LP2960

SNVS112C – APRIL 1999 – REVISED APRIL 2013

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OUTPUT ISOLATION

If the LP2960 output is connected to an active voltage source (such as a battery) the regulator input should not

be shorted to ground, as this will cause a large current to flow from the battery into the LP2960 output lead.

If the LP2960 input is left floating with the output connected to a battery, a small current (a few mA) will flow into

the output lead.

The “reverse” current flowing from the battery into the LP2960 output can be prevented by using a blocking diode

between the output and the battery.

REDUCING OUTPUT NOISE

In reference applications it may be desirabIe 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 Basic Application

Circuit).

A 0.1

μF capacitor connected across R1 will reduce the high frequency gain of the circuit to unity, lowering the

RMS output noise voltage from 210

μV to 130 μV (typical) using a 10 Hz–100 kHz bandwidth test measurement.

Also, output noise is no longer proportional to the output voltage, so improvements are more pronounced at

higher output voltages.

IMPORTANT: Since the 0.1

μF capacitor reduces the AC gain of the LP2960 to unity, the output capacitance

must be increased to at least 33

μF to assure regulator stability.

DROPOUT DETECTION COMPARATOR

The dropout detection comparator produces a logic “LOW” on the Error output whenever the LP2960 output

drops 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% below nominal” 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.

The figure below 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.

*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 26. Error Output Timing Diagram

The Error signal becomes low as VIN exceeds about 1.3V. 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, but the output voltage trip point does not.

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