RH5RH
APPLICATION HINTS
When using these ICs, be sure to take care of the following points :
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Set external components as close as possible to the IC and minimize the connection between the components
and the IC. In particular, when an external component is connected to OUT Pin, make minimum connection
with the capacitor.
Make sufficient grounding. A large current flows through Vss Pin by switching. When the impedance of the
Vss connection is high, the potential within the IC is varied by the switching current. This may result in
unstable operation of the IC.
Use capacitor with a capacity of 10µF or more, and with good high frequency characteristics such as tanta-
lum capacitor. We recommend the use of a capacitor with a resistance to the voltage being at least three
times the output set voltage. This is because there may be the case where a spike-shaped high voltage is gen-
erated by the inductor when Lx transistor is turned OFF.
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Take the utmost care when choosing a inductor. Namely, choose such an inductor that has sufficiently small
d.c. resistance and large allowable current, and hardly reaches magnetic saturation. When the inductance
value of the inductor is small, there may be the case where ILX exceeds the absolute maximum ratings at the
maximum load. Use an inductor with an appropriate inductance.
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Use a diode of a Schottky type with high switching speed, and also take care of the rated current.
These ICs are provided with a soft-start circuit. However, there may be the case where the overshoot of the
out put voltage takes place depending upon the peripheral circuits employed and the input/output condi-
tions. In particular, when the input voltage is increased slowly, the occurrence of the overshoot of the output
voltage becomes conspicuous. Therefore in the case where the overshoot becomes a problem, take a counter-
measure against this problem, for example, by clamping the output (OUT Pin) by use of a Zener diode.
The transient response characteristics corresponding to the variations in the input and output are set so as
to be slightly delayed by an internal phase compensation circuit in order to prevent the oscillation. because
of such setting of the transient response characteristics, take care of the occurrence of the overshoot and/or
undershoot of the output voltage.
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The internal phase compensation circuit is designed with the avoidance of the problem of the occurrence of
the oscillation fully taken into consideration. However, there may be the case the oscillation takes place
depending upon the conditions for the attachment of external components. In particular, take the utmost
care when an inductor with a large inductance is used.
The performance of power source circuits using these ICs largely depends upon the peripheral components. Take the utmost care
in the selection of the peripheral components. In particular, design the peripheral circuits in such a manner that the values such as volt-
age, current and power of each component, PCB patterns and the IC do not exceed their respective rated values.
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RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
