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ADP1073AR-12 データシート(PDF) 9 Page - Analog Devices |
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ADP1073AR-12 データシート(HTML) 9 Page - Analog Devices |
9 / 16 page ADP1073 –9– REV. 0 Circuit Operation, Step-Up (Boost) Mode In boost mode, the ADP1073 produces an output voltage that is higher than the input voltage. For example, +5 V can be derived from one alkaline cell (+1.5 V), or +12 V can be generated from a +5 V logic power supply. Figure 15 shows an ADP1073 configured for step-up operation. The collector of the internal power switch is connected to the output side of the inductor, while the emitter is connected to GND. When the switch turns on, Pin SW1 is pulled near ground. This action forces a voltage across L1 equal to VIN –VCE(SAT) and current begins to flow through L1. This current reaches a final value (ignoring second-order effects) of: IPEAK ≅ V IN –VCE(SAT ) L ×38 µs where 38 µs is the ADP1073 switch’s “on” time. 3 5 1 8 4 2 ILIM VIN SW1 FB SW2 GND ADP1073 L1 D1 VOUT C1 *OPTIONAL R3* R1 R2 VIN Figure 15. Step-Up Mode Operation When the switch turns off, the magnetic field collapses. The polarity across the inductor changes, current begins to flow through D1 into the load and the output voltage is driven above the input voltage. The output voltage is fed back to the ADP1073 via resistors R1 and R2. When the voltage at pin FB falls below 212 mV, SW1 turns “on” again and the cycle repeats. The output voltage is therefore set by the formula: VOUT = 212 mV × 1+ R1 R2 The circuit of Figure 15 shows a direct current path from VIN to VOUT, via the inductor and D1. Therefore, the boost converter is not protected if the output is short circuited to ground. Circuit Operation, Step-Down (Buck) Mode) The ADP1073’s step-down mode is used to produce an output voltage that is lower than the input voltage. For example, the output of four NiCd cells (+4.8 V) can be converted to a +3.3 V logic supply. A typical configuration for step-down operation of the ADP1073 is shown in Figure 16. In this case, the collector of the internal power switch is connected to VIN and the emitter drives the inductor. When the switch turns on, SW2 is pulled up toward VIN. This forces a voltage across L1 equal to (VIN – VCE) – VOUT, and causes current to flow in L1. This current reaches a final value of: IPEAK ≅ V IN –VCE –VOUT L ×38 µs where 38 µs is the ADP1073 switch’s “on” time. 5 1 8 4 2 ILIM VIN SW1 FB SW2 GND ADP1073 L1 D1 1N5818 VOUT C2 R3 220 R1 R2 VIN 3 C1 Figure 16. Step-Down Mode Operation When the switch turns off, the magnetic field collapses. The polarity across the inductor changes and the switch side of the inductor is driven below ground. Schottky diode D1 then turns on and current flows into the load. Notice that the Absolute Maximum Rating for the ADP1073’s SW2 pin is 0.5 V below ground. To avoid exceeding this limit, D1 must be a Schottky diode. Using a silicon diode in this application will generate forward voltages above 0.5 V, which will cause potentially dam- aging power dissipation within the ADP1073. The output voltage of the buck regulator is fed back to the ADP1073’s FB pin by resistors R1 and R2. When the voltage at pin FB falls below 212 mV, the internal power switch turns “on” again and the cycle repeats. The output voltage is set by the formula: VOUT = 212 mV × 1+ R1 R2 The output voltage should be limited to 6.2 V or less when using the ADP1073 in step-down mode. If the input voltage to the ADP1073 varies over a wide range, a current limiting resistor at Pin 1 may be required. If a particular circuit requires high peak inductor current with minimum input supply voltage the peak current may exceed the switch maximum rating and/or saturate the inductor when the supply voltage is at the maximum value. See the Limiting the Switch Current section of this data sheet for specific recommendations. Positive-to-Negative Conversion The ADP1073 can convert a positive input voltage to a negative output voltage, as shown in Figure 17. This circuit is essentially identical to the step-down application of Figure 16, except that the “output” side of the inductor is connected to power ground. When the ADP1073’s internal power switch turns off, current flowing in the inductor forces the output (–VOUT) to a negative ILIM VIN SW1 FB SW2 GND ADP1073 L1 D1 1N5818 VOUT C2 R3 R2 R1 VIN C1 Figure 17. A Positive-to-Negative Converter potential. The ADP1073 will continue to turn the switch on until its FB pin is 212 mV above its GND pin, so the output voltage is determined by the formula: |
同様の部品番号 - ADP1073AR-12 |
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同様の説明 - ADP1073AR-12 |
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