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ICL7673CBAZA Datasheet(PDF) 4 Page - Intersil Corporation |
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ICL7673CBAZA Datasheet(HTML) 4 Page - Intersil Corporation |
4 / 7 page 4 FN3183.4 July 22, 2005 Detailed Description As shown in the Functional Diagram, the ICL7673 includes a comparator which senses the input voltages VP and VS. The output of the comparator drives the first inverter and the open-drain N-Channel transistor PBAR. The first inverter drives a large P-Channel switch, P1, a second inverter, and another open-drain N-Channel transistor, SBAR. The second inverter drives another large P-Channel switch P2. The ICL7673, connected to a main and a backup power supply, will connect the supply of greater potential to its output. The circuit provides break-before-make switch action as it switches from main to backup power in the event of a main power supply failure. For proper operation, inputs VP and VS must not be allowed to float, and, the difference in the two supplies must be greater than 50mV. The leakage current through the reverse biased parasitic diode of switch P2 is very low. Output Voltage The output operating voltage range is 2.5V to 15V. The insertion loss between either input and the output is a function of load current, input voltage, and temperature. This is due to the P-Channels being operated in their triode region, and, the ON-resistance of the switches is a function of output voltage VO. The ON-resistance of the P-Channels have positive temperature coefficients, and therefore as temperature increases the insertion loss also increases. At low load currents the output voltage is nearly equal to the greater of the two inputs. The maximum voltage drop across switch P1 or P2 is 0.5V, since above this voltage the body- drain parasitic diode will become forward biased. Complete switching of the inputs and open-drain outputs typically occurs in 50 µs. Input Voltage The input operating voltage range for VP or VS is 2.5V to 15V. The input supply voltage (VP or VS) slew rate should be limited to 2V per microsecond to avoid potential harm to the circuit. In line-operated systems, the rate-of-rise (or fall) of the supply is a function of power supply design. For battery applications it may be necessary to use a capacitor between the input and ground pins to limit the rate-of-rise of the supply voltage. A low-impedance capacitor such as a 0.047 µF disc ceramic can be used to reduce the rate-of-rise. Status Indicator Outputs The N-Channel open drain output transistors can be used to indicate which supply is connected, or can be used to drive external PNP transistors to increase the power switching capability of the circuit. When using external PNP power transistors, the output current is limited by the beta and thermal characteristics of the power transistors. The application section details the use of external PNP transistors. Applications A typical discrete battery backup circuit is illustrated in Figure 6. This approach requires several components, substantial printed circuit board space, and high labor cost. It also consumes a fairly high quiescent current. The ICL7673 battery backup circuit, illustrated in Figure 7, will often replace such discrete designs and offer much better performance, higher reliability, and lower system manufacturing cost. A trickle charge system could be implemented with an additional resistor and diode as shown in Figure 8. A complete low power AC to regulated DC system can be implemented using the ICL7673 and ICL7663S micropower voltage regulator as shown in Figure 9. INPUT VP (V) 02 4 5 6 8 10 12 1mA 100mA 10nA 1nA 1000pA 10pA 1pA ILOAD = 10mA VS = 0V 85°C 25°C FIGURE 5. IS LEAKAGE CURRENT VP TO VS AS A FUNCTION OF INPUT VOLTAGE +5V PRIMARY DC POWER GND NiCAD BATTERY STACK VO +5V OR +3V STATUS INDICATOR FIGURE 6. DISCRETE BATTERY BACKUP CIRCUIT ICL7673 |
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