Audio ICs Fluorescent display tube level meter driver, 16-point 2 channel, VU scale, bar display BA6800AS The BA6800AS is a two-channel, 16-point fluorescent display tube driver for VU-scale bar-level meters. It uses a dynamic-drive system and has both AC and DC inputs. The AC input mode has a peak hold circuit. The IC features a power-on mute, and the output block can directly drive fluorescent display tubes, so few external components are required. The grid output duty cycle is 1 / 8. Applications Level meters for all types of AV equipment Features 1) Uses dynamic-drive system to display two 16-point channels. 30-pin SDIP package. 2) AC and DC inputs provided. Switching function allows two-mode display. 3) Upper 12 points have peak hold function in AC mode (two seconds). 4) Power-on mute function. 5) Dynamic-drive system reduces the power dissipation of the fluorescent display tube power supply. 6) Square root compression amplifier built in. Absolute maximum ratings (Ta = 25C) 797 Audio ICs Electrical characteristics (unless otherwise noted, Ta = 25C) Block diagram 798 BA6800AS Audio ICs BA6800AS Electrical characteristics (unless otherwise noted, Ta = 25C) 799 Audio ICs Measurement circuit Application example 800 BA6800AS Audio ICs Circuit operation (1) Input block The AC input pins are pins 1 and 3, and the DC input pins are pins 6 and 7. Pin 8 is used to switch between the AC and DC inputs. When the input to pin 8 is “H”, AC input is selected (pins 1 and 3). Therefore, by using pin 8 to switch between the AC and DC modes, the IC can do two jobs. For example, pins 1 and 3 can be used for audio signal input, and pins 6 and 7 can be used as the input for the signal meter output from a tuner (DC). The AC input impedance of pins 1 and 3 is a low 250Ω (typ.), so connect potentiometers (VR1 and VR2) in series with the inputs to adjust the sensitivity and ch1 and ch2 balance. (2) Peak hold circuit The BA6800AS features a peak hold circuit that temporarily holds peak signal levels in AC input mode. The peak hold function can be used with the upper 12 points (5 to 16). The peak hold time depends on the oscillator frequency. It is 2 sec. (typ.) for an oscillator frequency of 2kHz. DC mode does not have a peak hold function. (3) Grid output The pin 10 and 11 grid outputs are open-collector NPN transistors. The logic is active low (the fluorescent tube lights when the output is “L”), so connect two PNP transistors Q1 and Q2 as shown in the application example circuit to drive the fluorescent tubes (see Fig. 3). BA6800AS (4) Segment output block Pins 12 to 28 are the segment outputs. The output circuits are open-collector PNP transistors. When grid 1 is “L”, the ch1 level is output (pin 1 or 6), and when grid 2 is “L”, the ch2 level is output (pin 3 or 7). Refer to Fig. 4. (5) Grid and segment output timing chart The grid and segment output timing is shown in Fig. 5. (6) Attack and release times The response characteristic for AC input signals is set by resistor R1 and capacitor C3 for ch1 and resistor R2 and capacitor C4 for ch2 (pins 2 and 4). When R1 = 47kΩ and C3 = 22µF, the attack time is about 4ms, and the release time is about 1sec. (same for ch2). Attack time : Time for the voltage on pins 2 and 4 to rise from 1V to 2.5V when the input goes from no input to 2.6Vrms, then back to no input. Release time : Time for the voltage on pins 2 and 4 to fall from 2.5V to 1V when the input goes from 2.6Vrms to no input. (7) Oscillator frequency The resistor R26 and capacitor C5 connected to pin 30 determine the oscillator frequency. The oscillator frequency (fosc) and grid output period (T) are related as follows : T (ms) = 16 / fosc (kHz) 801 Audio ICs BA6800AS Timing chart (when oscillator frequency is 2kHz) Attached components (refer to “Circuit operation”) C1 and C2 : input coupling capacitors. VR1 and VR2 : AC sensitivity adjustment and balance adjustment (3kΩ recommended). C3, R1, C4 and R2 : set the response characteristics with respect to the AC input signal. In the example given, the attack time is about 4mS and the release time is about 1sec. R3 : Pullup resistor for the input switching terminal (pin 8). R6 and R7 : resistor for the grid leak current path (IGleak). R6 (R7) < 0.6V. Set so that IGleak R4 and R5 : base bias resistors for Q1 and Q2. Conditions for base bias current (IB) flow are VCC = 5V and VF = 0.6V : 5*0.6 < = 7.3 R6 0.6 the base current is given by the following formula. R4 Operation notes (1) Adjust the potentiometers VR1 and VR2 (connected to pins 1 and 3) to adjust the 0dB input level and the dispersion of ch1 and ch2. (2) The temperature characteristic for the lighting limit for the 16th LED is shown in Fig. 6. 802 IB (mA) 5*0.6 * 0.6 R6 (kΩ) R4 (kΩ) Set resistors R4 and R6 (R5 and R7) so that IB > Fluorescent tube grid current hFE Q1 and Q2 : grid output inverting transistors. Use transistors for which VCEO > VCC ) VEE. R8 to R25 : Resistors that reverse bias the segments and grid when the fluorescent tube is not lit. The application example given is for general cases. Select the resistors to suit the characteristics of the fluorescent tube used. C5 and R26 : set the oscillator frequency. Capacitor C5 should be a component with good temperature characteristics. (3) The external resistor R26 connected to the oscillator (pin 30) should be in the range of 20kΩ and 100kΩ. If it is outside this range, oscillation may stop due to the influence of temperature (see Fig. 7). Audio ICs BA6800AS Electrical characteristics curves External dimensions (Units: mm) 803