent of the programmed output voltage.

8.1.5 Dropout Detection Comparator

This comparator produces a logic low whenever the output falls out of regulation by more than about 5%. This

value results from the comparator built-in offset of 60 mV divided by the 1.23-V reference (see Functional Block

Diagrams). 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 32 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 5-V output. The ERROR

signal becomes low at about 1.3-V input. It goes high at about 5-V input, where the output equals 4.75 V.

Because 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 pullup 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 that occurs if it is pulled up to an external voltage while the regulator input voltage

is reduced below 1.3 V. In selecting a value for the pullup 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 VIN ≤ 1.3 V, the ERROR pin becomes a high impedance, allowing the error flag voltage to rise to its pullup

voltage. Using VOUT as the pullup voltage (rather than an external 5-V source) will keep the error flag voltage

below 1.2 V (typical) in this condition. The user may wish to divide down the error flag voltage using equal-value

resistors (10 k

Ω is suggested) to ensure a low-level logic signal during any fault condition, while still allowing a

valid logic high level during normal operation.

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