Selection of peripheral components, Rv5vh1, Rv5vh2 – Ricoh RV5VH User Manual
Page 26
RV5VH1
××
/RV5VH2
××
22
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 V
IN
is
decreased in an amount corresponding to the voltage reduction caused by LxSW when IL is large or V
IN
is
small. Furthermore, It is required that with respect to V
OUT
, Vf of the diode (about 0.3V in the case of a
Schottky type diode) be taken into consideration.
When I
LX
and V
LX
exceed their respective ratings, use the RV5VH with the attachment of an external tran-
sistor with a low saturation voltage thereto.
HINTS
When LxTr is on, the energy P
ON
charged in the inductor is provided by Equation 2 as follows :
P
ON
=
∫
0
ton
(V
IN
· IL (t)) dt=
∫
0
ton
(V
IN
2
· t/L) dt
=V
IN
2
· ton
2
/(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, P
OFF
=
∫
0
topen
(V
IN
· IL (t)) dt=
∫
0
topen
(V
IN
· (V
OUT
–V
IN
) · t/L)dt
=V
IN
· (V
OUT
–V
IN
) · topen
2
/(2 · L)
Here, topen=V
IN
· ton/(V
OUT
–V
IN
) from Equation 1, and when this is substituted into the above equation.
=V
IN
3
· ton
2
/(2 · L · (V
OUT
–V
IN
)) ............................................................................Equation 3
Input power P
IN
is (P
ON
+P
OFF
)/T. When this is converted in its entirely to the output.
P
IN
=(P
ON
+P
OFF
)/T=V
OUT
· I
OUT
=P
OUT
.........................................................................Equation 4
Equation 5 can be obtained as follows by solving Equation 4 for I
OUT
by substituting Equation 2 and 3 into
Equation 4 :
I
OUT
=V
IN
2
· ton
2
/(2 · L · T · (V
OUT
–V
IN
)
=V
IN
2
· maxdty
2
/(20000 · fosc · L · (V
OUT
–V
IN
)) ...................................................Equation 5
The peak current which flows through L · LxTr · SD is
ILmax=V
IN
· 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.
SELECTION OF PERIPHERAL COMPONENTS