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AN8037 Datasheet(PDF) 7 Page - Panasonic Semiconductor |
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AN8037 Datasheet(HTML) 7 Page - Panasonic Semiconductor |
7 / 28 page 7 Voltage Regulators AN8027, AN8037 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 VCC. 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.) • Functions This IC includes a function that monitors the VCC voltage. It starts IC operation when VCC 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 VCC . 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 CCF 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 (IFB) 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 VH-CF = 4 V (typical) and VFB . An approximate on-period of the power MOSFET is obtained from the following formula. Refer to figure 2. TON = CCF × (VH-CF − VFB)/ION whereas, VH-CF = 4 V typ. ION = ITR + 250 µA typ. ITR = (EIN × NB/NP − VZ)/RTR VFB = 0.7 V typ. (IFB ≤ 200 µA) VFB = 4 kΩ × IFB typ. (IFB > 200 µA) GND C8 b Figure 1 Startup failure Startup state Voltage supplied from bias winding Startup Standby Startup voltage Stop voltage a c Rectified AC Startup resistor R1 VCC VOUT |
Similar Part No. - AN8037 |
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Similar Description - AN8037 |
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