Voltage Regulators
s
Application Notes
[1] Operation descriptions
1. Start/stop circuit block
•
Startup mechanism
After the AC voltage is applied and the
supply voltage due to the current in the startup
resistor reaches the startup voltage and the IC
begins to operate, drive of the power MOSFET
begins. This causes a bias in the transformer, and
the supply voltage is provided to the IC from
the bias winding. (This is point a in figure 1.)
During the period between the point when the
startup voltage is reached, and the point when
the bias winding can generate a voltage enough
to supply the IC, the IC supply voltage is pro-
vided by the capacitor (C8) connected to V
CC
.
Since the supply voltage falls during this period
(area b in figure 1), if the supply voltage falls
below the IC stop-voltage before an adequate
supply voltage can be provided by the bias wind-
ing, it will not be possible to start the power sup-
ply. (This is the state at point c in figure 1.)
Rectified AC
Startup resistor
R1
AN8027, AN8037
V
CC
C8
V
OUT
GND
Standby
Startup
voltage
Stop
voltage
a
Startup
Voltage supplied
from bias winding
Startup state
b
c Startup failure
Figure 1
•
Functions
This IC includes a function that monitors the V
CC
voltage. It starts IC operation when V
CC
reaches the startup
voltage (14.9 V typical), and stops operation when the voltage falls below the stop voltage (8.6 V typical). Since
a large voltage difference (6.3 V typical) is taken between the start and stop voltages, it is easy to select values for
the start resistor and the capacitor connected to V
CC
.
Note) To start up the IC operation, the startup current which is a pre-start current plus a circuit drive current is necessary.
Set the resistance value so as to supply a startup current of 400
µA.
2.
Oscillation circuit
The oscillation circuit makes use of the charge and discharge of current to and from the capacitor C
CF
connected
to the CF pin (pin 3) to determine the switching timing of the power MOSFET.
The IC is in constant voltage control by changing the on-period of the power MOSFET without making off-
period change while the IC is in normal (RCC continuous) operation mode. At that time, the on-period is con-
trolled by directly changing the output pulse width of the oscillation circuit, and the maximum on-period can be
adjusted with input voltage compensation by detecting the input voltage with the flow of current from the TR pin
(TR source current). Refer to figure 2. When the IC is in standby mode (for light loads), the stable, efficient
control of the IC is ensured by detecting the flow of current from the FB pin (I
FB
) and changing the off-period for
a decrease in frequency. Refer to figure 3.
The following provides information on how to set on- and off-period.
•
Setting the on-period
The output on-period is the discharge period when the CF pin is between the peak value oft V
H-CF
=
4 V
(typical) and V
FB
.
An approximate on-period of the power MOSFET is obtained from the following formula. Refer to figure 2.
T
ON
=
C
CF
×
(V
H-CF
−
V
FB
)/I
ON
whereas, V
H-CF
=
4 V typ.
I
ON
=
I
TR
+
250
µA
typ.
I
TR
=
(E
IN
×
NB/NP
−
V
Z
)/R
TR
V
FB
=
0.7 V typ. (I
FB
≤
200
µA)
V
FB
=
4 kΩ
×
I
FB
typ. (I
FB
>
200
µA)
7
PANASONIC [ PANASONIC SEMICONDUCTOR ]
