
R1211x
Output Current and Selection of External Components
10
<Basic Circuit>
i1
CL
L
X
Tr
V
IN
I
OUT
V
OUT
i2
Inductor
GND
Diode
<Circuit through L>
Discontinuous Mode
Continuous Mode
IL
ILxmax
ILxmin
Tf
t
Ton
T=1/fosc
Toff
IL
ILxmax
ILxmin
t
Ton
T=1/fosc
Iconst
Toff
There are two modes, or discontinuous mode and continuous mode for the PWM step-up switching regulator
depending on the continuous characteristic of inductor current.
During on time of the transistor, when the voltage added on to the inductor is described as V
IN
, the current is
V
IN
×
t/L. Therefore, the electric power, P
ON
, which is supplied with input side, can be described as in next formula.
Ton
ON
×
.............................................................................................................................. Formula 1
dt
t/L
V
P
2
IN
0
=
∫
With the step-up circuit, electric power is supplied from power source also during off time. In this case, input
current is described as (V
OUT
V
IN
)
×
t/L, therefore electric power, P
OFF
is described as in next formula.
×
×
........................................................................................................ Formula 2
dt
t/L
)
V
(V
V
P
IN
OUT
IN
Tf
0
OFF
=
∫
In this formula, Tf means the time of which the energy saved in the inductance is being emitted. Thus average
electric power, P
AV
is described as in the next formula.
∫
∫
In PWM control, when Tf = Toff is true, the inductor current becomes continuos, then the operation of
switching regulator becomes continuous mode.
In the continuous mode, the deviation of the current is equal between on time and off time.
Toff/L
)
V
(V
/L
V
IN
OUT
ON
IN
×
=
×
T
................................................................................................... Formula 4
Further, the electric power, P
AV
is equal to output electric power, V
OUT
×
I
OUT
, thus,
{
}
/(2
T
V
)
V
(V
L
2
/
T
V
f
I
ON
IN
IN
OUT
ON
IN
OSC
OUT
×
=
×
×
×
×
=
dt}
t/L
)
V
(V
V
dt
t/L
V
{
)
T
1/(T
P
IN
OUT
IN
Tf
0
2
IN
Ton
0
OFF
ON
AV
×
×
+
×
×
+
=
................................................... Formula 3
)
V
L
OUT
2
2
2
×
×
.................................................... Formula 5