RH5RI251B データシートの表示（PDF） - RICOH Co.,Ltd.
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RH5RI251B Datasheet PDF : 24 Pages
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RH5RI
SELECTION OF PERIPHERAL COMPONENTS
When LxTr is on, the energy PON charged in the inductor is provided by Equation 2 as follows :
PON=∫0ton (VIN · IL (t)) dt=∫0ton (VIN2 · t/L) dt
=VIN2 · ton2/(2 · L)....................................................................................................Equation 2
In the case of the step-up DC/DC converter, the energy is also supplied from the input power source at the time
of OFF.
Thus, POFF = ∫0topen(VIN · IL (t)) dt=∫0topen (VIN · (VOUT–VIN) · t/L)dt
=VIN · (VOUT–VIN) · topen2/(2 · L)
Here, topen=VIN · ton/(VOUT–VIN) from Equation 1, and when this is substituted into the above equation.
=VIN3 · ton2/(2 · L · (VOUT–VIN)) ............................................................................Equation 3
Input power PIN is (PON+POFF)/T. When this is converted in its entirely to the output.
PIN=(PON+POFF)/T=VOUT · IOUT=POUT .........................................................................Equation 4
Equation 5 can be obtained as follows by solving Equation 4 for IOUT by substituting Equation 2 and 3 into
Equation 4 :
IOUT=VIN2 · ton2/(2 · L · T · (VOUT–VIN)
=VIN2 · maxdty2/(20000 · fosc · L · (VOUT–VIN)) ...................................................Equation 5
The peak current which flows through L · LxTr · SD is
ILmax=VIN · ton/L..........................................................................................................Equation 6
Therefore, it is necessary that the setting of the input/output conditions and the selection of peripheral compo-
nents be made with ILmax taken into consideration.
HINTS
The above explanation is directed to the calculation in an ideal case where it is supposed that there is no
energy loss in the external components and LxSW. In an actual case, the maximum output current will be 50
to 80% of the above calculated maximum output current. In particular, care must be taken because VIN is
decreased in an amount corresponding to the voltage reduction caused by LxSW when IL is large or VIN is
small. Furthermore, It is required that with respect to VOUT, Vf of the diode (about 0.3V in the case of a
Schottky type diode) be taken into consideration.
When ILX and VLX exceed their respective ratings, use RH5RI××2B and RH5RI××3B ICs with the attach-
ment of an external transistor with a low saturation voltage thereto.
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