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ADP1073 Datasheet(PDF) 10 Page - Analog Devices |
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ADP1073 Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADP1073 –10– REV. 0 VOUT = 212 mV × 1+ R1 R2 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| and D1 must be a Schottky diode to prevent excessive power dissipation in the ADP1073. 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 16, except that the cur- rent through feedback resistor R1 is level-shifted below ground by a PNP transistor. The voltage across R1 is (VOUT – VBE(Q1)). 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 tempera- ture, which would otherwise be dominated by the –2 mV/ °C V BE contribution of Q1. The output voltage for this circuit is deter- mined by the formula: VOUT = 212 mV × 1+ 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 ILIM VIN SW1 FB SW2 GND ADP1073 L1 D2 1N4148 C2 RLIM R1 CL Q1 2N3906 AO SET NC NC R2 10k POSITIVE OUTPUT NEGATIVE INPUT Figure 18. A Negative-to-Positive Converter Limiting the Switch Current The ADP1073’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 ADP1073 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 4. The ILIM feature is also valuable for controlling inductor current when the ADP1073 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 ADP1073’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 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 main- tained 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) The internal structure of the ILIM circuit is shown in Figure 21. Q1 is the ADP1073’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 con- nected directly to VIN), the maximum switch current will be 1.5 A. Figure 4 gives RLIM values for lower current-limit values. 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 ADP1073 will still provide a properly regulated output voltage. |
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