BA3528AFP

Audio ICs Pre / power amplifier and motor governor for 3V headphone stereos BA3528AFP / BA3529AFP The BA3528AFP and AB3529AFP have been developed for headphone stereos. They run off a 3V power supply, and include dual pre- and power amplifiers, and a motor governor. The preamplifiers are direct-coupled, and the power amplifiers use a fixed-gain NF circuit. An on-chip VREF amplifier makes output coupling capacitors unnecessary, and the motor governor uses a bridge ratio system to minimize the external parts count and make reliable and compact designs possible. FApplications 3V portable stereo equipment FFeatures 1) All the functions required for headphone stereo units on a single chip. 2) Preamplifier includes a mute amplifier. 3) Direct-coupled preamplifier. 4) No output coupling capacitors required for the power amplifiers. 5) Power amplifiers do not require oscillation prevention measures. 6) Power amplifier gain allows use of noise reduction (BA3529AFP). FAbsolute maximum ratings (Ta = 25_C) FRecommended operating conditions (Ta = 25_C) 385 Audio ICs FBlock diagram BA3528AFP / BA3529AFP 386 Audio ICs BA3528AFP / BA3529AFP FElectrical characteristics (unless otherwise noted, Ta = 25_C, VCC = 3V, and f = 1kHz) BA3528AFP 387 Audio ICs BA3529AFP BA3528AFP / BA3529AFP 388 Audio ICs FMeasurement circuit BA3528AFP / BA3529AFP 389 Audio ICs FCircuit operation (1) Preamplifier In the preamplifier input stage the pin 22 bias is the input and the negative feedback virtual earth, and the bias for the input stage transistor is taken from pin 22 via the tape head to allow direct coupling. Connect a 1000pF capacitor in parallel with the tape head to prevent high-frequency interference (see Fig. 2). BA3528AFP / BA3529AFP (4) Power amplifier The power amplifier employs an NF circuit with fixed gain. G VC = 3 6dB (BA3528AFP) and G VC = 2 7dB (BA3529AFP). (2) Mute amplifier Preamplifier output muting can be switched on and off. The mute is off when the mute switch input (pin 1) is low or open, and on when the mute switch input is high (tied to VCC via a resistor), see Fig. 3. For the input stage, the pin 22 bias point is the input and the negative feedback virtual earth point, and the first stage transistor bias is taken from pin 22. The built-in VREF amplifier uses the same bias point as its input, and its output voltage is about the same as DC output voltage from the power amplifier. This becomes the virtual earth for the headphones (see Fig. 5). (3) Equalizer The preamplifier is based on an NAB120µs NF-type equalizer. It is possible to add a switching function for the equalizer using the mute amplifier. Switching of the equalizer constant is controlled by the voltage on pin 1 (low or high). Note, however, when this is done, preamplifier muting no longer operates (see Fig. 4). 390 Audio ICs (5) Motor controller circuit The motor controller circuit uses a resistance bridge to maintain uniform motor speed regardless of changes in supply voltage, ambient temperature and load torque. Speed control is performed by a comparator and a stable on-chip reference voltage (VREG = 1.23V). See Fig. 6. BA3528AFP / BA3529AFP FApplication examples 391 Audio ICs BA3528AFP / BA3529AFP 392 Audio ICs FAttached components (1) Preamplifier If the closed-loop voltage gain (GVC) of the preamplifier is below 30dB for a frequency of f = 1kHz, oscillation may occur. (2) Playback equalizer terminal (NAB) The playback equalizer characteristics are determined by the RC circuit connected between the output and NF pins. For the circuit in Fig. 9, with a closed-loop voltage gain of GVC at an input frequency of 1 kHz, the relationships between the values of the RC circuit components are as follows: C1 = R2 = 2 3180 10–6 R1)200)103 R1 10–GVC/20 BA3528AFP / BA3529AFP the voltage difference between the pre-output and pin 22 will generate switching noise (a “pop” sound) when the mute is switched on and off. To reduce the DC gain and reduce this switching noise, we recommend that you connect a resistor (R = 51 kΩ) as shown in Fig. 10. This resistor reduces the gain of the circuit in the bass region of the playback equalizer as shown in the graph in Fig. 11. By using different combinations of component values for R1 and C1, it is possible to compensate for this effect in the low-frequency region as shown