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LP2954, LP2954A

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SNVS096E – JUNE 1999 – REVISED JULY 2016

Product Folder Links: LP2954 LP2954A

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Copyright © 1999–2016, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 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 voltages

for various values of load current are listed under Electrical Characteristics.

If the regulator is powered from a rectified AC source with a capacitive filter, the minimum AC line voltage and

maximum load current must be used to calculate the minimum voltage at the input of the regulator. The minimum

input voltage, including AC ripple on the filter capacitor, must not drop below the voltage required to keep the

LP2954 in regulation. It is also advisable to verify operating at minimum operating ambient temperature, because

the increasing ESR of the filter capacitor makes this a worst-case test for dropout voltage due to increased ripple

amplitude.

7.3.2 Dropout Detection Comparator

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

5% value is from the comparators built-in offset of 60 mV divided by the 1.23-V 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 17 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 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 which 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 kΩ to 1 MΩ. This resistor is not

required if the output is unused.

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

resistors (10 kΩ suggested) to ensure a low-level logic signal during any fault condition, while still allowing a valid

high logic level during normal operation.

* In shutdown mode, ERROR goes 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 Dropout Voltage)

Figure 17. ERROR Output Timing

7.3.3 Output Isolation

The regulator output can be left connected to an active voltage source (such as a battery) with the regulator input

power turned 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.