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LM2750LDX-ADJ Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM2750LDX-ADJ Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 15 page Operation Description OVERVIEW The LM2750 is a regulated switched capacitor doubler that, by combining the principles of switched-capacitor voltage boost and linear regulation, generates a regulated output from an extended Li-Ion input voltage range. A two-phase non-overlapping clock generated internally controls the op- eration of the doubler. During the charge phase ( φ1), the flying capacitor (C FLY) is connected between the input and ground through internal pass-transistor switches and is charged to the input voltage. In the pump phase that follows ( φ2), the flying capacitor is connected between the input and output through similar switches. Stacked atop the input, the charge of the flying capacitor boosts the output voltage and supplies the load current. A traditional switched capacitor doubler operating in this manner will use switches with very low on-resistance to generate an output voltage that is 2x the input voltage. The LM2750 regulates the output voltage by controlling the re- sistance of the two input-connected pass-transistor switches in the doubler. PRE-REGULATION The very low input current ripple of the LM2750, resulting from internal pre-regulation, adds very little noise to the input line. The core of the LM2750 is very similar to that of a basic switched capacitor doubler: it is composed of four switches and a flying capacitor (external). Regulation is achieved by modulating the on-resistance of the two switches connected to the input pin (one switch in each phase). The regulation is done before the voltage doubling, giving rise to the term "pre-regulation". It is pre-regulation that eliminates most of the input current ripple that is a typical and undesirable characteristic of a many switched capacitor converters. INPUT, OUTPUT, AND GROUND CONNECTIONS Making good input, output, and ground connections is es- sential to achieve optimal LM2750 performance. The two input pads, pads 8 and 9, must be connected externally. It is strongly recommended that the input capacitor (C IN)be placed as close as possible to the LM2750, so that the traces from the input pads are as short and straight as possible. To minimize the effect of input noise on LM2750 performance, it is best to bring two traces out from the LM2750 all the way to the input capacitor pad, so that they are connected at the capacitor pad. Connecting the two input traces between the input capacitor and the LM2750 input pads could make the LM2750 more susceptible to noise-related performance deg- radation. It is also recommended that the input capacitor be on the same side of the PCB as the LM2750, and that traces remain on this side of the board as well (vias to traces on other PCB layers are not recommended between the input capacitor and LM2750 input pads). The two output pads, pads 1 and 2, must also be connected externally. It is recommended that the output capacitor (C OUT) be placed as close to the LM2750 output pads as possible. It is best if routing of output pad traces follow guidelines similar to those presented for the input pads and capacitor. The flying capacitor (C FLY) should also be placed as close to the LM2750 as possible to minimize PCB trace length between the capacitor and the IC. Due to the pad- layout of the part, it is likely that the trace from one of the flying capacitor pads (C+ or C-) will need to be routed to an internal or opposite-side layer using vias. This is acceptable, and it is much more advantageous to route a flying capacitor trace in this fashion than it is to place input traces on other layers. The GND pads of the LM2750 are ground connections and must be connected externally. These include pads 3 (LM2750-5.0 only), 5, 6 and the die-attach pad (DAP). Large, low impedance copper fills and via connections to an internal ground plane are the preferred way of connecting together the ground pads of the LM2750, the input capacitor, and the output capacitor, as well as connecting this circuit ground to the system ground of the PCB. SHUTDOWN When the voltage on the active-low-logic shutdown pin is low, the LM2750 will be in shutdown mode. In shutdown, the LM2750 draws virtually no supply current. There is a 200k Ω pull-down resistor tied between the SD pin and GND that pulls the SD pin voltage low if the pin is not driven by a voltage source. When pulling the part out of shutdown, the voltage source connected to the SD pin must be able to drive the current required by the 200k Ω resistor. For voltage man- agement purposes required upon startup, internal switches connect the output of the LM2750 to an internal pull-down resistor (1k Ω typ) when the part is shutdown. Driving the output of the LM2750 by another supply when the LM2750 is shutdown is not recommended, as the pull-down resistor was not sized to sink continuous current. SOFT START The LM2750 employs soft start circuitry to prevent excessive input inrush currents during startup. The output voltage is programmed to rise from 0V to the nominal output voltage (5.0V) in 500µs (typ.). Soft-start is engaged when a part, with input voltage established, is taken out of shutdown mode by pulling the SD pin voltage high. Soft-start will also engage when voltage is established simultaneously to the input and SD pins. OUTPUT CURRENT CAPABILITY The LM2750-5.0 is guaranteed to provide 120mA of output current when the input voltage is within 2.9V-to-5.6V. Using the LM2750 to drive loads in excess of 120mA is possible. IMPORTANT NOTE: Understanding relevant application is- sues is recommended and a thorough analysis of the appli- cation circuit should be performed when using the part out- side operating ratings and/or specifications to ensure satisfactory circuit performance in the application. Special care should be paid to power dissipation and thermal effects. These parameters can have a dramatic impact on high- current applications, especially when the input voltage is high. (see "Power Efficiency and Power Dissipation" section, to come). The schematic of Figure 1 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 (2x), and an output resistance (R OUT). Out- put resistance models the output voltage droop that is inher- ent to switched capacitor converters. The output resistance of the LM2750 is 5 Ω (typ.), 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 5.0V ± 4%. With increased output current, the voltage drop across R OUT increases. To prevent droop in output voltage, the voltage drop across the regulator is reduced, V’ in- creases, and V OUT remains at 5V. When the output current www.national.com 8 |
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