In the design of portable products, analog switches have been mainly used as audio signal switchers. Later, with the popularity of dual-card dual-mode mobile phones, analog switches have become an indispensable choice for dual-card switching; the latest digital TVs DVB, CMMB, etc. also require analog switches under certain conditions.
Low on-resistance is often the main selection factor for analog switches when switching audio. It is worth noting that the on-resistance is specified for specific VCC, VIN (voltage input) and a given load current. At the transistor level, RON is a function of device length (L), device width (W), electron and hole mobility, oxide layer capacitance, threshold voltage, and signal voltage.
RON will decrease as the supply voltage increases. Figure 1 shows the RON measured by Shanghai Yinglian UM4684 at different power supply voltages. On-resistance flatness (RFLAT) means that RON changes with VIN changing from 0V to VCC or V- to V +), which is the main factor causing THD.
Isolation is used to measure the "noise" between the conduction channel of the switch and the closed channel. It is measured at a specified frequency and is the coupling between the input and output of the closed channel. Crosstalk is the cross-coupling between an analog input channel and another channel. There are two forms: adjacent channels and non-adjacent channels. Both parameters are expressed in dB. In circuit design, we usually consider crosstalk and isolation indicators together with bandwidth. Although the input bandwidth of some analog switches is as high as several hundred megahertz, the bandwidth specification itself is not very meaningful. Because at high frequencies, the turn-off isolation and crosstalk indicators are significantly worse. For example, in the case of 1MHz, the typical value of the switch off isolation is 70dB, and the typical value of crosstalk is -85dB. Since these two indicators are reduced by 20dB / + frequency multiplication, at 100MHz, the turn-off isolation is reduced to 30dB, and the crosstalk is increased to -45dB. Therefore, it is not enough to consider the bandwidth alone, and it must be considered whether these two index drops can meet the application requirements under the required high-frequency operating conditions.
Figure 2 shows the significant difference in ICCT between the British Union UM3157 analog switch product using this improved technology and the common analog switch products of the same series.
In addition, British Union's low ICCT series analog switches have a lower VIH level value, and can use a lower control level to achieve high-level switching control of the switch. This feature of the product greatly reduces the complexity of the circuit designed by the system engineer, especially in portable product applications where the power supply and control signal levels do not match, the level conversion module can be eliminated, thereby effectively reducing costs. Taking an analog switch as an example of switching audio signals, as shown in FIG. 3, the 1.8V voltage transmitted from the ASIC can be used to directly control the control pins of the UM3157 without using a level shifter to achieve switch switching at the same time A very small leakage current is achieved. In addition, because the low ICCT analog switch does not consume much power (about 1uA), and the switch's on-resistance decreases with increasing operating voltage, designers can use a higher supply voltage to achieve a lower on-resistance. For typical audio switch or USB applications, low on-resistance is very important.
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