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ADP1073 Datasheet(PDF) 11 Page - Analog Devices |
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ADP1073 Datasheet(HTML) 11 Page - Analog Devices |
11 / 16 page ADP1073 –11– REV. 0 OSCILLATOR VIN SW2 SW1 RLIM (EXTERNAL) DRIVER ILIM Q1 ADP1073 Q2 Q3 R1 80 (INTERNAL) Figure 21. Current Limit Operation Programming the Gain Block The gain block of the ADP1073 can be used as a low battery detector, error amplifier or linear post regulator. The gain block consists of an op amp with PNP inputs and an open-collector NPN output. The inverting input is internally connected to the ADP1073’s 212 mV reference, while the noninverting input is available at the SET pin. The NPN output transistor will sink about 100 µA. Figure 22a shows the gain block configured as a low-battery monitor. Resistors R1 and R2 should be set to high values to reduce quiescent current, but not so high that bias current in the SET input causes large errors. A value of 100 k Ω for R2 is a good compromise. The value for R1 is then calculated from the formula: R1 = V LOBATT − 212 mV 212 mV R2 where VLOBATT is the desired low battery trip point. Since the gain block output is an open-collector NPN, a pull-up resistor should be connected to the positive logic power supply. VIN +5V GND ADP1073 R1 AO SET R2 100k TO PROCESSOR 212mV REF VBAT R1 = R2 ( –1 ) VLB 212mV VLB = BATTERY TRIP POINT Figure 22a. Setting the Low Battery Detector Trip Point VIN +5V GND ADP1073 R1 AO SET R2 33k 47k TO PROCESSOR 212mV REF VBAT R3 1.6M Figure 22b. Adding Hysteresis to the Low Battery Detector The circuit of Figure 22a may produce multiple pulses when approaching the trip point, due to noise coupled into the SET input. To prevent multiple interrupts to the digital logic, hyster- esis can be added to the circuit (Figure 22b). Resistor R HYS, with a value of 1 M Ω to 10 MΩ, provides the hysteresis. The addi- tion of R HYS will change the trip point slightly, so the new value for R1 will be: R1 = V LOBATT – 212 mV 212 mV R2 – V L – 212 mV RL + RHYS where VL is the logic power supply voltage, RL is the pull-up resistor and RHYS creates the hysteresis. The gain block can also be used as a control element to reduce output ripple. The ADP3000 is normally recommended for low- ripple applications, but its minimum input voltage is 2 V. The gain-block technique using the ADP1073 can be useful for step- up converters operating down to 1 V. A step-up converter using this technique is shown in Figure 23. This configuration uses the gain block to sense the output volt- age and control the comparator. The result is that the compara- tor hysteresis is reduced by the open loop gain of the gain block. Output ripple can be reduced to only a few millivolts with this technique, versus a typical value of 150 mV for a +5 V converter using just the comparator. For best results, a large output capacitor (1000 µF or more) should be specified. This tech- nique can also be used for step-down or inverting applications, but the ADP3000 is usually a more appropriate choice. See the ADP3000 data sheet for further details. D1 ILIM VIN SW1 FB SW2 GND ADP1073 L1 R1 C1 AO R2 R3 680k SET VBAT VOUT VOUT = ( +1 ) (212mV) R1 R2 Figure 23. Using the Gain Block to Reduce Output Ripple |
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