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LM2750 Datasheet(PDF) 11 Page - Texas Instruments |
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LM2750 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 29 page 4 176 = 2 × 4 59 + 1 ( 59 × % (.; + 4 × '54 %(.; + '54%176 Reg 2× V ' 2×V ' ROUT VIN VOUT Copyright © 2016, Texas Instruments Incorporated 8 176 = 2 ×8+0 F +176 × 4176 11 LM2750, LM2750-ADJ www.ti.com SNVS180N – APRIL 2002 – REVISED APRIL 2016 Product Folder Links: LM2750 LM2750-ADJ Submit Documentation Feedback Copyright © 2002–2016, Texas Instruments Incorporated Feature Description (continued) The schematic of Figure 11 is a simplified model of the LM2750 that is useful for evaluating output current capability. The model shows a linear pre-regulation block (Reg), a voltage doubler (2×), and an output resistance (ROUT). Output resistance models the output voltage droop that is inherent to switched capacitor converters. The output resistance of the LM2750 is 5 Ω (typical), and is approximately equal to twice the resistance of the four LM2750 switches. When the output voltage is in regulation, the regulator in the model controls the voltage V' to keep the output voltage equal to 5 V ± 4%. With increased output current, the voltage drop across ROUT increases. To prevent droop in output voltage, the voltage drop across the regulator is reduced, V' increases, and VOUT remains at 5V. When the output current increases to the point that there is zero voltage drop across the regulator, V' equals the input voltage, and the output voltage is "on the edge" of regulation. Additional output current causes the output voltage to fall out of regulation, and the LM2750 operation is similar to a basic open- loop doubler. As in a voltage doubler, increase in output current results in output voltage drop proportional to the output resistance of the doubler. The out-of-regulation LM2750 output voltage can be approximated by: (1) Again, Equation 1 only applies at low input voltage and high output current where the LM2750 is not regulating. See Figure 1 and Figure 2 in for more details. Figure 11. LM2750 Output Resistance Model A more complete calculation of output resistance takes into account the effects of switching frequency, flying capacitance, and capacitor equivalent series resistance (ESR). See Equation 2: (2) Switch resistance (5 Ω typical) dominates the output resistance equation of the LM2750. With a 1.7-MHz typical switching frequency, the 1/(F×C) component of the output resistance contributes only 0.6 Ω to the total output resistance. Increasing the flying capacitance only provides minimal improvement to the total output current capability of the LM2750. In some applications it may be desirable to reduce the value of the flying capacitor below 1 µF to reduce solution size and/or cost, but this must be done with care so that output resistance does not increase to the point that undesired output voltage droop results. If ceramic capacitors are used, equivalent series resistance (ESR) is a negligible factor in the total output resistance, as the ESR of quality ceramic capacitors is typically much less than 100 m Ω. 7.3.6 Thermal Shutdown The LM2750 implements a thermal shutdown mechanism to protect the device from damage due to overheating. When the junction temperature rises to 150°C (typical), the part switches into shutdown mode. The LM2750 releases thermal shutdown when the junction temperature of the part is reduced to 130°C (typical). Thermal shutdown is most-often triggered by self-heating, which occurs when there is excessive power dissipation in the device and/or insufficient thermal dissipation. LM2750 power dissipation increases with increased output current and input voltage (see Power Efficiency And Power Dissipation). When self-heating brings on thermal shutdown, thermal cycling is the typical result. Thermal cycling is the repeating process where the part self-heats, enters thermal shutdown (where internal power dissipation is practically zero), cools, turns on, and then heats up again to the thermal shutdown threshold. Thermal cycling is recognized by a pulsing output voltage and can be stopped be reducing the internal power dissipation (reduce input voltage and/or output current) or the ambient temperature. If thermal cycling occurs under desired operating conditions, thermal dissipation performance must be improved to accommodate the power dissipation of the LM2750. The WSON package has excellent thermal properties that, when soldered to a PCB designed to aid thermal dissipation, allows the LM2750 to operate under very demanding power dissipation conditions. |
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