Voltage Regulators
s
Application Notes (continued)
[1] Operation descriptions (continued)
AN8027, AN8037
2.
Oscillation circuit (continued)
•
Setting the off-period
The minimum off-period is the charge period from V
L-CF
=
0.2 V (typical) to V
TH(OFF)
or V
FB
.
An approximate minimum off-period of the power MOSFET is obtained from the following formula.
Refer to figure 3.
T
OFF(min)
=
C
CF
×
V
TH(OFF)
/I
OFF1
(I
FB
≤
0.6 mA)
=
{C
CF
×
V
TH(OFF)
/ I
OFF1
}
+
{C
CF
×
(V
FB
−
V
TH(OFF)
) / I
OFF2
} (I
FB
>
0.6 mA)
V
TH(OFF)
=
2.4 V typ.
I
OFF1
=
660
µA
typ.
I
OFF2
=
160
µA
typ.
T
OFF(min)
: Minimum off-period
C
CF
: Value of a capacitor connected to the CF pin
V
FB
: Voltage internally converted from feedback signal I
FB
V
TH(OFF)
: Threshold voltage of V
FB
to extend off-period
I
OFF1
: Charge current until CF pin voltage increases from 0.2 V to 2.4 V
I
OFF2
: Charge current until CF pin voltage increases from 2.4 V to V
FB
When the IC is in local resonance operation, the off-period is determined by the longer one of either the
time required for the input voltage on the TR pin to drop below the threshold voltage or the minimum off-
period (T
OFF(min)
) specified by the C
CF
.
Thus the power MOSFET is in continuous on/off operation.
E
IN+
AN8027, AN8037
Current mirror
NP
V
CC
NB
PC
FB
1:5
20 kΩ
OFF
V
FB
(I
<
0.6 mA)
FB
I
=
660
µA
typ.
I
OFF
Out
GND
I
OFF
=
160
µA
typ.
(I
FB
>
0.6 mA)
CF
C
CF
V
H−CF
=
4 V
CF pin voltage
I
FB
<
0.6 mA
V
H−CF
=
4V
CF pin voltage
I
FB
>
0.6 mA
V
FB
I
FB
; large
T
OFF(min)
; large
V
TH(OFF)
=
2.4 V
V
L2−CF
=
0.2 V
0V
V
CC
−
1.5 V
Out pin voltage
V
FB
V
TH(OFF)
=
2.4 V
V
L2−CF
=
0.2 V
0V
V
CC
−
1.5 V
T
OFF(min)
T
OFF
On
Out pin voltage
T
OFF(min)
T
OFF
On
0V
0V
Figure 3. Off-period block diagram and control waveform
9
PANASONIC [ PANASONIC SEMICONDUCTOR ]
