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LM1086IS-1.8/NOPB Datasheet(PDF) 11 Page - Texas Instruments |
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LM1086IS-1.8/NOPB Datasheet(HTML) 11 Page - Texas Instruments |
11 / 24 page LM1086 www.ti.com SNVS039H – JUNE 2000 – REVISED MAY 2013 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 Typical Performance Characteristics 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 limiting 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 Typical Performance Characteristics 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 ( θJA). 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 package as junction to case ( θJC). In order to get the total resistance to ambient ( θJA), two other thermal resistance must be added, one for case to heat-sink (θCH) and one for heatsink to ambient ( θHA). The junction temperature can be predicted as follows: TJ = TA + PD (θJC + θCH + θHA) = TA + PD θJA where • TJ is junction temperature • TA is ambient temperature • PD is the power consumption of the device Device power consumption is calculated as follows: IIN = IL + IG PD = (VIN−VOUT) IL + VINIG Figure 23 shows the voltages and currents which are present in the circuit. Figure 23. Power Dissipation Diagram Once the device power is determined, the maximum allowable ( θJA(max)) is calculated as: θJA (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 junction to case thermal resistances for each of these sections, while the maximum junction temperatures (TJ(max)) for each section is listed in the Absolute Maximum section of the datasheet. TJ(max) is 125°C for the control section, while TJ(max) is 150°C for the output section. θJA (max) should be calculated separately for each section as follows: θJA (max, CONTROL SECTION) = (125°C for TA(max))/PD θJA (max, OUTPUT SECTION) = (150°C for TA(max))/PD Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM1086 |
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