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Moteur de recherche de fiches techniques de composants électroniques |
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ADP1173 Fiches technique(PDF) 9 Page - Analog Devices |
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ADP1173 Fiches technique(HTML) 9 Page - Analog Devices |
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9 / 16 page  ADP1173 –9– REV. 0 + ILIM VIN SW1 ADP1173 VIN L1 D1 C1 R2 R1 VOUT R3* FB * = OPTIONAL SW2 GND 4 5 1 2 3 8 Figure 14. 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 ADP1173 via resistors R1 and R2. When the voltage at pin FB falls below 1.245 V, SW1 turns “on” again and the cycle repeats. The output voltage is therefore set by the formula: VOUT =1.245 V × 1+ R1 R2 The circuit of Figure 14 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 ADP1173’s step-down mode is used to produce an output voltage 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 ADP1173 is shown in Figure 15. 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 ×23 µs where 23 µs is the ADP1173 switch’s “on” time. ILIM VIN FB ADP1173 SW2 GND 5 1 2 8 SW1 3 R3 100 Ω + C2 VIN L1 D1 1N5818 + C1 VOUT R2 R1 4 Figure 15. 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 ADP1173’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 damaging power dissipation within the ADP1173. The output voltage of the buck regulator is fed back to the ADP1173’s FB pin by resistors R1 and R2. When the voltage at pin FB falls below 1.245 V, the internal power switch turns “on” again and the cycle repeats. The output voltage is set by the formula: VOUT =1.245 V × 1+ R1 R2 When operating the ADP1173 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 16. If high output current is required in a step-down converter, the ADP1111 or ADP3000 should be considered. These devices offer higher frequency operation, which reduces inductor size, and an external pass transistor can be added to reduce RON of the switch. ILIM VIN FB ADP1173 SW2 GND 5 1 2 8 SW1 3 RLIM 100 Ω + C2 L1 D1 1N5818 + C1 VOUT R2 R1 1N5818 VIN 4 Figure 16. Step-Down Mode, VOUT > 6.2 V If the input voltage to the ADP1173 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. POSITIVE-TO-NEGATIVE CONVERSION The ADP1173 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 15, except that the “output” side of the inductor is connected to power ground. When the ADP1173’s internal power switch turns off, current flowing in the inductor forces the output (–VOUT) to a negative potential. The ADP1173 will continue to turn the switch on |
Numéro de pièce similaire - ADP1173 |
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Description similaire - ADP1173 |
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