LP2953QML, LP2953QML-SP

SNVS395C – NOVEMBER 2010 – REVISED APRIL 2013

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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 (refer to Block

Diagram). 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 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.

* 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 32. ERROR Output Timing

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 31). The formula

for selecting the capacitor to be used is:

(3)

16

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