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IR3820AMPBF Datasheet(PDF) 10 Page - International Rectifier |
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IR3820AMPBF Datasheet(HTML) 10 Page - International Rectifier |
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10 / 21 page  01/08/08 10 PD-60330 IR3820AMPbF Over-Current Protection The over-current protection is performed by sensing current through the R DS(on) of the low side MOSFET. This method enhances the converter’s efficiency and reduces cost by eliminating a current sense resistor. As shown in figure 7, an external resistor (R SET) is connected between OCSet pin and the inductor point which sets the current limit set point. The internal current source develops a voltage across R SET. When the low side MOSFET is turned on, the inductor current flows through the Q2 and results a voltage which is given by: The OCP circuit starts sampling current when the low gate drive is about 3V. The OCSet pin is internally clamped about 1.5V during on time of high side gate to prevent false trigging, figure 9 shows the OCSet pin during one switching cycle. As shown, there is about 150ns delay to mask the dead time. Since this node contains switching noises, this delay also functions as a filter. Fig. 7: Connection of over current sensing resistor Fig. 8: 3uA current source for discharging soft-start capacitor during hiccup An over-current is detected if the OCSet pin goes below ground. This trips the OCP comparator and cycles the soft start function in hiccup mode. The hiccup is performed by charging and discharging the soft-start capacitor in a certain slope rate. As shown in figure 8 a 3uA current source is used to discharge the soft-start capacitor. The OCP comparator resets after every soft start cycle. The converter stays in this mode until the overload or short circuit is removed. The converter will automatically recover. ) ) --( I (R ) R (I V L DS(on) OCSet OCSet OCSet 2 ∗ − ∗ = 0 ) I (R ) R (I V L DS(on) OCSet OCSet OCSet = ∗ − ∗ = ) 3 --( R I R I I on DS OCSet OCSet critical L SET ) ( ) ( ∗ = = Fig. 9: OCset pin during normal condition Ch1: Inductor point, Ch3:OCSet The value of R SET should be checked in an actual circuit to ensure that the over-current protection circuit activates as expected. The IR3820A current limit is designed primarily as disaster preventing, and doesn't operate as a precision current regulator. The critical inductor current can be calculated by setting: I OCSet*ROCSet Blanking time Deadtime Clamp voltage |
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