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LM2686 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LM2686 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Detailed Device Description The LM2686 CMOS charge pump voltage converter oper- ates as an input voltage doubler, +5V regulator for an input voltage in the range of +2.85V to +6.5V. It delivers maximum load currents of 50mA for the regulated +5V, with an operat- ing current of only 450µA. It also has a typical shutdown cur- rent of 6µA. All these performance qualities make the LM2686 an ideal device for battery powered systems. The LM2686 has two main functional blocks: a voltage dou- bler and a low dropout (LDO) regulator. Figure 1 shows the LM2686 functional block diagram. Voltage Doubler The voltage doubler ties directly to V IN and doubles the input voltage in the range from +2.85V to +5.4V up to 5.7V to 10.8V at the V DBL pin. For VIN above 5.4V, the doubler shuts off and the input voltage is passed directly to VDBL via an in- ternal power switch. The doubler contains four large CMOS switches which are switched in a sequence to double the input supply voltage. Figure 2 illustrates the voltage conversion scheme. When S2 and S4 are closed, C1 charges to the supply voltage V IN. During this time interval, switches S1 and S3 are open. In the next time interval, S2 and S4 are opened at the same time, S1 and S3 are closed, the sum of the input voltage V IN and the voltage across C1 gives the 2V In and the voltage across C2 gives the 2V IN at VDBL output. VDBL supplies the LDO regulator. It is recommended not to load V DBL when V05 has a load of 50mA. For proper operation, the sum of V DBL and V 05 loads must not be more than 50mA. +5 LDO Regulator V DBL is the input to an LDO regulator that regulates it to a +5 output voltage at V 05.VPSW is tied to V05 through a series switch PSW. The LDO output capacitor (4.7µF Tantalum) may be connected to either V 05 or VPSW. Shutdown and Load Disconnect In addition to the nominal charge pump and regulator func- tions, the LM2686 features shutdown and load disconnect circuitry. CE (chip enable) and SD (shutdown positive) per- form the same task with opposite input polarities. When CE is low or SD is high, all circuit blocks are disabled and V 05 falls to ground potential. This is the same result as when the die temperature exceeds 150˚C, and the device’s internal thermal shutdown is triggered. The LM2686 incorporates a low impedance switch tied to the V 05 output, because some special applications require load disconnect and this is achievable via the switch. Switch PSW connects V 05 to VPSW. In normal operation, this switch is closed, allowing 5V loads to be tied to either V 05 or VPSW. Forcing CE low or SD high opens the PSW. Application Information Capacitor Selection The output resistance and ripple voltage are dependent on the capacitance and ESR values of the external capacitors. Voltage Doubler External Capacitors The selection of capacitors are based on the specifications of the dropout voltage (which equals I OUT ROUT), the output voltage ripple, and the converter efficiency. where R SW is the sum of the ON resistance of the internal MOSFET switches as shown in Figure 2. The peak-to-peak output voltage ripple is determined by the oscillator frequency, the capacitance and ESR of the capaci- tor C3. DS101141-3 FIGURE 1. Functional Block Diagram DS101141-4 FIGURE 2. Voltage Doubler Principle www.national.com 6 |
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