LP2951-33-Q1 Datasheet, PDF (14/30 Pages) Texas Instruments – Adjustable Micropower Voltage Regulators With Shutdown

LP2951-33-Q1, LP2951-50-Q1
SLVSAW6E – JUNE 2011 – REVISED NOVEMBER 2014
www.ti.com
7.3 Feature Description
7.3.1 ERROR Function
The LP2951-xx-Q1 devices have 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 V_OUT 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 32 for ERROR vs V_OUT (5 V), as V_IN is ramped up and down. ERROR
becomes valid (low) when V_IN ≈ 1.3 V. When V_IN ≈ 5 V, V_OUT = 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 V_IN, depending
on the load current. For instance, at higher load current, ERROR goes high at a slightly higher value of V_IN, and
vice versa for lower load current. The output-voltage trip point remains at ~4.75 V, regardless of the load. Note
that when V_IN ≤ 1.3 V, the ERROR comparator output is turned off and pulled high to its pullup voltage. If V_OUT 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 kΩ 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.
Output
Voltage
4.75 V
ERROR
Input
Voltage
5V
1.3 V
Figure 32. ERROR Output Timing
Because the ERROR comparator has an open-collector output, an external pullup resistor is required to pull the
output up to V_OUT 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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