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| Audio ICs LED level meter driver, 12-point, power scale, dot or bar display BA681A The BA681A is a monolithic IC for LED power meter applications. The display level range is 6mVrms to 359mVrms (Typ.) divided into 12 points with repeated interval spacings of 3dB, 3dB and 4dB. The 0dB level is 180mVrms (Typ.) and the range is *30dB to +16dB. The constant current outputs can be set using external components allowing use of different color LEDs in various combinations. FApplications Level meters for radio cassettes, car stereos, and home stereos FFeatures 1) 12-point power meter driver for bar / dot displays. 2) Fixed-current outputs that can directly drive LEDs. 3) Output current can be set using external resistors allowing different types of LEDs to be used in combination. 4) Built-in half-wave rectifier amplifier. 5) LED on and off timing can be set using an external capacitor and resistor. 6) With bar-type display, by connecting four LEDs in series, power dissipation is reduced. 7) Each group of three LEDs is a 10dB step which is ideal for power meter applications. FBlock diagram 645 Audio ICs FAbsolute maximum ratings (Ta = 25_C) BA681A FElectrical characteristics (unless otherwise noted, Ta = 25_C, VCC = 12V, and f = 1kHz) FComparator level vs. AC and DC input levels 646 Audio ICs FMeasurement circuit BA681A FApplication example Fig.2 FAttached components (1) LED current setting resistor (R1) This resistor sets the LED current value. Refer to Fig. 3 for the relationship between the value of this resistor and the current value. The recommended value is 27k for green LEDs, and 39k for red LEDs. If the LED current is set too high, the allowable power dissipation of the package may be exceeded, so exercise due caution. (2) Input coupling capacitor (C1) This capacitor connects the BA681A to external input circuits. The recommended value is 2.2F. (3) Input bias resistor (R2) This resistor is the input impedance. If the value is set too large, the DC bias voltage will increase, and the input offset will increase and have an effect on the comparators. The recommended value is 10k. 647 Audio ICs (4) Resistor and capacitor that set the LED operation level discharge time constant (R3 and C2) These components set the discharge time constant for LED operation level. The recommended values are R3 = 10k and C2 = 10F. (5) Input offset adjustment resistors (R4 and VR1) These resistors are used to adjust the input offset for the rectifier amplifier. The recommended values are R4 = 240k, and VR1 = 500. If the value of VR1 is too high, adjustment becomes difficult, and if it is too small, adjustment may not be possible. (6) Power supply capacitor (C3) This capacitor stabilizes the power supply line. The recommended value is 10F. This capacitor will have no effect if its value is 1F or less. BA681A FOperation notes (1) LED connection Connect the LEDs as indicated in Fig. 2. Note that the connection methods are different for bar and dot displays, and that in the case of bar display, pin 18 is open, while for dot display, pin 18 is connected to GND. When using different color LEDs that have different current values (e.g. green for the lower 10 LEDs and red for the top two) set the value of the current to the larger LED current using the external resistor, and adjust the current value for LEDs that require a smaller current by connecting resistors in parallel them. If you wish to remove LEDs, do so from the lowest level (except when the 12th LED is not used for display alignment). Be certain to short unused LEDs. (2) LED current adjustment The LED current is set using R1. The relationship between the value of R1 and the typical LED current is given in Fig. 3. (3) Comparator voltage In the case of the BA681A, the comparator voltage is the input voltage when about half LED current setting value (Typ.) is flowing in an LED. Therefore, if the LED current is set to 16mA, the comparator voltage is the value when 8mA is flowing in the LED. (4) 0dB and input offset adjustment First, adjust the input signal so that the 10th LED lights. This is the 0dB input signal level. Next, lower the input signal level to *27dB, and adjust the offset adjustment potentiometer VR1 so that the 2nd LED lights. Repeat the 0dB and *27dB adjustments alternately to accurately set the levels. The input level at which an LED lights is the comparator voltage. (5) The BA681A GND (pin 9) and the external component earths should connected together at one point. 648 Audio ICs (6) Application example (display) BA681A FTape deck application example 649 Audio ICs FExternal dimensions (Units: mm) BA681A 650 |
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