Communication ICs Cross point mixer for telephones BU8241F / BU8241FS / BU8242F / BU8244F The BU8241F, BU8241FS, BU8242F, and BU8244F are ICs developed for use with cordless telephones, and are equipped with switching and mixing functions. In addition, these ICs are provided with an internal power save function which enables the circuit to be run on line current if the power supply fails, by connecting a switch between the circuit and a new handset, reducing line current consumption. A series of these ICs is also under development, based on the number of circuits. Applications Telephones, telephone answering machines, cordless telephones Features 1) Eight input signals are selected and mixed by a cross-point, and output to eight pins (BU8241F / BU8241FS). 2) A power save mode enables the circuit to run on line current if the power supply fails. 1. Line current consumption reduced (IDD2 = 580µA) 2. IN1 OUT1 and IN2 OUT2 only are on 266 3) Serial binary data input. 4) Low voltage and current consumption. Communication ICs BU8241F / BU8241FS / BU8242F / BU8244F Block diagram 267 Communication ICs FAbsolute maximum ratings (Ta = 25_C) FRecommended operating range (Ta = 25_C) 268 BU8241F / BU8241FS / BU8242F / BU8244F Communication ICs BU8241F / BU8241FS / BU8242F / BU8244F FElectrical characteristics (unless otherwise noted, Ta = 25_C, VDD = 5V) 269 Communication ICs Pin descriptions Input / output circuits 270 BU8241F / BU8241FS / BU8242F / BU8244F Communication ICs BU8241F / BU8241FS / BU8242F / BU8244F Circuit operation Sections (1) to (3) below describe operation using the BU8241F and BU8241FS as examples. (1) Analog signals pass through an input amplifier and are supplied to eight switches. The analog input signals are turned on and off by the switches and can thus be directed to any desired output amplifier. Using mixing resistors connected to the outputs of the switches, and feedback resistors, each output amplifier can output a signal that is a mix of up to eight input signals. (2) The switch states are set using a 64-bit shift register and data latch. The shift register reads the data at the rising edge of the shift clock, and stores it until the next data is read at the rising edge of the next shift clock. (3) The power save mode is set as long as the PS pin is kept HIGH. (See Table 2 for the power save setting logic.) In the power save mode, the following are fixed : IN1 OUT1 (S1 = ON) and IN2 OUT2 (S10 = ON). (The other 62 switches are off.) In addition, the line current is reduced. When the PS pin goes from HIGH to LOW, the IC state switches from the power save mode back to normal operation. At that point, the state of the switch remains in the power save mode (only S1 and S10 are on). (See Figure 3 for the timing waveform in the power save mode.) (4) With the BU8242F, of the 64 data bits, the following should be input at LOW level : D3, D4, D11, D12, D17 to D32, D35, D36, D43, D44, D51, D52, D59, and D60. After the 64 data bits have been read, supply of the rising edge of the clock is stopped, the LATCH pin is set to HIGH, and the switches are set. (See Figure 2 for the timing waveform.) To change a switch setting, new setting data and shift clocks must be supplied for 64 bits of data, and the LATCH pin set to HIGH. If the LATCH pin is set to HIGH before 64 shift clocks have been supplied, or after more than 64 have been supplied, the switches cannot be set properly. (See Table 1 for the logic of switch settings.) Table.1 Switch Sn stage (Note 6) (5) With the BU8244F, DIN is 16 bits. The ON path in the power save mode is the same for the BU8241F and BU8241FS, but the pertinent switches are S1 and S6. Table.2 Power save setting theory 271 Communication ICs Application example 272 BU8241F / BU8241FS / BU8242F / BU8244F Communication ICs BU8241F / BU8241FS / BU8242F / BU8244F Selecting attached components 1) Components related to power supply and bias C602: VDD bias capacitor; normally 10µF C603: VB decoupling capacitor; normally 10µF 2) Components related to input pins 3) Components related to output pins C613 to C620: DC cutoff capacitor. A value should be selected which does not attenuate voice signals in the high band (300 Hz ). R613 to R620: Output load resistors. This should normally be left open. If using R615 to R620, select values that result in a total input impedance of 10kΩ or greater on the output side. If the total impedance is less than 10kΩ, the load on the output amplifier of the cross-point mixer will be excessive, and signals may be lost. C605 to C612: DC cutoff capacitor. A value should be selected which does not attenuate voice signals in the high band (300 Hz ). R605 to R612: Resistor which sets the input level. This should normally be left open. The constants shown in Figure 8 are set to match the oscillator output impedance. 273 Communication ICs BU8241F / BU8241FS / BU8242F / BU8244F Operation notes (1) Turning on the power supply When the power supply is turned on, the shift register is in an indeterminate state. After turning on the power supply, switch settings should be entered by supplying shift clock pulses to shift 64 bits of data into the shift register, and then setting the LATCH pin to HIGH. (For the BU8244, send 16 clock pulses.) (2) Setting serial data If the number of bits shifted or clock pulses applied is less than or more than the number stated above, the switches will not be set properly. Never apply shift clock pulses when the LATCH pin is HIGH. Doing so will change the data latch contents. (3) Drive impedance The input impedance Zin is 80 ± 32kΩ. To suppress input gain and loss, we recommend using the drive impedance shown in the table below. If the level is to be attenuated at the input pin, make sure the level is not affected by the internal resistance of the IC, as shown in Figure 10. This can cause gain and / or loss. Electrical characteristic curves 274 (4) Gain and noise when mixing signals Increasing the number of signals mixed in a single output (increasing the number of switches connected to the same output amplifier that are on) causes a slight drop in gain. With the BU8241 and BU8241FS / BU8242F, increasing the number of signals mixed in a single output (increasing the number of switches connected to the same output amplifier that are on) causes a slight increase in the noise level in terms of logic. If noise is a problem, we recommend limiting the number of signals mixed in a single output. (5) Output load capacitance The capacitance connected to GND in relation to an output pin (the output load capacitance) should be kept to 50pF or lower. (BU8241F / BU8241FS / BU8242F) Communication ICs BU8241F / BU8241FS / BU8242F / BU8244F Electrical characteristic curves 275 Communication ICs External dimensions (Units: mm) 276 BU8241F / BU8241FS / BU8242F / BU8244F