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NCP346SN2T1G Datasheet(PDF) 6 Page - ON Semiconductor |
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NCP346SN2T1G Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 15 page NCP346 http://onsemi.com 6 APPLICATION INFORMATION − + OUT IN Vref GND CNTRL + VCC Logic FET Driver LOAD AC/DC Adapter or Accessory Charger Zener Diode (optional) C1 Schottky Diode Microprocessor port P−CH NCP346 Zener Diode (optional) Figure 3. (optional) (opt.) MBRM130LT1 NHTS4101PT1 Introduction In many electronic products, an external AC/DC wall adapter is used to convert the AC line voltage into a regulated DC voltage or a current limited source. Line surges or faults in the adapter may result in overvoltage events that can damage sensitive electronic components within the product. This is becoming more critical as the operating voltages of many integrated circuits have been lowered due to advances in sub−micron silicon lithography. In addition, portable products with removable battery packs pose special problems since the pack can be removed at any time. If the user removes a pack in the middle of charging, a large transient voltage spike can occur which can damage the product. Finally, damage can result if the user plugs in the wrong adapter into the charging jack. The challenge of the product designer is to improve the robustness of the design and avoid situations where the product can be damaged due to unexpected, but unfortunately, likely events that will occur as the product is used. Circuit Overview To address these problems, the protection system above has been developed consisting of the NCP346 Overvoltage Protection IC and a P−channel MOSFET switch such as the MGSF3441. The NCP346 monitors the input voltage and will not turn on the MOSFET unless the input voltage is within a safe operating window that has an upper limit of the overvoltage detection threshold. A Zener diode can be placed in parallel to the load to provide for secondary protection during the brief time that it takes for the NCP346 to detect the overvoltage fault and disconnect the MOSFET. The decision to use this secondary diode is a function of the charging currents expected, load capacitance across the battery, and the desired protection voltage by analyzing the dV/dT rise that occurs during the brief time it takes to turn−off the MOSFET. For battery powered applications, a low−forward voltage Schottky diode such as the MBRM120LT3 can be placed in series with the MOSFET to block the body diode of the MOSFET and prevent shorting the battery out if the input is accidentally shorted to ground. This provides additional voltage margin at the load since there is a small forward drop across this diode that reduces the voltage at the load. When the protection circuit turns off the MOSFET, there can be a sudden rise in the input voltage of the device. This transient can be quite large depending on the impedance of the supply and the current being drawn from the supply at the time of an overvoltage event. This inductive spike can be clamped with a Zener diode from IN to ground. This diode breakdown voltage should be well above the worst case supply voltage provided from the AC/DC adapter or Cigarette Lighter Adapter (CLA), since the Zener is only intended to clamp the transient. The NCP346 is designed so that the IN and VCC pin can safely protect up to 25 V and withstand transients to 30 V. Since these spikes can be very narrow in duration, it is important to use a high bandwidth probe and oscilloscope when prototyping the product to verify the operation of the circuit under all the transient conditions. A similar problem can result due to contact bounce as the DC source is plugged into the product. For portable products it is normal to have a capacitor to ground in parallel with the battery. If the product has a battery pack that is easily removable during charging, this scenario should be analyzed. Under that situation, the charging current will go into the capacitor and the voltage may rise rapidly depending on the capacitor value, the charging current and the power supply response time. |
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