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RM4190D Datasheet(PDF) 11 Page - Raytheon Company |
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RM4190D Datasheet(HTML) 11 Page - Raytheon Company |
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11 / 25 page  RC4190 PRODUCT SPECIFICATION 11 Figure 13. Low Battery Detector When the battery voltage drops below this threshold Q2 will turn on and sink over 1500 µA typically. The low battery detector circuitry may also be used for other, less conven- tional applications (see Figures 19 and 20). Automatic Shutdown The bias control current for the reference is externally set by a resistor from the IC pin to the battery. This current can vary from 1.0 µA to 50 µA without affecting the operation of the IC. Interrupting this current will disable the entire circuit, causing the output voltage to go to 0V for step-down appli- cations, and reducing the supply current to less than 1.0 µA. Automatic shutdown of the RC4190 can be achieved using the circuit of Figure 14. Figure 14. Automatic Shutdown A resistor is placed from the IC pin to ground, creating a voltage divider. When the voltage at the IC pin is less than 1.2V, the RC4190 will begin to turn off. This scheme should only be used in limited temperature range applications since the “turn off” voltage at the IC pin has a temperature coeffi- cient of -4.0 mV/ °C. At 25°C, typically 250 nA is the mini- mum current required by the IC pin to sustain operation. A 5.0 µA voltage divider works well taking into account the sustaining current of 250 nA and a threshold voltage of 0.4V at turn off. As an example, if 3.0V is to be the turn off volt- age, then R9 = 1.1/4.75 µA and R1 = (3.0 – 1.1) 5.0 µA or about 240 k Ω and 390 kΩ respectively. The tempco at the top of the divider will be -4.0 mV (R1 + R9)/R9 or -10.5 mV/ °C, an acceptable number for many applications. +Vs R4 R5 1 C2 V 1.31V REF Q2 8 I LBD 65-1651 LBR LBD 65-2678 4190 +V I GND 3 6 5 R1 R9 V BAT S C Another method of automatic shutdown without temperature limitations is the use of a zener diode in series with the IC pin and set resistor. When the battery voltage falls below VZ + 1.2V the circuit will start to shut down. With this connec- tion and the low battery detector, the application can be designed to signal a display when the battery voltage has dropped to the first programmed level, then shut itself off as the battery reaches the zener threshold. The set current can also be turned off by forcing the IC pin to 0.2V or less using an external transistor or mechanical switch. An example of this is shown in Figure 15. In this circuit an external control voltage is used to determine the operating state of the RC4190. If the control voltage VC is a logic 1 at the input of the 4025 (CMOS Triple NOR Gate), the voltage at the IC pin will be less than 0.5V forcing the 4190 off (<0.1 µA ICC). Both the 2N3904 and 2N2907 will be off insuring long shelf for the battery since less than 1.0 µA is drawn by the circuit. When VC goes to a logic 0, 2.0 µA is forced into the IC pin through the 2.2 M Ω resistor and the NOR gate, and at the same time the 2N3904 and 2N2907 turn on, connecting the battery to the load. As long as VC remains low the circuit will regulate the output to 5.0V. This type of circuit is used to back up the main supply voltage when line interruptions occur, a particu- larly useful feature when using volatile memory systems. 9.0V Battery Life Extender Figure 16 shows a common application: a circuit to extend the lifetime of a 9.0V battery. The regulator remains in its quiescent state (drawing only 215 µA) until the battery volt- age decays below 7.5V, at which time it will start to switch and regulate the output at 7.0V until the battery falls below 2.2V. If this circuit operates at its typical efficiency of 80%, with an output current of 10 mA, at 5.0V battery voltage, then the average input current will be IIN = (VOUT x IL) ÷ (VBAT x ef) or (7.0V x 10 mA) ÷ (5.0V x 0.8 mA) = 17.5 mA. Bootstrapped Operation (Step-Up) In step-up applications, power to the RC4190 can be derived from the output voltage by connecting the +VS pin and the top of R1 to the output voltage (Figure 17). One requirement for this circuit is that the battery voltage must be greater than 3.0V when it is energized or else there will not be enough voltage at pin 5 to start up the IC. The big advantage of this circuit is the ability to operate down to a discharged battery voltage of 1.0V. |
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