LP3963 Datasheet, PDF (13/25 Pages) National Semiconductor (TI) – 3A Fast Ultra Low Dropout Linear Regulators

LP3963, LP3966
www.ti.com
SNVS067H – APRIL 2000 – REVISED APRIL 2013
The primary source of noise in low-dropout regulators is the internal reference. In CMOS regulators, noise has a
low frequency component and a high frequency component, which depend strongly on the silicon area and
quiescent current. Noise can be reduced in two ways: by increasing the transistor area or by increasing the
current drawn by the internal reference. Increasing the area will decrease the chance of fitting the die into a
smaller package. Increasing the current drawn by the internal reference increases the total supply current
(ground pin current). Using an optimized trade-off of ground pin current and die size, LP3963/LP3966 achieves
low noise performance and low quiescent current operation.
The total output noise specification for LP3963/LP3966 is presented in the Electrical Characteristics table. The
Output noise density at different frequencies is represented by a curve under typical performance characteristics.
SHORT-CIRCUIT PROTECTION
The LP3963 and LP3966 is short circuit protected and in the event of a peak over-current condition, the short-
circuit control loop will rapidly drive the output PMOS pass element off. Once the power pass element shuts
down, the control loop will rapidly cycle the output on and off until the average power dissipation causes the
thermal shutdown circuit to respond to servo the on/off cycling to a lower frequency. Please refer to the POWER
DISSIPATION/HEATSINKING for power dissipation calculations.
ERROR FLAG OPERATION
The LP3963/LP3966 produces a logic low signal at the Error Flag pin when the output drops out of regulation
due to low input voltage, current limiting, or thermal limiting. This flag has a built in hysteresis. The timing
diagram in Figure 21 shows the relationship between the ERROR flag and the output voltage. In this example,
the input voltage is changed to demonstrate the functionality of the Error Flag.
The internal Error flag comparator has an open drain output stage. Hence, the ERROR pin should be pulled high
through a pull up resistor. Although the ERROR flag pin can sink current of 1mA, this current is energy drain
from the input supply. Hence, the value of the pull up resistor should be in the range of 10kΩ to 1MΩ. The
ERROR pin must be connected to ground if this function is not used. It should also be noted that when the
shutdown pin is pulled low, the ERROR pin is forced to be invalid for reasons of saving power in shutdown
mode.
Figure 21. Error Flag Operation
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