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ADP1108AN-12 데이터시트(PDF) 11 Page - Analog Devices |
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ADP1108AN-12 데이터시트(HTML) 11 Page - Analog Devices |
11 / 12 page ADP1108 –11– REV. 0 PROGRAMMING THE GAIN BLOCK The gain block of the ADP1108 can be used as a low battery de- tector, 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 ADP1108’s 1.245 V reference, while the noninverting input is available at the SET Pin. The NPN output transistor will sink about 300 µA. Figure 24a shows the gain block configured as a low-battery monitor. Resistors R1 and R2 should be set to high values to re- duce quiescent current, but not so high that bias current in the SET input causes large errors. A value of 33 k for R2 is a good compromise. The value for R1 is then calculated from the formula: R1 = V LOBATT – 1.245 V 1.245 V 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. 2 VIN +5V GND ADP1108 R1 AO SET 5 R2 33k Ω 47k Ω TO PROCESSOR 6 1.245V REF 7 VBAT VLB –1.245V 37.7µA VLB = BATTERY TRIP POINT R1 = Figure 24a. Setting the Low Battery Detector Trip Point The circuit of Figure 24a may produce multiple pulses when ap- proaching the trip point, due to noise coupled into the SET in- put. To prevent multiple interrupts to the digital logic, hysteresis can be added to the circuit (Figure 24b). Resistor R3, with a value of 1 M to 10 M , provides the hysteresis. The addition of R3 will change the trip point slightly, so the new value for R1 will be: R1 = V LOBATT – 1.245 V 1.245 V R2 – V L – 1.245 V R L + R3 where VL is the logic power supply voltage, RL is the pull-up resistor and R3 creates the hysteresis. 2 VIN +5V GND ADP1108 R1 AO SET 5 R2 33k Ω 47k Ω TO PROCESSOR 6 1.245V REF 7 VBAT R3 1.6M Ω Figure 24b. Adding Hysteresis to the Low Battery Detector L1* 100 H 5VOUT 200mA AT 6.5V 500mA AT 8V 100 Ω 0.22 Ω ZETEX ZTX749 *L1 = COILTRONICS CTX100-4 100 Ω 5 8 2 ILIM VIN SW1 SW2 GND ADP1108-5 D1 1N5818 C2 VIN 6.5V TO 20V 3 C1 220 Ω 1 4 SENSE Figure 25. 6.5 V to 5 V Step-Down Converter ILIM VIN SW1 ADP1108-5 L1* 300µH 220 Ω SENSE SW2 GND 4 5 1 2 3 8 *L1 = COILTRONICS CTX300-4 –5V OUTPUT 150mA + C2 VIN 6.5V TO 20V MBRS130T3 + 330µF Figure 26. Positive to –5 V Converter |
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