LP3962 Datasheet, PDF (13/22 Pages) National Semiconductor (TI) – 1.5A Fast Ultra Low Dropout Linear Regulators

Applications Information (Continued)
the LP396X. Derating must be applied to the manufacturer’s
ESR specification, since it is typically only valid at room
temperature.
Any applications using aluminum electrolytics should be
thoroughly tested at the lowest ambient operating tempera-
ture where ESR is maximum.
PCB LAYOUT
Good PC layout practices must be used or instability can be
induced because of ground loops and voltage drops. The
input and output capacitors must be directly connected to the
input, output, and ground pins of the LP3962/5 using traces
which do not have other currents flowing in them Kelvin
connect).
The best way to do this is to lay out C_IN and C_OUT near the
device with short traces to the V_IN, V_OUT, and ground pins.
The regulator ground pin should be connected to the exter-
nal circuit ground so that the regulator and its capacitors
have a ’single point ground’.
It should be noted that stability problems have been seen in
applications where ’vias’ to an internal ground plane were
used at the ground points of the LP3962/5 IC and the input
and output capacitors. This was caused by varying ground
potentials at these nodes resulting from current flowing
through the ground plane. Using a single point ground tech-
nique for the regulator and it’s capacitors fixed the problem.
Since high current flows through the traces going into V_IN
and coming from V_OUT, Kelvin connect the capacitor leads to
these pins so there is no voltage drop in series with the input
and output capacitors.
RFI/EMI SUSCEPTIBILITY
RFI (radio frequency interference) and EMI (electromagnetic
interference) can degrade any integrated circuit’s perfor-
mance because of the small dimensions of the geometries
inside the device. In applications where circuit sources are
present which generate signals with significant high fre-
quency 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 ce-
ramic 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 capaci-
tors 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 pos-
sible, and grounded through a separate path. At MHz fre-
quencies, 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
1.215V 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.
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 spe-
cific frequency (measured with a 1Hz bandwidth). This type
of noise is usually plotted on a curve as a function of fre-
quency.
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).
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, LP3962/LP3965
achieves low noise performance and low quiescent current
operation.
The total output noise specification for LP3962/LP3965 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 LP3962and LP3965 is short circuit protected and in the
event of a peak over-current condition, the short-circuit con-
trol 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 section on thermal information for power
dissipation calculations.
Error Flag Operation
The LP3962/LP3965 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
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