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ADP1108 データシート(PDF) 10 Page - Analog Devices |
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ADP1108 データシート(HTML) 10 Page - Analog Devices |
10 / 12 page ADP1108 –10– REV. 0 Negative-to-Positive Conversion The circuit of Figure 20 converts a negative input voltage to a posi- tive output voltage. Operation of this circuit configuration is similar to the step-up topology of Figure 15, except that 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 would otherwise be dominated by the 2 mV/ C VBE contribution of Q1. The output voltage for this circuit is determined by the formula: VOUT =1.245V × 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. D1 1N5818 1 8 2 ILIM VIN SW1 FB SW2 GND ADP1108 L1 D2 1N4148 C2 RLIM R1 3 CL Q1 2N3906 6 AO SET 7 NC NC 5 4 R2 10k Ω POSITIVE OUTPUT NEGATIVE INPUT Figure 20. A Negative-to-Positive Converter Limiting the Switch Current The ADP1108’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 ex- ceed 1.6 A. The ADP1108 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 Figures 3 and 4. The ILIM feature is also valuable for controlling inductor current when the ADP1108 goes into continuous-conduction mode. This occurs in the step-up mode when the following condition is met: VOUT +VDIODE V IN –VSW < 1 1– DC where DC is the ADP1108’s duty cycle. When this relationship exists, the inductor current does not go all the way to zero dur- ing 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 21). This increases output ripple and can require a larger inductor and capacitor. By controlling switch current with the ILIM resis- tor, output ripple current can be maintained at the design val- ues. Figure 22 illustrates the action of the ILIM circuit. 0% 100 90 10 50µs 200mA VOUT = + 5V IL = 100mA L = 120µH RLIM = 0 VIN = 2.23V Figure 21. (ILIM Operation, RLIM = 0 Ω) 0% 100 90 10 50µs 200mA VOUT = + 5V IL = 100mA L = 120µH RLIM = 120 VIN = 2.23V Figure 22. (ILIM Operation, RLIM = 120 Ω) The internal structure of the ILIM circuit is shown in Figure 23. Q1 is the ADP1108’s internal power switch, which is paralleled by sense transistor Q2. The relative sizes of Q1 and Q2 are scaled so that IQ2 is 0.5% of IQ1. Current flows to Q2 through an internal 80 Ω resistor and through the R LIM resistor. These two resistors parallel the base-emitter junction of the oscillator- disable transistor, Q3. When the voltage across R1 and RLIM exceeds 0.6 V, Q3 turns on and terminates the output pulse. If only the 80 Ω internal resistor is used (i.e. the I LIM pin is connected directly to VIN), the maximum switch current will be 1.5 A. OSCILLATOR VIN SW2 SW1 RLIM (EXTERNAL) DRIVER ILIM Q1 ADP1108 Q2 Q3 R1 80 Ω (INTERNAL) Figure 23. ADP1108 Current Limit Operation The delay through the current limiting circuit is approximately 2 µs. If the switch ON time is reduced to less than 5 µs, accu- racy of the current trip-point is reduced. Attempting to program a switch ON time of 2 µs or less will produce spurious responses in the switch ON time. However, the ADP1108 will still provide a properly regulated output voltage. |
同様の部品番号 - ADP1108 |
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同様の説明 - ADP1108 |
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