AN8027, AN8037
s
Application Notes (continued)
[1] Operation descriptions (continued)
2.
Oscillation circuit (continued)
•
Setting the on-period (continued)
T
ON
: On-period
C
CF
: Value of a capacitor connected to CF pin
V
FB
: Voltage internally converted from feedback signal I
FB
V
H-CF
: CF upper limit voltage
I
ON
: On-period discharge current
I
TR
: Flowing current at TR pin
E
IN
: Voltage on primary winding
NB : Number of turns in the bias winding
NP : Number of turns in the primary winding
V
Z
: Voltage on Zener diode connected to bias winding
R
TR
: Value of a resistor connected to the TR pin
Voltage Regulators
The power MOSFET is turned off if the voltage at the CLM pin reaches the pulse-by-pulse overcurrent
protection threshold voltage (i.e.,
−200
mV typical) when the overcurrent protection function of the IC is
operating.
T
ON
is shortened
NP
E
IN+
because large E
IN
increases I
TR
.
I
TR
R
TR
V
Z
V
CC
NB
AN8027, AN8037
TR
I
FB
PC
FB
Current mirror
Current mirror
I
TR
=I
ON
1:5
20 kΩ
V
FB
250
µA
typ.
CF
I
ON
Out
GND
E
IN−
The minimum current is guaranteed
to prevent a limitless increase in T
ON
.
C
CF
E
IN
; large
I
TR
(I
ON
); large
V
H-CF
=
4 V
CF pin voltage
V
FB
0V
V
CC
−
1.5 V
Out pin voltage
Off
0V
On
Figure 2. On-period block diagram and control waveform
The maximum on-period can be used for overcurrent protection.
When the input voltage is low, the maximum on-period overcurrent protection is possible.
When the input voltage is high, the CLM pulse-by-pulse overcurrent protection is possible.
8
PANASONIC [ PANASONIC SEMICONDUCTOR ]
