Audio ICs Stereo pre-amplifiers with mute detection circuits for car stereos BA3430S / BA3430F / BA3430FS The BA3430S, BA3430F, and BA3430FS are stereo pre-amplifiers with mute detection circuits which have been designed for use in car stereos. The analog circuitry including the pre-amplifier and mute detection and the digital circuitry such as control logic circuits have been combined together on a single chip. This has made it possible to reduce the interface with the microcontroller to five parallel inputs and outputs, and enables a very compact circuit configuration around the tape head. The BA3430S, BA3430F, or BA3430FS makes it possible to easily incorporate an ultra-compact, high-precision pre-amplifier board into the mechanical components of a car stereo. Absolute maximum ratings (Ta = 25C) Recommended operating conditions (Ta = 25C) 269 Audio ICs Block diagram 270 BA3430S / BA3430F / BA3430FS Audio ICs BA3430S / BA3430F / BA3430FS Pin descriptions 271 Audio ICs BA3430S / BA3430F / BA3430FS Electrical characteristics (unless otherwise noted, Ta = 25C and VCC = 9V, f = 1kHz, Rg = 680Ω, VO = 10dBV) 272 Audio ICs BA3430S / BA3430F / BA3430FS Measurement circuit Audio signal amplification circuit (1) Input switch circuit This circuit switches between head A and head B as needed for the auto reverse function. A 10kΩ or less resistor must be connected to the input pin to limit the input current. The electrolytic capacitor between the input pin and ground is for preventing switch noise. Although it is possible to prevent switch noise in the audio output without this capacitor by using the mute circuit, switch noise in the mute detection circuit will not be prevented and noise will be recognized as a song. Equivalent circuit schematic Truth table 273 Audio ICs (2) Equalizer amplifier and metal switch circuits The signal input from the magnetic head will be amplified based on the NAB characteristics for normal tape or metal tape. The equalizer characteristics are switched between normal and metal according to the logic of the metal switch pin. A 22kΩ or less resistor must be connected BA3430S / BA3430F / BA3430FS to the input pin to limit the input current. The capacitor between the input pin and ground is for preventing switch noise. It can be omitted without causing incorrect song detection. Input switch truth table Equivalent circuit schematic (3) Mute circuit This circuit mutes the output of the equalizer amplifier. A 22kΩ or less resistor must be connected to the input pin to limit the input current. The electrolytic capacitor between the input pin and ground is for preventing switch noise. It can be omitted without causing incorrect song detection, however, it is best to use it to ensure proper operation of the mute circuit. Input switch truth table 274 Equivalent circuit schematic Audio ICs BA3430S / BA3430F / BA3430FS Mute detection circuit (1) Mute detection based on noise and music signals The signals of each block in the mute detection circuit are shown in the diagram below. (2) Sensitivity switch amplifier circuit The noise level and signal frequency of the tape differ during playback and fast forward. Therefore, this signal amplifier includes a circuit for switching the detection frequency and gain for playback and fast forward. Input switch truth table (1) Sensitivity switch circuit This circuit switches the detection frequency and gain for playback and fast forward. A 10kΩ or less resistor must be connected to the input pin to limit the input current. The electrolytic capacitor between the input pin and ground is for preventing incorrect detection. Without this capacitor, sensitivity switch noise may cause incorrect detection of songs. 2) Input low pass filter High frequency noise increases during fast forward. This filter reduces high frequency noise. It consists of an internal 30kΩ resistor which combines the left and right equalizer output and an external capacitor connected to Pin 5. The cutoff frequency of the low pass filter fH is 1 fH = (Hz) 2 π C 30k / / 30k C = External capacitor connected to Pin 5 275 Audio ICs (3) Sensitivity switch amplifier Equivalent circuit schematic BA3430S / BA3430F / BA3430FS R2 is a resistor inside the IC which is 100Ω during playback and 360Ω during fast forward. The gain of the amplifier is G = 20 30k ) R1 ) R2 log R1 ) R2 (dB) The low cutoff frequency fL of the amplifier is fL = (4) Comparator circuit This circuit detects the presence or not of the signal and outputs a detection signal. The circuit consists of two comparators. The first comparator detects the level of the signal and the second comparator detects the duration of the signal. When a signal above VCC / 30 (VO-P) is input, the first comparator turns on and draws a constant current I from the time constant pin. This causes the voltage of the time constant pin to fall, and when it drops below VCC - 2.7(V) the next comparator turns on, changing the mute detection output pin to low. If the signal level drops below VCC / 30 (VO-P), the first comparator turns off and the voltage of the time constant pin rises due to the external discharge resistor. When this voltage rises above VCC - 2.0(V), the next comparator turns off, changing the mute detection output pin to high. The time ι1 from the arrival of the signal to the changeover of the mute detection output pin to low is given as follows: 5.4 C τ1= (S) I 276 1 2 π C (R1 ) R2) (Hz) This equation holds when the input signal is sufficiently large. See Fig. 20 for the small signal time τ1. The time τ2 from the ceasing of the signal to the changeover of the mute detection output pin to high is as follows: τ 2 = C R In 1/2 VCC)2.0 2.0 (S) I: Output current of first comparator = approx. 