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ADP1111 Fiches technique(PDF) 11 Page - Analog Devices |
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ADP1111 Fiches technique(HTML) 11 Page - Analog Devices |
11 / 15 page ADP1111 –11– REV. 0 POSITIVE-TO-NEGATIVE CONVERSION The ADP1111 can convert a positive input voltage to a negative output voltage as shown in Figure 22. This circuit is essentially identical to the step-down application of Figure 19, except that the “output” side of the inductor is connected to power ground. When the ADP1111’s internal power switch turns off, current flowing in the inductor forces the output (–VOUT) to a negative potential. The ADP1111 will continue to turn the switch on until its FB pin is 1.25 V above its GND pin, so the output voltage is determined by the formula: VOUT = 1.25 V • 1+ R2 R1 ⎛ ⎝⎜ ⎞ ⎠⎟ ILIM VIN SW1 SW2 FB GND SET AO ADP1111 NC L1 D1 1N5818 RLIM 1 INPUT 2 3 6 7 5 4 8 NC CINPUT + R1 R2 CL + OUTPUT NEGATIVE OUTPUT Figure 22. 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 20.) Also, D1 must again be a Schottky diode to prevent excessive power dissipation in the ADP1111. NEGATIVE-TO-POSITIVE CONVERSION The circuit of Figure 23 converts a negative input voltage to a positive output voltage. Operation of this circuit configuration is similar to the step-up topology of Figure 18, except the current through feedback resistor R2 is level-shifted below ground by a PNP transistor. The voltage across R2 is VOUT –VBEQ1. How- ever, diode D2 level-shifts the base of Q1 about 0.6 V below ground thereby cancelling the VBE of Q1. The addition of D2 also reduces the circuit’s output voltage sensitivity to tempera- ture, which otherwise would be dominated by the –2 mV VBE contribution of Q1. The output voltage for this circuit is determined by the formula: VOUT = 1.25 V • R2 R1 Unlike the positive step-up converter, the negative-to-positive converter’s output voltage can be either higher or lower than the input voltage. ILIM VIN SW1 SW2 FB GND SET AO ADP1111 NC D1 1N5818 1 2 3 6 7 5 4 8 NC C2 + R1 10k Ω CL + POSITIVE OUTPUT R2 MJE210 RLIM NEGATIVE INPUT L1 D2 2N3906 Q1 Figure 23. ADP1111 Negative-to-Positive Converter LIMITING THE SWITCH CURRENT The ADP1111’s RLIM pin permits the switch current to be limited with a single resistor. This current limiting action occurs on a pulse by pulse basis. This feature allows the input voltage to vary over a wide range without saturating the inductor or exceeding the maximum switch rating. For example, a particular design may require peak switch current of 800 mA with a 2.0 V input. If VIN rises to 4 V, however, the switch current will exceed 1.6 A. The ADP1111 limits switch current to 1.5 A and thereby protects the switch, but the output ripple will increase. Selecting the proper resistor will limit the switch current to 800 mA, even if VIN increases. The relationship between RLIM and maximum switch current is shown in Figure 6. The ILIM feature is also valuable for controlling inductor current when the ADP1111 goes into continuous-conduction mode. Table I. Component Selection for Typical Converters Input Output Output Circuit Inductor Inductor Capacitor Voltage Voltage Current (mA) Figure Value Part No. Value Notes 2 to 3.1 5 90 mA 4 15 μH CD75-150K 33 μF* 2 to 3.1 5 10 mA 4 47 μH CTX50-1 10 μF 2 to 3.1 12 30 mA 4 15 μH CD75-150K 22 μF 2 to 3.1 12 10 mA 4 47 μH CTX50-1 10 μF 5 12 90 MA 4 33 μH CD75-330K 22 μF 512 30 mA 4 47 μH CTX50-1 15 μF 6.5 to 11 5 50 mA 5 15 μH47 μF** 12 to 20 5 300 mA 5 56 μH CTX50-4 47 μF** 20 to 30 5 300 mA 5 120 μH CTX100-4 47 μF** 5–5 7 mA 6 56 μH CTX50-4 47 μF 12 –5 250 mA 6 120 μH CTX100-4 100 μF** NOTES CD = Sumida. CTX = Coiltronics. **Add 47 Ω from ILIM to VIN. **Add 220 Ω from ILIM to VIN. REV. A |
Numéro de pièce similaire - ADP1111_09 |
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Description similaire - ADP1111_09 |
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