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LD7531 Datasheet(PDF) 10 Page - Leadtrend Technology |
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LD7531 Datasheet(HTML) 10 Page - Leadtrend Technology |
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10 / 18 page  LD7531 10 Leadtrend Technology Corporation www.leadtrend.com.tw LD7531-DS-00 June 2008 Oscillator and Switching Frequency The switching frequency of LD7531 is programmed as an external resistor on RT to provide the optimized operations by considering the EMI performance, thermal treatment, component sizes and transformer design. Internal Slope Compensation In the conventional application, the problem of the stability is a critical issue for current mode controlling, when it operates in higher than 50% of the duty-cycle. As UC384X, It takes slope compensation from injecting the ramp signal of the RT/CT pin through a coupling capacitor. It therefore requires no extra design for the LD7531 since it has integrated it already. On/Off Control The LD7531 can be turned off by pulling COMP pin lower than 1.2V. The gate output pin of the LD7531 will be disabled immediately under such condition. The off-mode can be released when the pull-low signal is removed. Dual-Oscillator Green-Mode Operation There are many different topologies has been implemented in different chips for the green-mode or power saving requirements such as “burst-mode control”, “skipping-cycle mode”, “variable off-time control “…etc. The basic operation theory of all these approaches intended to reduce the switching cycles under light-load or no-load condition either by skipping some switching pulses or reduce the switching frequency. By using this dual-oscillator control, the green-mode frequency can be well controlled and further to avoid the generation of audible noise. OVP (Over Voltage Protection) on Vcc The VGS ratings of the nowadays power MOSFETs are often limited up to max. 30V. To prevent the VGS from the fault condition, LD7531 is implemented an OVP function on Vcc. Whenever the Vcc voltage is higher than the OVP threshold voltage, the output gate drive circuit will be shutdown simultaneously thus to stop the switching of the power MOSFET until the next UVLO(on). The Vcc OVP function in LD7531 is an auto-recovery type protection. If the OVP condition, usually caused by the feedback loop opened, is not released, the Vcc will tripped the OVP level again and re-shutdown the output. The Vcc is working as a hiccup mode. The Figure 18 shows its operation. On the other hand, if the OVP condition is removed, the Vcc level will get back to normal level and the output will automatically return to the normal operation. Fig. 18 Over Load Protection (OLP) To protect the circuit from being damaged under over load condition or short condition, a smart OLP function is implemented in the LD7531. The Figure 19 shows the waveforms of the OLP operation. In this case, the feedback system will force the voltage loop proceed toward the saturation and then pull up the voltage on COMP pin (VCOMP). Whenever the VCOMP trips up to the OLP threshold 5V and stays longer than the OLP delay time, the protection will activate and then turn off the gate output to stop the switching of power circuit. The OLP delay time is to prevent |
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