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LD7531 Datasheet(PDF) 8 Page - Leadtrend Technology |
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LD7531 Datasheet(HTML) 8 Page - Leadtrend Technology |
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8 / 18 page  LD7531 8 Leadtrend Technology Corporation www.leadtrend.com.tw LD7531-DS-00 June 2008 Application Information Operation Overview The LD7531 meets the green-power requirement and is intended for the use in those modern switching power suppliers and adaptors which demand higher power efficiency and power-saving. It integrated more functions to reduce the external components counts and the size. Its major features are described as below. Under Voltage Lockout (UVLO) An UVLO comparator is implemented in it to detect the voltage on the VCC pin. It would assure the supply voltage enough to turn on the LD7531 PWM controller and further to drive the power MOSFET. As shown in Fig. 13, a hysteresis is built in to prevent the shutdown from the voltage dip during startup. The turn-on and turn-off threshold level are set at 16.0V and 10.0V, respectively. Vcc UVLO(on) UVLO(off) t t I(Vcc) startup current (~uA) operating current (~ mA) Fig. 13 Startup Current and Startup Circuit The typical startup circuit to generate the LD7531 Vcc is shown in Fig. 14. During the startup transient, the Vcc is lower than the UVLO threshold thus there is no gate pulse produced from LD7531 to drive power MOSFET. Therefore, the current through R1 will provide the startup current and to charge the capacitor C1. Whenever the Vcc voltage is high enough to turn on the LD7531 and further to deliver the gate drive signal, the supply current is provided from the auxiliary winding of the transformer. Lower startup current requirement on the PWM controller will help to increase the value of R1 and then reduce the power consumption on R1. By using CMOS process and the special circuit design, the maximum startup current of LD7531 is only 20 μA. If a higher resistance value of the R1 is chosen, it usually takes more time to start up. To carefully select the value of R1 and C1 will optimize the power consumption and startup time. Fig. 14 Current Sensing and Leading-edge Blanking The typical current mode of PWM controller feedbacks both current signal and voltage signal to close the control loop and achieve regulation. As shown in Fig. 15, the LD7531 detects the primary MOSFET current from the CS pin, which is not only for the peak current mode control but also for the pulse-by-pulse current limit. The maximum voltage threshold of the current sensing pin is set at 0.85V. From above, the MOSFET peak current can be obtained from below. |
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