1mA C: External capacitor connected to time constant pin R: External resistor connected to time constant pin (this should be 22kΩ or higher) Equivalent circuit schematic Audio ICs BA3430S / BA3430F / BA3430FS 5) Gain settings for the mute detection circuit The gain settings for the mute detection circuit are shown below. The reference voltage of the comparator circuit is set at VCC / 30. As an example, when VCC = 9V and f = 5kHz, the mute detection level during playback is obtained as follows: a. The reference voltage of the comparator circuit is V = VCC / 30 = 0.3V, therefore, the circuit will operate at an AC input of 0.3 / 2Vrms = 13.5dBV or more. b. From the above chart, the gain at 5kHz during playback is 80dB. c. Therefore, the output / input level of mute detection is 13.5 80 = 93.5dBV. 277 Audio ICs Application example 278 BA3430S / BA3430F / BA3430FS Audio ICs Operation notes (1) Application circuit The application circuit is an example of a circuit we recommend, however, before using it further examination of characteristics and patent issues is necessary. If any of the values of the external connections are changed, be sure to take into consideration both the quiescent and transient characteristics of the IC and external components, and also deviations in the characteristics. (2) Resistor values for switch pins To prevent incorrect operation of the switch pins, use a 3k to 10kΩ resistor at Pins 20 and 23, and a 2k to 22kΩ resistor at Pins 21 and 22. (3) Capacitors for switch pins The capacitors connected to Pins 20 and 23 are for preventing incorrect operation of the mute detection circuit. Without these capacitors, the switching sound which occurs when the internal switches are switched will cause the mute detection output pin (Pin 24) to go low even when there is no signal, and this error will continue throughout the mute detection time. If a short mute detection time is set and the error can be tolerated, the capacitors can be omitted. The capacitors at Pins 21 and 22 are for preventing a “pop” sound in the audio signal. If the pop sound is muted at a latter stage, the capacitors can be omitted. Naturally, the mute detection circuit will still operate correctly if they are omitted. BA3430S / BA3430F / BA3430FS (4) Song detection output / input level The song detection output / input level can be set with the resistors connected to Pins 3 and 4. Small values for these resistors will create a large amplifier gain, lowering the song detection output / input level. The song detection output / input level in the application circuit is 93.5dBV (5kHz) during playback and 81dBV (10kHz) during fast forward. (5) Frequency characteristics of sensitivity switch amplifier The low cutoff frequency during playback can be set with the capacitor and resistor values of Pin 3. The low cutoff frequency during fast forward can be set with the capacitor and resistor values of Pin 4, and the high cutoff frequency can be set with the capacitor connected to Pin 5 and the internal resistor. (6) Song detection time The song detection time can be set with the capacitor connected to Pin 2. If the time is too short, short noise signals will also cause the circuit to operate. Set the time as long as possible. (7) Mute detection time The mute detection time is proportional to the product of the capacitor and resistor between the supply voltage and Pin 2. Taking into consideration the allowed IC current, select a resistor which is 22kΩ or more. 279 Audio ICs Thermal derating curves As the allowed power dissipation of the BA3430F / BA3430FS is small (BA3430F = 550mW, BA3430FS = 800mW), high temperature or high current may cause the allowed power dissipation to be exceeded. Fig. 8 shows the internal power dissipation vs. supply voltage (Pd vs. VCC), and Fig. 9 shows the allowed power dissipation vs. ambient temperature (PdMAX. vs. Ta). Allowed supply voltage vs. ambient temperature (VCCMAX. vs. Ta) can be obtained from these curves, and the result is shown in Fig. 10. Refer to Fig. 10 to set the supply voltage when designing the application. Electriclal charecteristics curves 280 BA3430S / BA3430F / BA3430FS Audio ICs BA3430S / BA3430F / BA3430FS 281 Audio ICs External dimensions (Units: mm) 282 BA3430S / BA3430F / BA3430FS