RH5RH
Therefore it is necessary that the setting of the input/output conditions and the selection of peripheral compo-
nents should be made with ILmax taken into consideration.
•
Output Current in Continuous Conduction Mode
When the operation enters into the continuous conduction mode by increasing the I
OUT
, ILmin becomes equal
to Iconst (> 0), and this current always flows through the inductor. Therefore, V
IN
· Iconst is added to P
IN
in
Equation 5.
Thus, P
IN
=V
IN
· Iconst+(P
ON
+P
OFF
)/T=V
OUT
· I
OUT
=P
OUT
When the above Equation is solved for I
OUT
,
I
OUT
=V
IN
2
· tonc
2
/(2 · L · T · (V
OUT
–V
IN
))+V
IN
· Iconst/V
OUT
............................................Equation 8
The peak current which flows through L · LxTr · SD is
ILmax=V
IN
· ton/L+Iconst ...................................................................................................Equation 9
From Equations 6 and 9, the larger the value of L, the smaller the load current at which the operation enters
into the continuous mode, and the smaller the difference between ILmax and ILmin, and the smaller the value of
ILmax.
Therefore, when the load current is the same, the larger the value of L, the easier the selection of peripheral
components with a small allowable current becomes, and the smaller the ripple of the peripheral components can
be made. In this case, however, it must be noted from Equation 6 that I
OUT
becomes small when the allowable cur-
rent of the inductor is small or when V
IN
is so small that the operation cannot enter into the continuous mode.
HINTS
The above explanation is directed to the calculation in an ideal case where there is no energy loss caused by the
resistance 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 drop caused by LxSW when IL is large or V
IN
is low.
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.
12
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
