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ADP1108AR Arkusz danych(PDF) 9 Page - Analog Devices |
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ADP1108AR Arkusz danych(HTML) 9 Page - Analog Devices |
9 / 12 page ADP1108 –9– REV. 0 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 –V CE –VOUT L ×36 µs where 36 µs is the ADP1108 switch’s “on” time. 5 1 8 4 2 ILIM VIN SW1 FB SW2 GND ADP1108 L1 D1 1N5818 VOUT C2 RLIM 100 Ω R1 R2 VIN 3 C1 Figure 16. Step-Down Mode Operation When the switch turns off, the magnetic field collapses. The po- larity across the inductor changes and the switch side of the in- ductor is driven below ground. Schottky diode D1 then turns on and current flows into the load. Notice that the Absolute Maxi- mum Rating for the ADP1108’s SW2 pin is 0.5 V below ground. To avoid exceeding this limit, D1 must be a Schottky diode. If a silicon diode is used for D1, Pin SW2 can go to 0.8 V, which will cause potentially damaging power dissipation within the ADP1108. The output voltage of the buck regulator is fed back to the ADP1108’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: V OUT = 1.245 V × 1 + R1 R2 When operating the ADP1108 in step-down mode, the output voltage is impressed across the internal power switch’s emitter- base junction while the switch is off. To protect the switch, the output voltage should be limited to 6.2 V or less. If a higher out- put voltage is required, a Schottky diode should be placed in se- ries with SW2, as shown in Figure 17. 5 1 8 4 2 ILIM VIN SW1 FB SW2 GND ADP1108 L1 D1 1N5818 VOUT C2 RLIM 100 Ω R1 R2 VIN 3 C1 D2 Figure 17. Step-Down Model, VOUT > 6.2 V If the input voltage to the ADP1108 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, then 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. Increasing Output Current in the Step-Down Regulator Unlike the boost configuration, the ADP1108’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 ADP1108 when high peak current is required. To increase the output current, an external PNP switch can be added (Figure 18). In this circuit, the ADP1108 provides base drive to Q1 through R3 while R4 ensures that Q1 turns off rapidly. The ADP1108’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 ADP1108, this circuit also reduces the switch voltage. When selecting an inductor value for the circuit of Fig- ure 18, the switch voltage can be calculated from the formula: V SW = V R5 + V Q1(SAT ) ≅ 0.6 V + 0. 4 V ≅ 1V L1* 100 H 5VOUT 200mA AT 6.5V 500mA AT 8V R4 100 Ω R5 0.22 Ω Q1 ZETEX ZTX749 *L1 = COILTRONICS CTX100-4 R2 100 Ω 5 8 2 ILIM VIN SW1 SW2 GND ADP1108-5 D1 1N5818 C2 VIN 6.5V TO 20V 3 C1 R3 220 Ω 1 4 SENSE Figure 18. High Current Step-Down Operation Positive-to-Negative Conversion The ADP1108 can convert a positive input voltage to a negative output voltage, as shown in Figure 19. 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 ADP1108’s internal power switch turns off, current flowing in the inductor forces the output ( VOUT) to a negative potential. The ADP1108 will continue to turn the switch on un- til its FB pin is 1.245 V above its GND pin, so the output volt- age is determined by the formula: V OUT = 1.245 V × 1 + R1 R2 5 1 8 4 2 ILIM VIN SW1 FB SW2 GND ADP1108 L1 D1 1N5818 –VOUT C2 R3 R1 R2 VIN 3 CL Figure 19. A Positive-to-Negative Converter The design criteria for the step-down application also apply to the positive-to-negative converter. The output voltage should be limited to |6.2 V|, unless a diode is inserted in series with the SW2 pin (see Figure 17). Also, D1 must again be a Schottky di- ode to prevent excessive power dissipation in the ADP1108. |
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