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LM2798MMX-2.0 Folha de dados(PDF) 9 Page - National Semiconductor (TI) |
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LM2798MMX-2.0 Folha de dados(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Operation Description OVERVIEW The LM2797/98 are switched capacitor converters that pro- duce a regulated low-voltage output. The core of the parts is a highly efficient charge pump that utilizes multiple fractional gains and pulse-frequency modulated (PFM) switching to minimize power losses over wide input voltage and output current ranges. A description of the principal operational characteristics of the LM2797/98 is broken up into the fol- lowing sections: PFM Regulation, Fractional Multi-Gain Charge Pump, and Gain Selection for Optimal Efficiency. Each of these sections refers to the block diagram presented on the previous page. PFM REGULATION The LM2797/98 achieves tightly regulated output voltages with pulse-frequency modulated (PFM) regulation. PFM sim- ply means the part only pumps when it needs to. When the output voltage is above the target regulation voltage, the part idles and consumes minimal supply-current. In this state, the load current is supplied solely by the charge stored on the output capacitor. As this capacitor discharges and the output voltage falls below the target regulation voltage, the charge pump activates. Charge/current is delivered to the output (supplying the load and boosting the voltage on the output capacitor). The primary benefit of PFM regulation is when output cur- rents are light and the part is predominantly in the low- supply-current idle state. Net supply current is minimal be- cause the part only occasionally needs to recharge the output capacitor by activating the charge pump. FRACTIONAL MULTI-GAIN CHARGE PUMP The core of the LM2797/98 is a two-phase charge pump controlled by an internally generated non-overlapping clock. The charge pump operates by using the external flying ca- pacitors, C1 and C2, to transfer charge from the input to the output. During the charge phase, which doubles as the PFM "idle state", the flying capacitors are charged by the input supply. The charge pump will be in this state until the output voltage drops below the target regulation voltage, triggering the charge pump to activate so that it can deliver charge to the output. Charge transfer is achieved in the pump phase. In this phase, the fully charged flying capacitors are con- nected to the output so that the charge they hold can supply the load current and recharge the output capacitor. Input, output, and intermediary connections of the flying capacitors are made with internal MOS switches. The LM2797/98 utilizes two flying capacitors and a versatile switch network to achieve several fractional voltage gains: 1 ⁄2, 2⁄3, and 1. With this gain-switching ability, it is as if the LM2797/98 is three-charge-pumps-in-one. The "active" charge pump at any given time is the one that will yield the highest efficiency given the input and output conditions present. GAIN SELECTION AND GAIN HOPPING FOR OPTIMAL EFFICIENCY The ability to switch gains based on input and output condi- tions results in optimal efficiency throughout the operating ranges of the LM2797/98. Charge-pump efficiency is derived in the following two ideal equations (supply current and other losses are neglected for simplicity): I IN =GxIOUT E=(V OUT xIOUT)÷(VIN xIIN)=VOUT ÷(GXVIN) In the equations, G represents the charge pump gain. Effi- ciency is at its highest as GxV IN approaches VOUT. Optimal efficiency is achieved when gain is able to adjust depending on input and output voltage conditions. Due to the nature of charge pumps, G cannot adjust continuously, which would be ideal from an efficiency standpoint. But G can be a set of simple quantized ratios, allowing for a good degree of effi- ciency optimization. The gain set of the LM2797/98 consists of the gains 1/2, 2/3, and 1. An internal input voltage range detector, along with the nominal output voltage of a given LM2797/98 option, determines what is to be referred to as the "base gain" of the part, G B. The base gain is the default gain configuration of the part over a set V IN range. Table 1 lists GB of the LM2798- 1.8 over the input voltage range. For the remainder of this discussion, the 1.8V option of the LM2798 will be used as an example. The other voltage options of the LM2798 operate under the same principles as LM2798-1.8, the gain transi- tions merely occur at different input voltages. Since the only difference between the LM2797 and the LM2798 is start-up time, the modes of operation of the LM2798-1.8 discussed here are identical to those of the LM2797-1.8. TABLE 1. LM2798-1.8 Base Gain (G B) vs. VIN Input Voltage Base Gain (G B) 2.6V - 2.9V 1 2.9V - 3.8V 2 ⁄3 3.8V - 5.5V 1 ⁄2 Figure 1 shows the efficiency of the LM2798-1.8 versus input voltage, with output currents of 10mA and 120mA. The base gain regions (G B) are separated and labeled. There is also a set of ideal efficiency gradients, E IDEAL(G=xx) , showing the ideal efficiency of a charge pumps with gains of 1/2, 2/3, and 1. These gradients have been generated using the ideal efficiency equation presented above. 20044522 FIGURE 1. Efficiency of LM2798-1.8 with 10mA and 120mA output currents. Base-gain (G B) regions are separated and labeled. Ideal efficiency curves of charge pumps with G =1/2, 2/3, and 1 are included, and are labelled: E IDEAL(G=1),EIDEAL(G=2/3),EIDEAL(G=1/2) www.national.com 9 |
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