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SIP4613B Datasheet(PDF) 4 Page - Vishay Siliconix |
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SIP4613B Datasheet(HTML) 4 Page - Vishay Siliconix |
4 / 10 page www.vishay.com 4 Document Number: 69998 S-82461-Rev. B, 06-Oct-08 Vishay Siliconix SiP4613A, SiP4613B DETAILED DESCRIPTION The SiP4613A, SiP4613B limits load current by sampling the pass transistor current and passing that through an external resistor, RSET. The voltage across RSET, VSET, is then compared with an internal reference voltage, VREF. In the event that load current surpasses the set limit current, VSET will exceed VREF causing the pass transistor gate voltage to increase, thereby reducing the gate to source voltage of the PMOS switch and regulating its current back down to ILIMIT. Setting the Current Limit Level Setting the current limit level on the SiP4613A, SiP4613B requires some care to ensure the maximum current required by the load will not trigger the current limit circuitry. The minimum current limit threshold should be determined by taking the maximum current required by the load, ILOAD, and adding 25 % headroom. The SiP4613A, SiP4613B has a current limit tolerance of 25 %, which is largely a result of process variations from part to part, and also temperature and VIN/VOUT variances. Thus, to ensure that the actual current limit is never below the desired current limit a 1/0.80 = 1.25 coefficient needs to be added to the calculations. Knowing the maximum load current required, the value of RSET is calculated as follows. RSET = RSET coefficient/ILIMIT where ILIMIT = (ILOAD x 1.25) x 1.20 and RSET coefficient is 3460 for a 500 mA current limit. For typical RSET coefficient values given a limit current refer to the "Typical Characteristics" section. Operation at Current Limit and Thermal Shutdown In the event that a load higher than ILIMIT is demanded of the SiP4613A, SiP4613B the load current will stay fixed at the current limit established by RSET. However, since the required current is not supplied, the voltage at OUT will drop. The increase in VIN - VOUT will cause the chip to dissipate more heat. The power dissipation for the SiP4613A, SiP4613B can be expressed as P = ILOAD x (VIN - VOUT) Once this exceeds the maximum power dissipation of the package, the die temperature will rise. When the die temperature exceeds an over-temperature limit of 165 °C, the SiP4613A, SiP4613B will shut down until it has cooled down to 145 °C, before starting up again. As can be seen in the figure below, the SiP4613A, SiP4613B will continue to cycle on and off until the load is reduced or the part is turned off (see figure 4 on next page). The maximum power dissipation in any application is dependant on the maximum junction temperature, TJ(MAX) = 125 °C, the junction-to-ambient thermal resistance for the TSC75-6 package, θ J-A = 131 °C/W, and the ambient temperature, TA, which may be formulaically expressed as: It then follows that assuming an ambient temperature of 70 °C, the maximum power dissipation will be limited to about 419 mW. There is CL pin designed to indicate the current limit status of SiP4613. A typical 5 k Ω resistor is required to connect between CL pin and IN pin. In the event of the output over load, the current limit flag pin CL will go low to indicate the current limit status of the device. Figure 2 shows the voltage on CL pin go low after output short. Figure 2 The voltage signal in CL pin is not only used to indicate the output short circuit status. It is also used to indicate the thermal protection status of the device. Once the thermal protection is activated in the severe output short circuit condition, the voltage signal on the CL pin will run into the thermal protection cycling. Figure 3 shows the voltage waveform of CL pin after activation of the thermal protection circuit due to the severe short circuit status of the device’s output. Figure 3 The thermal protection is the final protection to the device. The device will be completely shut down including the open drain current limit indicator pin CL until the device temperature drop below 145 °C. 131 125 (max) (max) A A J A J T T T P - = - = - θ VOUT (500 mV/div) CL (500 mV/div) VOUT = 2.4 V RSET = 6.81 kΩ 50 µs/div VOUT (2 V/div) CL (2 V/div) VOUT = 5.5 V RSET = 6.81 kΩ 20 ms/div |
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Similar Description - SIP4613B |
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