LP38501-ADJ Datasheet, PDF (12/16 Pages) National Semiconductor (TI) – 3A FlexCap Low Dropout Linear Regulator for 2.7V to 5.5V Inputs

30028139
FIGURE 9. 10 µF Ceramic Paralleled By 220 µF Generic
Aluminum Electrolytic
A solid Tantalum should work better, so the aluminum elec-
trolytic is replaced by a 220 µF Tantalum (Figure 10). The
peak amplitude of the output transient is now reduced to about
130 mV, just slightly worse than the value of the 47 µF ceramic
alone.
30028141
FIGURE 10. 10 µF Ceramic Paralleled By 220 µF Tantalum
The OSCON (ultra low ESR) aluminum electrolytic is the best
of the electrolytics. Figure 11 shows the output voltage tran-
sient is reduced down to about 90 mV (about 5% of V_OUT)
when a 220 µF OSCON is added to the 10 µF ceramic. This
indicates that some kind of ultra-low ESR aluminum elec-
trolytic used in parallel with some ceramic capacitance is
probably the best approach for extremely fast transients, but
each application must be dialed in for it’s specific load re-
quirements.
30028140
FIGURE 11. 10 µF Ceramic Paralleled By 220 µF OSCON
PRINTED CIRCUIT BOARD LAYOUT
Good layout practices will minimize voltage error and prevent
instability which can result from ground loops. The input and
output capacitors should be directly connected to the IC pins
with short traces that have no other current flowing in them
(Kelvin connect).
The best way to do this is to place the capacitors very near
the IC and make connections directly to the IC pins via short
traces on the top layer of the PCB. The regulator’s ground pin
should be connected through vias to the internal or backside
ground plane so that the regulator has a single point ground.
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-Mag-
netic Interference) can degrade any integrated circuit's per-
formance because of the small dimensions of the geometries
inside the device. In applications where circuit sources are
present which generate signals with significant high frequen-
cy 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 capac-
itors must be used at the input pin of the IC to reduce the
amount of EMI conducted into the IC.
If the LP38501/3-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 reg-
ulator 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). Ce-
ramic 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 by-
pass 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
www.national.com
12