an8029 Panasonic Corporation of North America, an8029 Datasheet - Page 6

• Manufacturer Part Number: an8029
• Description: Self-excited Rcc Pseudo-resonance Type Ac-dc Switching Power Supply Control Ic
• Manufacturer: Panasonic Corporation of North America

AN8029
6
[1] Operation descriptions (continued)
1. Start/stop circuit block (continued)
2. Oscillation circuit
Application Notes (continued)
Start mechanism (continued)
is not able to start (state c in figure 1), if the stop voltage of the IC is reached before the sufficient supply voltage
is supplied from the bias coil.
Function
of IC when V
voltage (8.6 V typical). A large voltage difference is set between start and stop (6.3 V typical) so that it is easier
to select the start resistor and the capacitor to be connected to V
Note) To start up the IC operation, the startup current which is a pre-start current plus a circuit drive current is necessary.
discharge of the C2, R2 and C3 connected to TOFF (pin 2), TON (pin 3) respectively.
period of the power MOSFET and achieving the control by changing the on-period. This on-period control is
performed by directly changing the output pulse width of the oscillation circuit.
During the on-period of the power MOSFET, the C2 is charged to the constant voltage (approximately 0.9 V).
voltage across the both ends of the C3 reaches approximately 0.7 V (T
reversed and the power MOSFET is turned off. At the same time, the C3 is rapidly discharged by the discharge
circuit inside the IC and its voltage across the both ends becomes almost 0 V. The charge current from the TON
terminal is changed by the feedback signal to the FB terminal (pin 9). (Described later.)
discharging by the R2. When the voltage across the both ends of C2 decreases to approximately 0.1 V, the oscillation
circuit output is reversed again to turn on the power MOSFET. At the same time, the C2 is rapidly charged to
approximately 0.9 V. An equation for approximate calculation of the minimum off-period T
mined by the C2 and R2, the off-period in the pseudo-resonance circuit operation described below is determined
by the former.
oscillation waveform at the time when the TDL terminal is pulled down to the GND.
Since the supply voltage continuously decreases during the above period (area b in figure 1), the power supply
The start/stop circuit block is provided with the function to monitor the V
The oscillation circuit generates the pulse signal for turning on/off the power MOSFET by using charge/
The concept of constant voltage control at the time of making up the switching power supply is fixing the off-
On the other hand, the C3 is charged from almost 0 V by the charge current from the TON terminal. When the
An equation for approximate calculation of the maximum on-period T
When the power MOSFET is off, the TOFF terminal becomes a high impedance state and the C2 starts
However, when the voltage-time fed back to the TDL terminal (pin 1) is longer than the T
By repeating the above operation, the power MOSFET is turned on and off continuously. Figure 2 shows the
T
T
Set the resistance value so as to supply a startup current of 350 A.
ON(max)
OFF(min)
CC
6 500
voltage reaches the start voltage (14.9 V typical), and to stop when it decreases under the stop
2.2
C3
200
C2
C2
C3
R2
R2
Figure 2. Oscillation circuit operation
Voltage across
both ends of C2
Voltage across
both ends of C3
IC output
0.9 V
0.1 V
0.7 V
0 V
0 V
CC
.
On-period Off-period
ON(max)
a
75°C), the oscillation circuit output is
CC
of the power MOSFET is as follows:
voltage, and to start the operation
Voltage Regulators
OFF(min)
OFF
is as follows:
period deter-