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FS6X1220RTYDTU Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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FS6X1220RTYDTU Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 16 page FS6X1220R 9 3.2 Over Load Protection (OLP) : Overload means that the load current exceeds a pre-set level due to an abnormal situation. In this situation, protection circuit should be activated in order to protect the SMPS. However, even when the SMPS is in the normal operation, the over load protection circuit can be activated during the load transition. In order to avoid this undesired operation, the over load protection circuit is designed to be activated after a specified period to determine whether it is a transient situation or an overload situation. Because of the pulse-by-pulse current limit capability, the maximum peak current through the SMPS is limited, and therefore the maximum input power is restricted with a given input voltage. If the output consumes beyond this maximum power, the output voltage (Vo) decreases below the set voltage. This reduces the current through the opto-coupler diode, which also reduces opto-coupler transistor current increasing Vfb. If Vfb exceeds 2.6V, D1 is blocked and the 5 µA current source starts to charge Cfb slowly compared to when the 0.9mA current source charges Cfb. In this condition, Vfb continues increasing until it reaches 7.5V, and the switching operation is terminated at that time as shown in figure 4. The delay time for shutdown is the time required to charge Cfb from 2.6V to 7.5V with 5 µA. When Cfb is 10nF (103), T12 is approximately 9.8ms and when Cfb is 0.1 µF (104), T12 is approximately 98ms. These values are enough to prevent SMPS from being shut down during transient situations. Figure 4. Over load protection 3.3 Over voltage Protection (OVP) : In case of malfunction in the secondary side feedback circuit, or feedback loop open caused by a defect of solder, the current through the opto- coupler transistor becomes almost zero. Then, Vfb climbs up in a similar manner to the over load situation, forcing the preset maximum current to be supplied to the secondary side until the over load protection is activated. Because energy more than required is provided to the output, the output voltage may exceed the rated voltage before the over load protection is activated, resulting in the breakdown of the devices in the secondary side. In order to prevent this situation, an over voltage protection (OVP) circuit is employed. In general, Vcc is proportional to the output voltage and FS6X1220R uses Vcc instead of directly monitoring the output voltage. If VCC exceeds 25V, OVP circuit is activated resulting in termination of switching. In order to avoid undesired activation of OVP during normal operation, Vcc should be properly designed to be below 25V. 3.4 Thermal Shutdown (TSD) : The thermal shutdown circuitry senses the junction temperature. The threshold is set at 160 °C. When the junction temperature rises above this threshold (160 °C) the power MOSFET is disabled. 4. The Line UVLO and Sleep Mode According to the voltage of Line Sense pin, three operation modes are defined; Normal operation mode, Line under voltage lock out mode and Sleep mode as shown in figure 5. When the voltage of this pin is over 2.55V, FS6X1220R operates in normal mode. When the voltage of this pin is smaller than 2.55V, it goes into line under voltage lock out mode terminating switching operation. When the voltage of this pin is smaller than 1.8V, it enters into sleep mode. During sleep mode, reference voltage generation circuit including shunt regulator is disabled and only 300 µA operation current is required. Figure 5. Line Sense block V FB t 2.6V 7.5V Over load protection T 12= Cfb*(7.5-2.6)/Idelay T 1 T 2 V SL (1.8v) V LU (2.5V) 9V/15V 3 5 Vref Line UVLO Enable Internal Bias Vcc good Line Sense Vcc Vin R1 R2 Sleep ON OFF |
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