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R1212D
13
In the continuous mode, the deviation of the current is equal between on time and off time.
L
/
Toff
)
V
V
(
L
/
T
V
IN
OUT
ON
IN
×
=
=
.............................................................................Formula 4
Further, the electric power, P
AV
is equal to output electric power, V
OUT
×
I
OUT
, thus,
{
(
L
2
/
T
V
f
I
ON
IN
OSC
OUT
×
×
×
×
=
}
)
V
L
2
/(
T
V
)
V
V
OUT
ON
2
IN
IN
OUT
2
2
×
×
×
=
.....................Formula 5
When I
OUT
becomes more than formula 5, the current flows through the inductor, then the mode becomes
continuous. The continuous current through the inductor is described as Iconst, then,
{
}
IN
OUT
ON
IN
OSC
OUT
)
V
V
(
L
2
/
T
V
f
I
+
×
×
×
×
=
In this moment, the peak current, ILxmax flowing through the inductor and the driver Tr. is described as
follows:
L
/
T
V
Iconst
max
ILx
ON
+
=
.....................................................................................Formula 7
With the formula 4,6, and ILxmax is,
)
L
2
/(
T
V
I
V
/
V
max
ILx
ON
IN
OUT
IN
OUT
×
×
+
×
=
................................................................Formula 8
OUT
IN
2
2
V
/
Iconst
V
×
................................Formula 6
IN
×
Therefore, peak current is more than I
OUT
. Considering the value of ILxmax, the condition of input and output,
and external components should be selected.
In the formula 7, peak current ILxmax at discontinuous mode can be calculated. Put Iconst
=
0 in the formula.
The explanation above is based on the ideal calculation, and the loss caused by L
X
switch and external
components is not included. The actual maximum output current is between 50% and 80% of the calculation.
Especially, when the IL
X
is large, or V
IN
is low, the loss of V
IN
is generated with the on resistance of the switch. As
for V
OUT
, Vf (as much as 0.3V) of the diode should be considered.