LP2950-N, LP2951-N
SNVS764N –JANUARY 2000–REVISED MAY 2013
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value output capacitors when the condition is removed, and the recommended generic solution is to set the
output capacitor to a value not less than 10 μF. Although the 10 μF value for COUT may not eliminate the output
voltage over-shoot in all cases, it should lower it to acceptable levels (<10% of VOUT(NOM)) in the majority of
cases. In all three of these conditions, applications with lighter load currents are more susceptible to output
voltage over-shoot than applications with higher load currents.
1) At power-up, with the input voltage rising faster than output stage can charge the output capacitor.
VIN tRISE(MIN) > ((COUT / 100 mA) x ΔVIN)
where
•
ΔVIN = VOUT(NOM) + 1.0V
(4)
(5)
(6)
2) Recovery from an output short circuit to ground condition.
C
OUT(MIN)≈ (160 mA - ILOAD(NOM))/((VOUT(NOM)/10)/25 μs))
3) Toggling the LP2951-N SHUTDOWN pin from high (i.e. OFF) to low (i.e. ON).
C
OUT(MIN)≈ (160 mA - ILOAD(NOM))/((VOUT(NOM)/10)/25 μs))
Figure 45. LP2951-N Enable Transient
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.
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 60mV 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 4.75V
for a 5V output or 11.4V for a 12V output. The out of regulation condition may be due either to low input voltage,
current limiting, or thermal limiting.
Figure 46 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 5V versions, the ERROR signal becomes valid (low) at about 1.3V
input. It goes high at about 5V input (the input voltage at which VOUT = 4.75V). Since the LP2951-N's dropout
voltage is load-dependent (see curve in typical performance characteristics), the input voltage trip point (about
5V) will vary with the load current. The output voltage trip point (approx. 4.75V) 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 100k to 1 MΩ. The resistor is not required if this output is
unused.
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