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LM2760M5 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM2760M5 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 10 page Application Information (Continued) 1mA). The simplified power model of the LM2760, in Figure 2, will be used to discuss power efficiency and power dissi- pation. In calculating power efficiency, output power (P OUT) is easily determined as the product of the output current and the 3.3V output voltage. Like output current, input voltage is an application-dependent variable. The input current can be calculated using the principles of linear regulation and switched capacitor conversion. In an ideal linear regulator, the current into the circuit is equal to the current out of the circuit. The principles of power conservation mandate the ideal input current of a voltage converter must be equal to the product of the gain of the converter and the output current. When the input voltage is low (V IN ≤ 3.4V) and the LM2760 is in doubler mode (2x), the gain of the converter is 2. At higher input voltages (V IN > 3.4V typ.) where the LM2760 is in pass mode (1x), the gain of the converter is 1. Adding a correction factor for supply current when the pump is continuously active (I PUMP, 0.3mA typ.) gives an approxi- mation for total input current which, when combined with the other input and output parameter(s), yields the following equations for efficiency: A plot of efficiency versus input voltage, contained in Figure 4, gives a clear indication of how the pass mode improves efficiency of the part at higher input voltages. Efficiency of the part in both doubler and pass modes is inversely propor- tional to input voltage, highest when the input voltage is low. When the part transitions to pass mode, however, there is a boost in efficiency that is a result of the reduced input current needed for the 1x-configured converter. This multi-gain to- pology maximizes LM2760 efficiency over the Li-Ion battery input range: the average efficiency of the part over a 3.0V- to-4.2V input range is 75% (1mA ≤ I OUT ≤ 20mA). Comparisons of LM2760 efficiency measurements to calcu- lations using the above equations have shown the equations to be quite accurate approximations of actual efficiency when output currents are at or above 1mA. Under these conditions, the pump is on nearly continuously, and PFM- related rest states are infrequent. At lighter loads, however, pulse frequency modulation has a greater impact on part performance. With PFM, the part only pumps when it needs to boost the output voltage. Otherwise, it idles in a low- current rest mode. PFM reduces the input current of the part (effectively reducing I PUMP), improving the efficiency of the part at lighter loads. POWER DISSIPATION LM2760 power dissipation (P D) is calculated simply by sub- tracting output power from input power. Maximum power dissipation of the LM2760 occurs at the high end of the doubler mode (V IN= 3.4V). The following equation can be used to estimate power dissipation of the LM2760 when in doubler mode: P D-2X =PIN -POUT =[VIN x (2·IOUT +IPUMP)]-[VOUT xIOUT], where I PUMP is the supply current when the pump is continu- ously active (0.3mA typ.). Neglecting conditions outside the specified operating rat- ings, maximum power dissipation of the LM2760 is approxi- mately 70mW (V IN ) 3.4V, I OUT = 20mA, IPUMP = 0.3mA). When the junction-to-ambient thermal resistance of the LM2760 in the application is 220oC/W (matches JEDEC JESD51-3), power dissipation results in a 15oC elevation above ambient in the junction temperature of the part. Thus, when the ambient temperature is 85oC, the junction tem- perature of the LM2760 under these conditions will be 100oC. LM2760 performance limits are guaranteed over a junction temperature range of 0oCto+70oC. The LM2760 must be operated with a junction temperature within this range for the guaranteed performance limits listed in this datasheet to apply. 20044304 FIGURE 2. LM2760 Power Model 20044307 FIGURE 3. 20044305 FIGURE 4. LM2760 Power Efficiency vs. Input Voltage (T A =25 oC) www.national.com 9 |
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