Logic Level Conversion in Smartphones

　　Figure 1: Logic level converter application diagram.

　　Therefore, there is an input/output voltage mismatch problem between different ICs and peripheral modules in portable products such as smartphones. To enable these devices and modules to communicate with each other, efficient logic voltage level conversion is required. The so-called logic level converter connects ICs and modules or printed circuit boards (PCBs) with different operating voltages to provide system integration solutions.

　　Traditional logic level conversion methods and their advantages and disadvantages

　　Table 1: Traditional logic level conversion methods and their advantages and disadvantages.

　　Since transistor-transistor logic (TTL) and complementary metal oxide semiconductor (CMOS) are standard levels in logic circuits, TTL-CMOS input conversion is very common in traditional logic level conversion methods. This conversion method is simple and low-cost, and is mainly used for low-level to high-level conversion, and can also be used to convert high-level to low-level. This conversion method also has some disadvantages. Other traditional logic level conversion methods include overvoltage tolerance (OVT) voltage conversion, open drain (OD)/active pull-down conversion, and discrete I2C conversion, each with its own advantages and disadvantages, see Table 1.

　　Dual-power supply logic level conversion and its application

　　Power is consumed in logic level conversion. For example, in low-to-high level conversion, in order to output a high logic level, the input voltage (Vin) is lower than VCC, and the power supply current change (ΔICC) is always high, so the power consumption is also high. To solve the problem of high power consumption, a dual power supply voltage (VCCA and VCCB) logic level converter can be used. When the logic power supply voltage (VL) is equal to Vin, ΔICC is 0, thereby reducing power consumption.

　　Common dual-supply logic level conversions include unidirectional conversion, bidirectional conversion with direction control pins, automatic sensing bidirectional conversion (push-pull output), and automatic sensing bidirectional conversion for open-drain applications (such as I2C). The structural diagram is shown in Figure 2.

　　Figure 2: Schematic diagram of several dual-supply logic-level converters.

　　Among these dual-power supply logic level conversion methods, the principle of unidirectional logic level conversion is to provide conversion from point A to point B when the output enable (Output Enable) is low; when the output enable is high, A and B are in a high impedance state (Hi-Z), which is usually treated as infinite resistance, equivalent to no connection. Common dual-power supply unidirectional logic level converters include ON Semiconductor's NLSV1T34AMX1TCG, NLSV2T244MUTAG, NLSV4T3234FCT1G, NLSV8T244MUTAG, NLSV22T244MUTAG, etc. The applications of these dual-power supply unidirectional logic level converters include general-purpose input and output (GPIO) ports, serial peripheral interface (SPI) ports, and universal serial bus (USB) ports.

　　The working principle of the bidirectional logic level converter with direction control pin is: when the pin and the direction control (DIRection, T/) pin are both low, it provides point B to point A conversion; when the pin is low and the T/ pin is high, it provides point A to point B conversion; and when the pin is high, the direction from point A to point B and the direction from point B to point A are both in high impedance state, which is equivalent to not connected. ON Semiconductor is about to launch a bidirectional logic level converter with direction control pin. Common applications of this type of converter are memory and I/O devices that are accessed by byte.

　　自动感测双向逻辑电平转换器（推挽型输出）的工作原理是：启用（EN）引脚为低电平时，转换器处于待机状态；EN引脚为高电平、I/O电平不变时，转换器处于稳态；EN引脚为高电平、I/O电平变化时，转换器检测到变化，并产生脉冲，I/O藉P沟道MOSFET（PMOS）上拉至更快。典型的自动感测方向双向逻辑电平转换器（推挽型输出）有如安森美半导体的NLSX3012MUTAG、NLSX3013FCT1G、NLSX3013BFCT1G、NLSX4014MUTAG和NLSX3018MUTAG等。这类转换器的常见应用包括通用异步收发器（UART）、USB端口、4线SPI端口和3线SPI端口等。

　　The auto-sensing bidirectional logic level converter for open-drain applications (such as I2C) also contains three states: when the EN pin is high and the NMOS is turned on, it is in the working state, and the input I/O level is pulled down to ground, that is, the input low level; when the EN pin is high and the NMOS is in high impedance state, it is in the working state, and the output I/O level is pulled up to VCC, that is, the input high level; when the EN pin is low, the converter is in standby state. Typical auto-sensing bidirectional logic level converters for open-drain applications (such as I2C) include NLSX4373MUTAG, NLSX4348FCT1G and NSLX4378BFCT1G from ON Semiconductor. Common applications of this type of converter include I2C bus, Subscriber Identity Module (SIM) card, Single Wire (1-Wire) bus, display module, Secure Digital Input Output (SDIO) card, etc.

　　Of the above dual-supply logic level converters, auto-sensing converters without direction control pins and converters with direction control pins each have their own advantages and disadvantages. The advantage of auto-sensing converters is that they minimize the I/O lines of the microcontroller and are a simple solution for asynchronous communication. The disadvantage is that they are more expensive and have lower bandwidth than converters with direction control pins. The advantage of converters with direction control pins is that they are commodity components, low cost, and a simple solution for memory-mapped I/O. The disadvantage is that the number of microcontroller pins is large.

　　In the auto-sensing converters without direction control pins, there is also a difference between integrated solutions (such as NLSX3373) and discrete solutions (such as NTZD3154N). The integrated solution NLSX3373 is a single IC, and it is estimated that the printed circuit board (PCB) space occupied is only 2.6 mm2; the discrete solution NTZD3154N uses dual MOSFETs and 4 resistors in 01005 package (ie 0402), and the total PCB space occupied is estimated to be 3.3 mm2. The integrated solution provides a low-power standby mode, while the discrete solution does not provide a high impedance/standby mode. The low-voltage operating characteristics, bandwidth and circuit characteristics of these two different solutions are also different.