LP2950-N, LP2951-N
SNVS764Q –JANUARY 2000–REVISED DECEMBER 2017
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Figure 38. LP2951-N Enable Transient
9.2.1.2.2 Input Capacitor Requirements
A minimum 1 µF tantalum, ceramic or aluminum electrolytic capacitor should be placed from the LP2950-
N/LP2951-N input pin to ground if there is more than 10 inches of wire between the input and the AC filter
capacitor or if a battery is used as the input.
9.2.1.2.3 Error Detection Comparator Output
The comparator produces a logic low output whenever the LP2951-N output falls out of regulation by more than
approximately 5%. This figure is the comparator's built-in offset of about 60 mV divided by the 1.235 reference
voltage. (Refer to the block diagram in the front of the datasheet.) This trip level remains “5% below normal”
regardless of the programmed output voltage of the 2951. For example, the error flag trip level is typically 4.75 V
for a 5-V output or 11.4 V for a 12-V output. The out of regulation condition may be due either to low input
voltage, current limiting, or thermal limiting.
Figure 39 below gives a timing diagram depicting the ERROR signal and the regulated output voltage as the
LP2951-N input is ramped up and down. For 5 V versions, the ERROR signal becomes valid (low) at about 1.3-V
input. It goes high at about 5-V input (the input voltage at which VOUT = 4.75 V). Because the LP2951-N dropout
voltage is load-dependent (see curve in typical performance characteristics), the input voltage trip point (about
5 V) varies with the load current. The output voltage trip point (approx. 4.75 V) does not vary with load.
The error comparator has an open-collector output which requires an external pull up resistor. This resistor may
be returned to the output or some other supply voltage depending on system requirements. In determining a
value for this resistor, note that while the output is rated to sink 400 µA, this sink current adds to battery drain in
a low battery condition. Suggested values range from 100 k to 1 MΩ. The resistor is not required if this output is
unused.
*When VIN ≤ 1.3 V, the error flag pin becomes a high impedance, and the error flag voltage rises to its pullup voltage.
Using VOUT as the pullup voltage (see Figure 40), rather than an external 5-V source, keeps the error flag voltage
under 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.
Figure 39. ERROR Output Timing
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