LP38691-ADJ Datasheet, PDF (10/13 Pages) National Semiconductor (TI) – 500mA Low Dropout CMOS Linear Regulators with Adjustable Output / Stable with Ceramic Output Capacitors

Application Hints
EXTERNAL CAPACITORS
Like any low-dropout regulator, external capacitors are re-
quired to assure stability. These capacitors must be correctly
selected for proper performance.
INPUT CAPACITOR: An input capacitor of at least 1µF is
required (ceramic recommended). The capacitor must be
located not more than one centimeter from the input pin and
returned to a clean analog ground.
OUTPUT CAPACITOR: An output capacitor is required for
loop stability. It must be located less than 1 centimeter from
the device and connected directly to the output and ground
pins using traces which have no other currents flowing
through them.
The minimum amount of output capacitance that can be
used for stable operation is 1µF. Ceramic capacitors are
recommended (the LP38691/3-ADJ was designed for use
with ultra low ESR capacitors). The LP38691/3-ADJ is stable
with any output capacitor ESR between zero and 100 Ohms.
SETTING THE OUTPUT VOLTAGE: The output voltage is
set using the external resistors R1 and R2 (see Typical
Application Circuit). The output voltage will be given by the
equation:
V_OUT = V_ADJ X (1 + R1/R2)
Because the part has a minimum load current requirement of
100 µA, it is recommended that R2 always be 12k Ohms or
less to provide adequate loading. Even if a minimum load is
always provided by other means, it is not recommended that
very high value resistors be used for R1 and R2 because it
can make the ADJ node susceptible to noise pickup. A
maximum Ohmic value of 100k is recommended for R2 to
prevent this from occurring.
ENABLE PIN (LP38693-ADJ only): The LP38693-ADJ has
an enable pin which turns the regulator output on and off.
Pulling the enable pin down to a logic low will turn the part
off. The voltage the pin has to be pulled up to in order to
assure the part is on depends on input voltage (refer to
Electrical Characteristics section). This pin should be tied to
V_IN if the enable function is not used.
FOLDBACK CURRENT LIMITING: Foldback current limit-
ing is built into the LP38691/3-ADJ which reduces the
amount of output current the part can deliver as the output
voltage is reduced. The amount of load current is dependent
on the differential voltage between V_IN and V_OUT. Typically,
when this differential voltage exceeds 5V, the load current
will limit at about 350 mA. When the V_IN -V_OUT differential is
reduced below 4V, load current is limited to about 850 mA.
SELECTING A CAPACITOR
It is important to note that capacitance tolerance and varia-
tion with temperature must be taken into consideration when
selecting a capacitor so that the minimum required amount
of capacitance is provided over the full operating tempera-
ture range.
Capacitor Characteristics
CERAMIC: For values of capacitance in the 10 to 100 µF
range, ceramics are usually larger and more costly than
tantalums but give superior AC performance for bypassing
high frequency noise because of very low ESR (typically less
than 10 mΩ). However, some dielectric types do not have
good capacitance characteristics as a function of voltage
and temperature.
Z5U and Y5V dielectric ceramics have capacitance that
drops severely with applied voltage. A typical Z5U or Y5V
capacitor can lose 60% of its rated capacitance with half of
the rated voltage applied to it. The Z5U and Y5V also exhibit
a severe temperature effect, losing more than 50% of nomi-
nal capacitance at high and low limits of the temperature
range.
X7R and X5R dielectric ceramic capacitors are strongly rec-
ommended if ceramics are used, as they typically maintain a
capacitance range within ±20% of nominal over full operat-
ing ratings of temperature and voltage. Of course, they are
typically larger and more costly than Z5U/Y5U types for a
given voltage and capacitance.
TANTALUM: Solid Tantalum capacitors have good tempera-
ture stability: a high quality Tantalum will typically show a
capacitance value that varies less than 10-15% across the
full temperature range of -40˚C to 125˚C. ESR will vary only
about 2X going from the high to low temperature limits.
The increasing ESR at lower temperatures can cause oscil-
lations when marginal quality capacitors are used (if the ESR
of the capacitor is near the upper limit of the stability range at
room temperature).
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 regulator 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 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 technique 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 regulator
(such as applications where the input source comes from the
output of a switching regulator), good ceramic bypass ca-
pacitors must be used at the input pin of the IC.
If a load is connected to the IC 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 IC output. Since the bandwidth of the regulator loop is
less than 100 kHz, the control circuitry cannot respond to
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