Basic Knowledge of Voltage Regulator（1/4）
CMOS Linear Regulator Overview
The history of CMOS linear regulators is relatively new. They have developed with battery-powered portable electronics devices. Since CMOS processes have been used in large-scale integrated circuits like LSI and microprocessors, they have been miniaturized constantly. Taking full advantage of the miniaturization technology, CMOS linear regulators have become the power management ICs that are widely used in portable electronics products to realize low profile, low dropout, and low supply current.
How Are They Different from Bipolar Linear Regulators?
In general, a CMOS linear regulator offers lower supply current compare to a bipolar linear regulator. This is because bipolar process is current-driven, while CMOS process is voltage-driven. [See Figure 1]
[Figure 1] Current-Driven Device and Voltage-Driven Device
Current runs between the emitter and the collector when base current is on. Base current must be on to get output current.
Current runs between the source and drain when voltage is charged at the gate. Once electric charge is charged, current is not needed to turn on.
Linear regulators, which do not require clock operation, are especially suitable to attain low supply current because the operating current of the regulators can be nearly zero in the circuits other than analog operating circuits.
One example of bipolar linear regulators is 78 series, multipurpose 3-pin regulators. Since the input voltage range of the series is as high as 30V ~ 40V and the series can pull more than 1A of current, the series are used in various white goods and industrial equipment. Nevertheless, the series are not low dropout because the series’ output structure is NPN Darlington Output. Table 1 shows some main characteristics of the series.
|78Lxx||100mA||30V, 35V, 40V||6~6.5mA||1.7V@40mA|
Still, the number of process needed for bipolar linear regulators is about a half or two thirds of CMOS process, and therefore a bipolar linear regulator is more cost-effective than a CMOS regulator even if its die-size is larger. Thus, a bipolar linear regulator is better suited for large current or high voltage use. On the other hand, CMOS process’s miniaturization technologies are well developed and have advantages such as low voltage, low dropout, small size, and low power consumption.
Where and How Is CMOS Used?
CMOS linear regulators are widely used in battery-powered portable electronics devices because of their low dropout and low supply current characteristics. LDO (Low Dropout) regulators enable battery to be used up to the limit, and therefore the regulators are now essential power management ICs for the devices like mobile phones, digital cameras, and laptop PCs to have long battery life. Because LDO regulators feature to pull large current with small input-output voltage differential while minimizing heat losses, they can meet the wide range of current requirements of each device.
Some low supply current types of regulators use lower than 1µA of self-supply current. Because of this feature, those types of regulators can maintain supply current of the electronics devices and wireless applications like mobile phones as low as possible when these devices are in sleep mode. Since these regulators can also provide the benefit of the CMOS miniaturization technology, they offer a great potential to mobile electronics devices that require low profile and high precision.
Standard packages used for CMOS linear regulators are SOT-23 and SOT-89. Recently, ultra small packages like CSP (chip scale package) have also become available. Because the development of the power management ICs is led by the progress of mobile devices, they are typically sealed in surface-mount small packages. Picture 1 shows the representative packages.
[Picture 1] Examples of CMOS Regulator Packages
Features: What Can CMOS Do?
The premise of linear regulators as the power management ICs is that they are directly connected to a battery or an AC adapter, so you must pay attention to the maximum input voltage. The ICs design rules of CMOS processes vary depending on maximum input voltage, and maximum input voltage and microminiaturization technology are in an inverse relationship; they do not mutually act like “the greater serves for the lesser”. If you choose high input voltage, then the ICs size will be bigger and its performance diminishes, and if you choose small sized ICs then you need to be careful about maximum input voltage. There are various CMOS regulators with various maximum input voltages for different applications. You should choose the most appropriate ones by carefully examining the types of power source and desired performances of your device [See Table 2].
|Operating Voltage||Product Series||Package|
|1.5V ~ 6V||XC6218||○|
|2V ~ 10V||XC6201||○|
|2V ~ 20V||XC6202||○||○||○|
CMOS linear regulators can be categorized as low supply current, large current, high voltage, high-speed, LDO, and so on. There is no strict definition for these categories, but usually “low supply current” are the ones with the supply current of a few μA, “large current” are the ones that can pull 500mA or more, “high voltage” are the ones with the voltage of 15V to 20V or more, and “high-speed” are the ones with the ripple rejection rate of approximately 60dB@1kHz. “LDO” does not have an exact definition either. Originally it referred to the low dropout output of PNP output and P-ch MOSFET output, in comparison to the dropout of NPN emitter follower output and NPN Darlington output of a bipolar linear regulator. Figure 2 shows the types of output transistors. These days, the value of less than 2Ω@3.3V in on-resistance conversion is becoming one standard of definition.
[Figure 2] Output Driver Models
NPN Emitter Follower Output
Control circuit must be higher by 0.6V (base voltage) than the output pin, in order to flow base current. The control circuit is operated by input power source, so dropout voltage of 0.6V is needed.
NPN Darlington Output
1.2V or more dropout voltage is needed as the circuit consists of 2 emitter follower circuits. The circuit can output large current because the base current of load transistor can be amplified by the predriver.
PNP Transistor Output
PMOS Transistor Output
A transistor turns on when input voltage is lower than base voltage and/or gate voltage is applied. There is no limit on input power source voltage in relative to output pin voltage. The dropout voltage is small because the circuit operates if there is the base voltage or gate voltage, and input power voltage that can operate control circuit.
Other than the above types of regulators, there are regulators with an ON/OFF function by Chip Enable pin according to need, composite regulators with 2 or 3 channels, regulators with a build-in voltage detector, and more. Such wide variety is another feature of CMOS. This is attributed to the fact that CMOS process can easily scale up circuits and lower supply current because it can completely shut down specific blocks of ICs when circuits are turned off separately. Figure 3 shows the block diagram of XC6415 series, 2-channel output regulators. This product can turn on and off VR1 and VR2 independently.
[Figure 3] Block Diagram of 2-Channel Regulator (XC6415 Series)
Internal Circuit and Basic Structure
An internal circuit consists of a reference voltage source, an error amplifier, an output voltage preset resistor, and an output P-ch MOSFET transistor. Some circuits also have a constant current limiter, a foldback circuit and a thermal shutdown function for protection purpose. Since it is difficult to build bandgap reference circuits that are used for bipolar processes as a reference voltage source, usually the reference voltage sources used are unique to CMOS process. For this reason, the output voltage temperature characteristics tend to be slightly inferior compare to bipolar linear regulators.
Also, internal phase compensation and circuitries vary depending on the regulator types such as low supply current, high-speed, and low ESR capacitor compatible. For instance, while a low supply current regulator normally uses two amplifiers, a high-speed regulator sometimes contains three amplifiers. Figure 4 indicates the basic circuitry block diagram of the high-speed regulator.
By adding a buffer amplifier between a preamplifier and an output P-ch MOSFET transistor, the buffer amplifier can drive the load P-ch MOSFET transistor in higher speed despite the large gate capacity. The output voltage can be determined by the values of divided resistors, R1 and R2, and the current limit value is determined by the values of divided resistors, R3 and R4. Each value is precisely set by trimming. Many high-speed type regulators are compatible with low ESR type capacitors, such as ceramic capacitors, because they are mostly used for wireless applications and portable electronics devices and therefore downsizing is necessary.
[Figure 4] Basic Circuitry Block Diagram of High-Speed Type Regulator