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LM1086CSX-3.3 bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI) |
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LM1086CSX-3.3 bảng dữ liệu(HTML) 10 Page - National Semiconductor (TI) |
10 / 15 page Application Note (Continued) OVERLOAD RECOVERY Overload recovery refers to regulator’s ability to recover from a short circuited output. A key factor in the recovery process is the current limiting used to protect the output from drawing too much power. The current limiting circuit reduces the output current as the input to output differential increases. Refer to short circuit curve in the curve section. During normal start-up, the input to output differential is small since the output follows the input. But, if the output is shorted, then the recovery involves a large input to output differential. Sometimes during this condition the current lim- iting circuit is slow in recovering. If the limited current is too low to develop a voltage at the output, the voltage will stabilize at a lower level. Under these conditions it may be necessary to recycle the power of the regulator in order to get the smaller differential voltage and thus adequate start up conditions. Refer to curve section for the short circuit current vs. input differential voltage. THERMAL CONSIDERATIONS ICs heats up when in operation, and power consumption is one factor in how hot it gets. The other factor is how well the heat is dissipated. Heat dissipation is predictable by knowing the thermal resistance between the IC and ambient ( θ JA). Thermal resistance has units of temperature per power (C/ W). The higher the thermal resistance, the hotter the IC. The LM1086 specifies the thermal resistance for each pack- age as junction to case ( θ JC). In order to get the total resistance to ambient ( θ JA), two other thermal resistance must be added, one for case to heat-sink ( θ CH) and one for heatsink to ambient ( θ HA). The junction temperature can be predicted as follows: T J =TA +PD ( θ JC + θ CH + θ HA)= TA +PD θ JA T J is junction temperature, TA is ambient temperature, and P D is the power consumption of the device. Device power consumption is calculated as follows: I IN =IL +IG P D =(VIN−VOUT)IL +VINIG Figure 6 shows the voltages and currents which are present in the circuit. Once the device power is determined, the maximum allow- able ( θ JA(max)) is calculated as: θ JA (max) =TR(max)/PD =TJ(max) −TA(max))/PD The LM1086 has different temperature specifications for two different sections of the IC: the control section and the output section. The Electrical Characteristics table shows the junc- tion to case thermal resistances for each of these sections, while the maximum junction temperatures (T J(max)) for each section is listed in the Absolute Maximum section of the datasheet. T J(max) is 125˚C for the control section, while T J(max) is 150˚C for the output section. θ JA (max) should be calculated separately for each section as follows: θ JA (max, CONTROL SECTION) = (125˚C for TA(max))/PD θ JA (max, OUTPUT SECTION) = (150˚C for TA(max))/PD The required heat sink is determined by calculating its re- quired thermal resistance ( θ HA(max)). θ HA(max) = θ JA(max) −( θ JC + θ CH) θ HA (max) should be calculated twice as follows: θ HA (max) = θ JA(max, CONTROL SECTION) - ( θ JC (CON- TROL SECTION) + θ CH) θ HA (max)= θ JA(max, OUTPUT SECTION) - ( θ JC(OUTPUT SECTION) + θ CH) If thermal compound is used, θ CH can be estimated at 0.2 C/W. If the case is soldered to the heat sink, then a θ CH can be estimated as 0 C/W. After, θ HA (max) is calculated for each section, choose the lower of the two θ HA (max) values to determine the appropri- ate heat sink. If PC board copper is going to be used as a heat sink, then Figure 7 can be used to determine the appropriate area (size) of copper foil required. 10094815 FIGURE 5. Regulator with Protection Diode 10094816 FIGURE 6. Power Dissipation Diagram 10094864 FIGURE 7. Heat sink thermal Resistance vs. Area www.national.com 10 |
Số phần tương tự - LM1086CSX-3.3 |
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Mô tả tương tự - LM1086CSX-3.3 |
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