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ADP1173AR-33 データシート(PDF) 10 Page - Analog Devices |
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ADP1173AR-33 データシート(HTML) 10 Page - Analog Devices |
10 / 16 page ADP1173 –10– REV. 0 until its FB pin is 1.245 V above its GND pin, so the output voltage is determined by the formula: –VOUT =1.245 V × 1+ R1 R2 ILIM VIN FB ADP1173 SW2 GND 5 1 2 8 SW1 3 R3 + C2 +VIN L1 D1 1N5818 + C1 –VOUT R2 R1 4 Figure 17. 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 16). Also, D1 must again be a Schottky diode to prevent excessive power dissipation in the ADP1173. NEGATIVE-TO-POSITIVE CONVERSION The circuit of Figure 18 converts a negative input voltage to a positive output voltage. Operation of this circuit configuration is similar to the step-up topology of Figure 14, except that the current through feedback resistor R1 is level-shifted below ground by a PNP transistor. The voltage across R1 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/ °C V BE contribution of Q1. The output voltage for this circuit is deter- mined by the formula: V OUT = 1.245 V × 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 ADP1173 L1 CL RLIM FB SW2 GND 4 5 1 2 3 8 SET 7 AO 6 + C2 NEGATIVE INPUT NC NC R1 1N5818 D1 R2 10k Ω Q1 2N3906 1N4148 D2 POSITIVE OUTPUT Figure 18. A Negative-to-Positive Converter LIMITING THE SWITCH CURRENT The ADP1173’s RLIM pin permits the switch current to be lim- ited 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 ex- ceeding 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 ADP1173 limits switch current to 1.5 A and thereby protects the switch, but increases the output ripple. 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 Figures 4 and 5. The ILIM feature is also valuable for controlling inductor current when the ADP1173 goes into continuous-conduction mode. This occurs in the step-up mode when the following condition is met: VOUT +V DIODE V IN –VSW < 1 1– DC where DC is the ADP1173’s duty cycle. When this relationship exists, the inductor current does not go all the way to zero dur- ing the time 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 in- ductor current will increase to a high level (see Figure 19). 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 20 illustrates the action of the ILIM circuit. Figure 19. (ILIM Operation, RLIM = 0 Ω) Figure 20. (ILIM Operation, RLIM = 240 Ω) |
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同様の説明 - ADP1173AR-33 |
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