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ADP1110AR-5 Fiches technique(PDF) 11 Page - Analog Devices |
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ADP1110AR-5 Fiches technique(HTML) 11 Page - Analog Devices |
11 / 16 page ADP1110 –11– REV. 0 POSITIVE-TO-NEGATIVE CONVERSION The ADP1110 can convert a positive input voltage to a negative output voltage as shown in Figure 23. 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 ADP1110’s internal power switch turns off, current flowing in the inductor forces the output (–VOUT) to a negative potential. The ADP1110 will continue to turn the switch on until its FB pin is 220 mV above its GND pin, so the output voltage is determined by the formula: VOUT = 220 mV • 1+ R1 R2 ILIM VIN SW1 SW2 FB GND SET AO ADP1110 NC NC L1 CL R1 R2 D1 1N5818 OUTPUT CINPUT RLIM INPUT 6 7 8 1 2 4 5 3 NEGATIVE OUTPUT Figure 23. 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 21.) Also, D1 must again be a Schottky diode to prevent excessive power dissipation in the ADP1110. NEGATIVE-TO-POSITIVE CONVERSION The circuit of Figure 24 converts a negative input voltage to a positive output voltage. Operation of this circuit configuration is similar to the step-up topology of Figure 19, except the current through feedback resistor R1 is level-shifted below ground by a PNP transistor. The voltage across R1 is VOUT – VBEQ1. However, 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 temperature, which other- wise would be dominated by the –2 mV VBE contribution of Q1. The output voltage for this circuit is determined by the formula: VOUT = 220 mV • R1 R2 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 ADP1110 NC NC L1 CL R2 D1 CINPUT RLIM 6 7 8 1 2 4 5 3 POSITIVE OUTPUT 10K NEGATIVE INPUT Q1 2N3906 D2 1N4148 R1 Figure 24. A Negative-to-Positive Converter LIMITING THE SWITCH CURRENT The ADP1110’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 ADP1110 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 ADP1110 goes into continuous-conduction mode. This occurs in the step-up mode when the following condition is met: VOUT +VD IODE V IN – V SW < 1 1– DC where DC is the ADP1110’s duty cycle. When this relationship exists, the inductor current does not go all the way to zero during the time that the switch is OFF. When the switch turns on for the next cycle, the inductor current begins to ramp up from the residual level. If the switch ON time remains constant, the inductor current will increase to a high level (see Figure 25). This increases output ripple and can require a larger inductor and capacitor. By controlling switch current with the ILIM resistor, output ripple current can be maintained at the design values. Figure 26 illustrates the action of the ILIM circuit. 10 0% 100 90 10µs 10mV 200mA/div. Figure 25. ILIM Operation—IL Characteristic 10 0% 100 90 10µs 10mV 200mA/div. Figure 26. ILIM Operation—IL Characteristic |
Numéro de pièce similaire - ADP1110AR-5 |
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Description similaire - ADP1110AR-5 |
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