in the graph in Fig. 12. The equalizer can be switched on and off using the mute amplifier. If equalization for metal tape is added, determine R3 as follows: R3 = 1.4 R1 (3) Pre-mute switching noise If you use the mute amplifier for pre-muting, 393 Audio ICs (4) Mute amplifier output To switch the mute amplifier on and off, switch the constant-current supply for the mute amplifier off and on by switching the voltage on pin 1 (Pre-mute SW) high or low. When the mute is switched on, the mute amplifier output goes open circuit and the output voltage is unstable resulting in the generation of an audible “pop” sound. To prevent this, bias pin 22 through the volume control as shown in Fig. 13. In applications that use a directly connected output coupling capacitor, connect a resister as shown in the circuit diagram in Fig. 14 to reduce the pre-mute switching noise described in (3) above. BA3528AFP / BA3529AFP (5) Preventing oscillation Connect a capacitor of approximately 1000pF between the preamplifier input and pin 22 to prevent oscillation, and as a countermeasure against strong electric fields. This capacitor can also be used for treble-region compensation. In this case, decide on a value for it based on the relationship with the impedance of the magnetic head (see Fig. 15). When countermeasures against strong electric fields for the power amplifiers are required, connect bypass capacitors between each input pin and pin 22, and connect choke coils in series with the output pins and the headphones. The component values should be about 330pF for the bypass capacitors, and the 10µH for the choke coils so that they do not effect the audible frequency range. Another effective measure is to connect a bypass capacitor of about 1000pF in parallel with the filter capacitor between pin 22 and ground (pin 21). Refer to the circuit diagram in Fig. 16. 394 Audio ICs (6) Motor speed setting To control the motor speed, the stable built-in reference voltage VREG is divided across R4 and R5, and this voltage is used as the speed control voltage. The balance conditions for the bridge circuit are as follows: Ea = R1 R2 R3 R2 R3 *Ra Ia R5 R4)R5 BA3528AFP / BA3529AFP R1, the amount of positive feedHowever, if Ra t 10 back increases, and the circuit will be unstable, so within the operating temperature range, always make Ra y 10 R1 (see Fig. 17). ) 1) VREG (however, Ir tt Ia) From this, the balance conditions for the load fluctuation zero are: Ra = 10 Ea = 11 R1 R5 R4)R5 VREG FOperation notes (1) Application circuits Provided the recommended circuit constants are used, the application circuits should function correctly. However, we recommend that you confirm the characteristics of the circuits in actual use. If you change the circuit constants, check both the static and transient characteristics of the circuit, and allow sufficient margin to accommodate variations between both ICs and external components. (2) Recommended supply voltage The values given in the electrical characteristics table are guaranteed only for Ta = 25_C, and VCC = 3V. However, as long as the IC is operated within the recommended operating temperature and supply voltage ranges, the general circuit functions are guaranteed to operate correctly, and there will not be significant changes in the electrical characteristics. (3) Power dissipation The internal power dissipation of the IC is depends strongly on the value of the load resistance and the supply voltage. For this reason, when designing sets for mass production, pay due consideration to the power dissipation characteristics of the IC with respect to ambient temperature and supply voltage (see Figs. 18 and 19). Note, that the maximum allowed power dissipation is 1.7W at 25_C, and this decreases by 13.6mW for each increase in temperature of 1_C over this. (4) PCB layout In certain cases, the external circuit wiring can induce oscillations in the IC or degrade circuit performance. To avoid this, design the PCB wiring in such a way as to keep external wiring as short as possible, and ensure that it does not have common impedance. 395 Audio ICs FElectrical characteristic curves BA3528AFP / BA3529AFP 396 Audio ICs BA3528AFP / BA3529AFP FApplication board patterns FApplication board component layout 397 Audio ICs FExternal dimensions (Units: mm) BA3528AFP / BA3529AFP 398