LP3963 Datasheet, PDF (12/25 Pages) National Semiconductor (TI) – 3A Fast Ultra Low Dropout Linear Regulators

LP3963, LP3966
SNVS067H – APRIL 2000 – REVISED APRIL 2013
www.ti.com
RFI/EMI SUSCEPTIBILITY
RFI (radio frequency interference) and EMI (electromagnetic interference) can degrade any integrated circuit's
performance because of the small dimensions of the geometries inside the device. In applications where circuit
sources are present which generate signals with significant high frequency energy content (> 1 MHz), care must
be taken to ensure that this does not affect the IC regulator.
If RFI/EMI noise is present on the input side of the LP396X regulator (such as applications where the input
source comes from the output of a switching regulator), good ceramic bypass capacitors must be used at the
input pin of the LP396X.
If a load is connected to the LP396X output which switches at high speed (such as a clock), the high-frequency
current pulses required by the load must be supplied by the capacitors on the LP396X output. Since the
bandwidth of the regulator loop is less than 100 kHz, the control circuitry cannot respond to load changes above
that frequency. The means the effective output impedance of the LP396X at frequencies above 100 kHz is
determined only by the output capacitor(s).
In applications where the load is switching at high speed, the output of the LP396X may need RF isolation from
the load. It is recommended that some inductance be placed between the LP396X output capacitor and the load,
and good RF bypass capacitors be placed directly across the load.
PCB layout is also critical in high noise environments, since RFI/EMI is easily radiated directly into PC traces.
Noisy circuitry should be isolated from "clean" circuits where possible, and grounded through a separate path. At
MHz frequencies, ground planes begin to look inductive and RFI/EMI can cause ground bounce across the
ground plane.
In multi-layer PCB applications, care should be taken in layout so that noisy power and ground planes do not
radiate directly into adjacent layers which carry analog power and ground.
OUTPUT ADJUSTMENT
An adjustable output device has output voltage range of 1.216V to 5.1V. To obtain a desired output voltage, the
following equation can be used with R1 always a 10kΩ resistor.
For output stability, C_F must be between 68pF and 100pF.
TURN-ON CHARACTERISTICS FOR OUTPUT VOLTAGES PROGRAMMED TO 2.0V OR BELOW
As Vin increases during start-up, the regulator output will track the input until Vin reaches the minimum operating
voltage (typically about 2.2V). For output voltages programmed to 2.0V or below, the regulator output may
momentarily exceed its programmed output voltage during start up. Outputs programmed to voltages above 2.0V
are not affected by this behavior.
OUTPUT NOISE
Noise is specified in two ways-
Spot Noise or Output noise density is the RMS sum of all noise sources, measured at the regulator output, at
a specific frequency (measured with a 1Hz bandwidth). This type of noise is usually plotted on a curve as a
function of frequency.
Total output Noise or Broad-band noise is the RMS sum of spot noise over a specified bandwidth, usually
several decades of frequencies.
Attention should be paid to the units of measurement. Spot noise is measured in units µV/√Hz or nV/√Hz and
total output noise is measured in µV(rms).
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