an8038 Panasonic Corporation of North America, an8038 Datasheet - Page 10

• Manufacturer Part Number: an8038
• Description: Ac-dc Switching Power Supply Control Ic With Rcc Local Resonance Circuit For Improved Conformance With Energy Conservation Laws
• Manufacturer: Panasonic Corporation of North America

AN8038, AN8038S
10
[2] Operation descriptions
1. Start/stop circuit block
2. Oscillator circuit
Application Notes (continued)
Startup mechanism
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 fig-
ure 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 sup-
ply voltage is provided by the capacitor (C8)
connected to V
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 winding, it will not be
possible to start the power supply. (This is the
state at point c in figure 1.)
Functions
voltage (14.4 V typical), and stops operation when the voltage falls below the stop voltage (8.9 V typical). Since
a large voltage difference (5.5 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
flows in this resistor. (To use a smaller startup resistor.) To achieve this, the circuit current at startup is set to as
small as 190 A (typical), and temperature variations, and also sample-to-sample variations are reduced as well.
Since the bias current is reduced, the capacitor connected to V
charge and discharge of the capacitor C
in which the main switch on-period is the discharge period of the CF pin waveform, and the off-period is the
charge period of that waveform.
operation by holding the main switch off-period fixed and varying the on-period. This on-period is controlled by
directly varying the output pulse-width of the oscillator circuit.
voltage through the flowing out current at the TR pin. (See figure 2.)
the off period, and lowers the operating frequency to reduce power loss during standby (light load) mode and to
prevent being out of control.
voltage reaches the threshold voltage of 205 mV (typical).
After the AC voltage is applied and the
This IC includes a function that monitors the V
Since high voltages are applied across the startup resistor, measures must be taken to minimize the current that
The oscillator circuit determines the pulse width with which the main switch is turned on and off using the
Constant-voltage control in a switching power supply using this IC is implemented during RCC (continuous)
Additionally, the IC reduces the maximum on-period when the input voltage increases by detecting the input
Furthermore, this IC features an added function that detects increases of the IFB feedback current, and reduces
During overcurrent protection operation, the IC performs a rapid discharge operation where the CLM pin
CC
. Since the supply voltage
CF
connected to the CF pin (pin 3). This IC implements a control scheme
Startup
voltage
Stop
voltage
CC
CC
.
voltage. It starts IC operation when V
After AC rectification
Before
startup
CC
can be miniaturized as well.
V
V
OUT
CC
a
Figure 1
C8
b
Voltage Regulators
Startup resistor
R1
CC
c
reaches the startup
Startup failure
Voltage supplied
from the bias
winding
Startup
Startup state