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| Audio ICs Mute detector IC for 3V sets BA3708F The BA3708F is a mute detector IC for 3V tape players. When the signal is higher than the input decision level VIN and longer than the song detect time, a pulse of width TW is output after the pulse delay time TD which begins when the input signal ends. The output is an open collector which enables direct drive of the plunger. A mute function is included which makes it possible to stop detection. FApplications 3V tape players FFeatures 1) Low supply voltage (can be operated at 1.8V; recommended operating voltage is 2.0 to 5.0V). 2) Uses the song detection method to minimize incorrect detection caused by noise between songs. 3) The pulse delay time TD, pulse width TW, and song detect time TC can be set using external CR time constants. FAbsolute maximum ratings (Ta = 25_C) 4) Includes a mute circuit which makes it possible to stop song selection. 5) Built-in output transistor enables direct drive of the plunger. (IO Max. = 100mA) FRecommended operating conditions 738 Audio ICs FBlock diagram BA3708F FElectrical characteristics (unless otherwise noted, Ta = 25_C and VCC = 3V) 739 Audio ICs FMeasurement circuit Switch operations BA3708F FApplication example 740 Audio ICs FCircuit operation (1) Input amplifier The input amplifier is a PNP differential amplifier, and the input pin (Pin 3) must be directly connected to ground through a 10k or less bias resistor (RIN). If RIN is too large, an input offset may occur and operation will become unstable. The gain and frequency characteristics of the amplifier are determined by CIN and RIN connected to the input pin (Pin 3), and CNF and RNF connected to the NFB pin (Pin 4). (Figure 3) (2) Timing An output pulse of width TW is generated from the output pin (Pin 7) after the pulse delay time TD which begins when the input signal ends. The values of TD and TW are determined by the RC time constants of Pin 2 and Pin 1. TD (ms) 1.7 CD (F) RD (k) TW (ms) 1.6 CW (F) RW (k) BA3708F 741 Audio ICs (3) Song detect time TC and mute circuit To prevent incorrect detection due to noise between songs, a song detect function has been included. With this function, the plunger activate pulse is only output when the input signal is longer than the song detect time TC, and therefore a song. A pulse is not output for noise signals shorter than TC. The length of TC is set by the value of the capacitor CC connected to Pin 6. (See Fig. 18 for the relation between CC and TC.) BA3708F (4) Output circuit The output circuit is an open collector which is suitable for mechanical systems where the plunger is on during song selection. The pulse width TW is 200 ms and the duty cycle is 30%, and drive is possible up to an output current IOUT = 100mA. The output is OFF (high) while the mute function operates. A discharge diode must be added in parallel with the plunger solenoid. If Pin 6 is connected to ground, the mute circuit will operate and song detection will stop. In this case, the value of the input resistor connected to Pin 3 will be smaller (approximately 1k), and this will prevent the increase of crosstalk between the left and right channels. FElectrical characteristic curves 742 Audio ICs BA3708F 743 Audio ICs BA3708F 744 Audio ICs FApplication board pattern and component layout BA3708F FExternal dimensions (Units: mm) 745 |
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