TK11812 Ver la hoja de datos (PDF) - Toko America Inc
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TK11812 Datasheet PDF : 8 Pages
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TK11812
ADDITIONAL INFORMATION
APPLICATION INFORMATION
Maximize copper foil area connecting to all IC pins for
optimum performance. Place input and output bypass
capacitors close to the GND pin. For best transient behavior
and lowest output impedance, use as large of a capacitor
value as possible. The temperature coefficient of the
capacitance and Equivalent Series Resistance (ESR)
should be taken into account. These parameters can
influence power supply noise and ripple rejection. In extreme
cases, oscillation may occur. In order to maintain stability
the output bypass capacitor value should be a minimum of
10 µF in the case of tantalum electrolytic, or 15 µF in the
case of aluminum electrolytic.
HANDLING MOLDED RESIN PACKAGES
All plastic molded packages absorb some moisture from
the air. If moisture absorption occurs prior to soldering the
device into the printed circuit board, increased separation
of the lead from the plastic molding may occur, degrading
the moisture barrier characteristics of the device. This
property of plastic molding compounds should not be
overlooked, particularly in the case of very small packages
where the plastic is very thin.
In order to preserve the original moisture barrier properties
of the package, devices are stored and shipped in moisture
proof bags filled with dry air. The bags should not be
opened or damaged prior to actual use of the devices. If
this is unavoidable, the devices should be stored in a low
relative humidity environment (40 to 65%) or in an enclosed
environment with desiccant.
INDUCTOR NOTES
The output current and efficiency are largely dependent on
the coil used. A coil with lower DC resistance is generally
better in efficiency than one with a higher DCR (DC
Resistance). The recommended TOKO 395KN-0369 AQ
is a 1:1 turns ratio transformer with an inductance of 110
µH, and Toko 395KN-0370UG is a 1:3 transformer ratio
with the inductance of 60 µH: 600 µH. These coils or
equivalent should be used. Smaller coils with higher DCR
will not perform as well as the above coil, and the oscillator
will not be stable.
FEEDBACK RC TIME CONSTANT
If a different coil is used other than the one mentioned, the
RC time constant of the feedback loop will need to be
adjusted for optimum performance. Generally, a lower
resistance will give more output current. In fact, R can be
zero; however, lower resistance will sacrifice efficiency at
low output currents as the quiescent current increases. If
the capacitance is increased beyond or below a certain
value, the oscillator will become unstable. The optimum
RC values depend upon the operating voltage range,
output voltage and operating current, and should be chosen
experimentally using the given values of 3300 pF and
22 KΩ as a starting point.
INPUT/OUTPUT DECOUPLING CAPACITORS
DC-DC converters generate a large ripple current on both
the input and output of the circuit. The capacitors used
should be as large as possible and have low impedance in
the 300 kHz range. Since low temperatures cause
capacitors to decrease capacitance and increase
Equivalent Series Resistance (ESR) care should be taken
to choose capacitors that have acceptable characteristics
over the temperature range you intend to use. This should
be done experimentally to verify results, as capacitor
performance varies widely from manufacturer to
manufacturer. Tantalum capacitors are generally the best
choice and 10 µF should be adequate for most applications.
LOAD CHARACTERISTICS
The TK11812 should not be allowed to start up under full
load conditions. If this occurs, the output may not stabilize
to the correct output voltage. This can be compensated for,
somewhat, by adjusting the RC of the feedback loop or by
decreasing the output decoupling capacitor. Maximum
current can be drawn after the oscillator has started and
the output has reached nominal output voltage. This is
more critical with input voltages under 1.3 V as the converter
needs to generate sufficient output to ensure correct
internal bandgap and bias voltages.
June 1999 TOKO, Inc.
Page 5
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