应用笔记 130

How Dual-Supply Analog Switches Can Operate with a Single 3V or 5V Supply


摘要 : By adding a voltage doubler and voltage inverter, a single 3V or 5V power supply can produce the voltages necessary to improve the performance on a dual voltage analog switch. With the higher power supplies and wider range, the on-resistance and timing performance are enhanced.

By adding a single component to a 3V-only or 5V-only board, you can operate conventional CMOS analog switches with performance approaching that specified with dual ±15V supplies. This simple modification provides fast switching, low on-resistance, CMOS/TTL compatibility, low-power consumption, and a signal range (±VCC) that exceeds the input supply range (VCC to ground).

Simply add a dual-output charge-pump voltage converter (IC1), which produces ±2VCC outputs from a single VCC input. These unregulated voltages ensure reliable switch operation for VCC levels as low as 2.5V.

Figure 1. The MAX865 dual-output charge pump (IC1) provides a local bipolar power supply for the MAX4659 CMOS analog switch (IC2).
Figure 1. The MAX865 dual-output charge pump (IC1) provides a local bipolar power supply for the MAX4659 CMOS analog switch (IC2).

A VCC supply voltage of 3V for the MAX865 charge pump produces ±6V rails for the switch (IC2). Using the MAX4659 CMOS analog switch results in a switch on-resistance of less than 30Ω, a transition time of less than 200ns, and low leakage current. Raising the VCC voltage to 5V generates ±10V rail supplies for the switch, reducing the on-resistance to less than 25Ω and transition times to less than 100ns. See the Typical Operating Characteristics in the MAX4659 data sheet for more information.

While the charge pump is capable of powering additional switches and/or low-power op amps, more than a few milliamps of load current degrades performance by lowering the unregulated supply rails.