LP38500-ADJ Datasheet, PDF (12/23 Pages) National Semiconductor (TI) – 1.5A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs

LP38500-ADJ, LP38502-ADJ
SNVS539F – NOVEMBER 2007 – REVISED APRIL 2013
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The external resistors which set the output voltage must also be located very near the IC with all connections
directly tied via short traces to the pins of the IC (Kelvin connect). Do not connect the resistive divider to the load
point or DC error will be induced.
RFI/EMI SUSCEPTIBILITY
RFI (Radio Frequency Interference) and EMI (Electro-Magnetic 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 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 IC
to reduce the amount of EMI conducted into the IC.
If the LP38500/2-ADJ output is connected to a load 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 IC output. Since the
bandwidth of the regulator loop is less than 300 kHz, the control circuitry cannot respond to load changes above
that frequency. This means the effective output impedance of the IC at frequencies above 300 kHz is determined
only by the output capacitor(s). Ceramic capacitors provide the best performance in this type of application.
In applications where the load is switching at high speed, the output of the IC may need RF isolation from the
load. In such cases, it is recommended that some inductance be placed between the 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 PC Board
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 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 voltage or Broadband 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 generally be reduced in two ways: increase the transistor area or increase the reference current.
However, enlarging the transisitors will increase die size, and increasing the reference current means higher total
supply current (ground pin current).
SHORT-CIRCUIT PROTECTION
The LP38500/2-ADJ contains internal current limiting which will reduce output current to a safe value if the output
is overloaded or shorted. Depending upon the value of V_IN, thermal limiting may also become active as the
average power dissipated causes the die temperature to increase to the limit value (about 170°C). The hysteresis
of the thermal shutdown circuitry can result in a “cyclic” behavior on the output as the die temperature heats and
cools.
ENABLE OPERATION (LP38502-ADJ Only)
The Enable pin (EN) must be actively terminated by either a 10 kΩ pull-up resistor to V_IN, or a driver which
actively pulls high and low (such as a CMOS rail to rail comparator). If active drive is used, the pull-up resistor is
not required. This pin must be tied to V_IN if not used (it must not be left floating).
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