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ADP1110AN-12 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADP1110AN-12 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page ADP1110 –10– REV. 0 turns “on” again, and the cycle repeats. The output voltage is therefore set by the formula: VOUT = 220 mV • 1+ R1 R2 The circuit of Figure 19 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 ADP1110’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 V logic supply. A typical configuration for step-down operation of the ADP1110 is shown in Figure 20. 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 towards 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 •10 µs where 10 µs is the ADP1110 switch’s “on” time. ILIM VIN SW1 SW2 GND SET AO ADP1110 NC NC L1 C1 R1 R2 D1 1N5818 VOUT C2 VIN 2 RLIM 100 Ω FB 1 3 6 75 4 8 Figure 20. 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 ADP1110’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 that will cause potentially damaging power dissipation within the ADP1110. The output voltage of the buck regulator is fed back to the ADP1110’s FB pin by resistors R1 and R2. When the voltage at pin FB falls below 220 mV, the internal power switch turns “on” again and the cycle repeats. The output voltage is set by the formula: VOUT = 220 mV • 1+ R1 R2 When operating the ADP1110 in step-down mode, the output voltage is impressed across the internal power switch’s emitter- base junction when the switch is off. To protect the switch, the output voltage should be limited to 6.2 V or less. If a higher output voltage is required, a Schottky diode should be placed in series with SW2, as shown in Figure 21. ILIM VIN SW1 SW2 FB GND SET AO ADP1110 NC NC L1 CL R1 R2 D1 1N5818 OUTPUT CINPUT INPUT 6 7 8 1 2 3 4 5 RLIM Figure 21. Step-Down Mode, VOUT > 6.2 V If the input voltage to the ADP1110 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 maxi- mum 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. INCREASING OUTPUT CURRENT IN THE STEP-DOWN REGULATOR Unlike the boost configuration, the ADP1110’s internal power switch is not saturated when operating in step-down mode. A conservative value for the voltage across the switch in step-down mode is 1.5 V. This results in high power dissipation within the ADP1110 when high peak current is required. To increase the output current, an external PNP switch can be added (Figure 22). In this circuit, the ADP1110 provides base drive to Q1 through R3, while R4 ensures that Q1 turns off rapidly. Because the ADP1110’s internal current limiting function will not work in this circuit, R5 is provided for this purpose. With the value shown, R5 limits current to 2 A. In addition to reducing power dissipation on the ADP1110, this circuit also reduces the switch voltage. When selecting an inductor value for the circuit of Figure 22, the switch voltage can be calculated from the formula: V SW =V R5 +VQ1(SAT ) ≅ 0.6V + 0.4V ≅1V ILIM VIN SW1 SW2 FB GND SET AO ADP1110 NC NC L1 CL R1 R2 D1 1N5821 OUTPUT CINPUT RLIM INPUT 6 7 8 1 2 4 5 3 R4 220 Ω R3 330 Ω MJE210 0.3 Ω R5 Figure 22. High Current Step-Down Operation |
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