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LTC1155M Datasheet(PDF) 7 Page - Linear Technology |
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LTC1155M Datasheet(HTML) 7 Page - Linear Technology |
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7 / 16 page  7 LTC1155 S APPLICATI I FOR ATIO For this example, we assume a worst-case scenario; i.e., that the power supply to the power MOSFET is “hard” and provides a constant 5V regardless of the current. In this case, the current is limited by the RDS(ON) of the MOSFET and the drain sense resistance. Therefore: IPEAK = VSUPPLY/0.08Ω = 62.5A The drop across the drain sense resistor under these conditions is much larger than 100mV and is equal to the drain current times the sense resistance: VDROP = (IPEAK)(RSEN) = 1.88V By consulting the power MOSFET data sheet SOA graph, we note that the IRLZ34 is capable of delivering 62.5A at a drain-to-source voltage of 3.12V for approximately 10ms. An RC time constant can now be calculated which satisfies this requirement: Graphical Approach to Selecting RDLY and CDLY Figure 2 is a graph of normalized overcurrent shutdown time versus normalized MOSFET current. This graph can be used instead of the above equation to calculate the RC time constant. The Y axis of the graph is normalized to one RC time constant. The X axis is normalized to the set current. (The set current is defined as the current required to develop 100mV across the drain sense resistor). This time constant should be viewed as a maximum safe delay time and should be reduced if the competing requirement of starting a high inrush current load is less stringent; i.e., if the inrush time period is calculated at 20ms, the RC time constant should be set at roughly two or three times this time period and not at the maximum of 182ms. A 60ms time constant would be produced with a 270k resistor and a 0.22 µF capacitor (as shown in Figure 1). RC t In V RI RC In ms SEN SEN MAX = − • = − • = = – –. . .. –. /– . 1 001 1 010 0 030 62 5 0 01 0 054 182 Note that the shutdown time is shorter for increasing levels of MOSFET current. This ensures that the total energy dissipated by the MOSFET is always within the bounds established by the MOSFET manufacturer for safe operation. In the example presented above, we established that the power MOSFET should not be allowed to pass 62.5A for more than 10ms. 62.5A is roughly 18 times the set current of 3.3A. By drawing a line up from 18 and reflecting it off the curve, we establish that the RC time constant should be set at 10ms divided by 0.054, or 180ms. Both methods result in the same conclusion. Using a Speed Up Diode A way to further reduce the amount of time that the power MOSFET is in a short-circuit condition is to “bypass”the delay resistor with a small signal diode as shown in Figure 3. The diode will engage when the drop across the drain sense resistor exceeds 0.7V, providing a direct path to the MOSFET CURRENT (1 = SET CURRENT) 1 0.01 0.1 1 10 5 10 20 100 1155 F02 250 Figure 2. Shutdown Time vs MOSFET Current |
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