TK712xx
APPLICATION INFORMATION
INPUT/OUTPUT
DECOUPLING
CAPACITOR
OPTIMUM PERFORMANCE
CONSIDERATIONS
Optimum performance can only be achieved when the IC
is mounted on a PC board according to the diagram below.
Thisisbecauseoftheextremelysmallpackageandlimited
power dissipation. Shape the metal portion of the PCB as
shown in the following drawing.
Voltage regulators require input and output decoupling
capacitors. The required value of these capacitors vary
with application. Capacitors made by different
manufacturers can have different characteristics,
particularly with regard to high frequencies and Equivalent
Series Resistance (ESR) over temperature. The type of
capacitorisalsoimportant. Forexample,a4.7µFaluminum
electrolytic may be required for a certain application. If a
tantalum capacitor is used, a lower value of 2.2 µF would
be adequate. It is important to consider the temperature
characteristics of the decoupling capacitors. While Toko
regulators are designed to operate as low as -40 °C, many
capacitors will not operate properly at this temperature.
The capacitance of aluminum electrolytic capacitors may
decrease to 0 at low temperatures. This may cause
oscillation on the output of the regulator since some
capacitance is required to guarantee stability. Thus, it is
important to consider the characteristics of the capacitor
over temperature when selecting decoupling capacitors.
V
IN
V
OUT
+
+
GND
GND
C
N
SOT-25 BOARD LAYOUT
The ESR is another important parameter. The ESR will
increase with temperature but low ESR capacitors are
oftenlargerandmorecostly.Ingeneral,tantalumcapacitors
offer lower ESR than aluminum electrolytic, but new low
ESR aluminum electrolytic capacitors are now available
from several manufacturers. Usually a bench test is
sufficient to determine the minimum capacitance required
for a particular application. After taking thermal
characteristics and tolerance into account, the minimum
capacitance value should be approximately two times this
value. The recommended minimum capacitance for the
TK712xx is 2.2 µF for a tantalum capacitor or 3.3 µF for an
aluminum electrolytic. Please note that linear regulators
with a low dropout voltage have high internal loop gains
which require care in guarding against oscillation caused
by insufficient decoupling capacitance. The use of high
quality decoupling capacitors suited for your application
will guarantee proper operation of the circuit. Pay attention
to temperature characteristics of the capacitor, especially
the increase of ESR and decrease of capacitance in low
temperatures. Oscillation, reduction of ripple rejection and
increased noise may occur in some cases if the proper
capacitorisnotused. Anoutputcapacitormorethan1.0µF
isrequiredtomaintainstability.Thestandardtestcondition
is 3.3 µF (TA = 25 °C).
Use a large bypass capacitor and connect it in a place near
GNDoftheIC.Payattentiontotemperaturecharacteristics
of the capacitor, especially the increase of ESR and
decrease of capacitance in low temperatures. Oscillation,
reduction of ripple rejection and increased noise may
occur in some cases if the proper capacitor is not used. An
output capacitor more than 1.0 µF is required to maintain
stability. The standard test condition is 3.3 µF (TA = 25 °C).
January 1999 TOKO, Inc.
Page 17
TOKO [ TOKO, INC ]
