LP2950 datasheet(14/32 Pages) TI

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5 V

1.3 V

Input

Voltage

Output

Voltage

ERROR

4.75 V

LP2950, LP2951

SLVS582I – APRIL 2006 – REVISED NOVEMBER 2014

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7.4 Feature Description

7.4.1

ERROR Function (LP2951 Only)

The LP2951 device has a low-voltage detection comparator that outputs a logic low when the output voltage

drops by

≈6% from its nominal value, and outputs a logic high when VOUT has reached ≈95% of its nominal

value. This 95% of nominal figure is obtained by dividing the built-in offset of

≈60 mV by the 1.235-V bandgap

reference, and remains independent of the programmed output voltage. For example, the trip-point threshold

(ERROR output goes high) typically is 4.75 V for a 5-V output and 11.4 V for a 12-V output. Typically, there is a

hysteresis of 15 mV between the thresholds for high and low ERROR output.

A timing diagram is shown in Figure 31 for ERROR vs VOUT (5 V), as VIN is ramped up and down. ERROR

becomes valid (low) when VIN ≈ 1.3 V. When VIN ≈ 5 V, VOUT = 4.75 V, causing ERROR to go high. Because the

dropout voltage is load dependent, the output trip-point threshold is reached at different values of VIN, depending

on the load current. For instance, at higher load current, ERROR goes high at a slightly higher value of VIN, and

vice versa for lower load current. The output-voltage trip point remains at ~4.75 V, regardless of the load. Note

that when VIN ≤ 1.3 V, the ERROR comparator output is turned off and pulled high to its pullup voltage. If VOUT is

used as the pullup voltage, rather than an external 5-V source, ERROR typically is ~1.2 V. In this condition, an

equal resistor divider (10 k

Ω is suitable) can be tied to ERROR to divide down the voltage to a valid logic low

during any fault condition, while still enabling a logic high during normal operation.

Figure 31. ERROR Output Timing

Because the ERROR comparator has an open-collector output, an external pullup resistor is required to pull the

output up to VOUT or another supply voltage (up to 30 V). The output of the comparator is rated to sink up to

400

μA. A suitable range of values for the pullup resistor is from 100 kΩ to 1 MΩ. If ERROR is not used, it can

be left open.

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