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LM5008 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LM5008 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
7 / 15 page Typical Performance Characteristics (Continued) Functional Description The LM5008 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient, Buck bias power converter. This high voltage regulator contains a 100 V N-Channel Buck Switch, is easy to implement and is provided in the MSOP-8 and the thermally enhanced LLP-8 packages. The regulator is based on a hysteretic control scheme using an on-time inversely proportional to V IN. The hysteretic control requires no loop compensation. Current limit is implemented with forced off-time, which is inversely proportional to V OUT. This scheme ensures short circuit pro- tection while providing minimum foldback. The Functional Block Diagram of the LM5008 is shown in Figure 1. The LM5008 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator is well suited for 48 Volt Telecom and the new 42V Automotive power bus ranges. Protection features include: Thermal Shutdown, V CC under-voltage lockout, Gate drive under- voltage lockout, Max Duty Cycle limit timer and the intelligent current limit off timer. Hysteretic Control Circuit Overview The LM5008 is a Buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with line volt- age (V IN). Control is based on a comparator and the on-time one-shot, with the output voltage feedback (FB) compared to an internal reference (2.5V). If the FB level is below the reference the buck switch is turned on for a fixed time determined by the line voltage and a programming resistor (R ON). Following the ON period the switch will remain off for at least the minimum off-timer period of 300ns. If FB is still below the reference at that time the switch will turn on again for another on-time period. This will continue until regulation is achieved. The LM5008 operates in discontinuous conduction mode at light load currents, and continuous conduction mode at heavy load current. In discontinuous conduction mode, cur- rent through the output inductor starts at zero and ramps up to a peak during the on-time, then ramps back to zero before 20097923 FIGURE 6. Efficiency vs V IN (Circuit of Figure 13) 20097927 FIGURE 7. Efficiency vs Load Current vs V IN (Circuit of Figure 13) 20097924 FIGURE 8. Output Voltage vs Load Current (Circuit of Figure 13) www.national.com 7 |
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