Basics of LDO Voltage Regulators
Time：2022.10.19
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The first lecture CMOS linear regulator basics
CMOS linear regulators do not have a long history and were developed with the growth of the portable electronic instrumentation business.
The CMOS process is becoming more microfine every day as it is applied to large-scale integrated circuits such as LSIs and memories. By applying the microfabrication technology, CMOS linear regulators have achieved small size, low dropout voltage, and low current consumption, and are widely used as power supply ICs for portable electronic devices.
Difference between CMOS linear regulator and bipolar linear regulator
Generally, CMOS linear regulators consume less current than bipolar linear regulators. This is because the bipolar process is a current-driven element and the CMOS process is a voltage-driven element. (Figure 1)
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(Figure 1)Current-Driven Device and Voltage-Driven Device
In particular, for applications such as linear regulators, which do not need to operate at clock frequencies, the operating current of circuits other than analog circuits can be suppressed to zero, making them suitable for circuits with low consumption current requirements.
Among the bipolar linear regulators are the general-purpose 78 series three-terminal regulators. With an input voltage range of 30V to 40V and a high current of 1A or more, they are mostly used in white goods and production machinery. There is no low saturation because the output terminal is an NPN compound transistor output structure. Table 1 shows some of the main characteristics of the 78 series, which is a representative bipolar linear regulator.
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Table 1 The Major Characteristics of Multipurpose 78 series Regulators| Product Series | Maximum Output Current | Rated Input Voltage | Operating Current | Dropout Voltage |
| 78xx | 1A | 35V, 40V | 4~8mA | 2V@1A |
| 78Mxx | 500mA | 35V, 40V | 6~7mA | 2V@350mA |
| 78Nxx | 300mA | 35V, 40V | 5~6mA | 1.7V@200mA 2V@300mA |
| 78Lxx | 100mA | 30V, 35V, 40V | 6~6.5mA | 1.7V@ 40mA |
The bipolar linear regulator process has fewer production procedures, one-half to one-third of the production process of the CMOS process, and has the advantage of low cost even with large chip sizes. Therefore, it can be considered as the most suitable for high-current and high-voltage applications.
On the contrary, the CMOS process has progressed in microfabrication and is characterized by low voltage, constant saturation, small size, and low current consumption.
Where CMOS is used
CMOS linear regulators, in particular, are active in portable electronic devices that use batteries, making full use of their low dropout voltage and low consumption characteristics. In addition, regulators with very low saturation called LDO (Low Dropout Linear Regulator) are available.
The LDO is a power supply IC necessary for cell phones, digital cameras, and personal computers because it can extend the battery life by exhausting the battery. In addition, the LDO can provide large currents with a small input and output voltage difference, which is useful for expanding the range of currents required by instruments by suppressing thermal losses and flowing large currents.
The low-current consumption regulator can suppress its own consumption current to approximately 1 μA, and is capable of reducing the consumption current of electronic devices in standby mode and wireless devices such as cellular phones in the wait-for-signal state.
Since they utilize the microfabrication technology of the CMOS process, they are valuable for portable electronic devices that require small size and high voltage accuracy.
●Appearance
Standard products are packaged in SOT-23 or SOT-89 small packages.
Recently, ultra-miniaturized packages such as the USP have been introduced, and many of the power supply ICs developed for use in portable devices are small packages enclosed in surface-mounted packages. A representative package is shown in Photo 1.
Photo 1 Examples of CMOS Regulator Packages
●Characteristics and classification
Since linear regulators for power supplies require direct connection to a battery or AC adapter, attention must be paid to the voltage resistance of the input power supply. Especially in CMOS process, the design rules of ICs differ according to the input power supply voltage resistance requirements, and there is a contradiction in the relationship between input power supply voltage resistance requirements and microtechnology. If you choose a product with a high input power supply voltage withstand, the size of the IC will increase and the performance will decrease.
Since there are various types of input power supply voltage withstand ICs available for various applications of CMOS linear regulators, please consider the type of instrument to be used and the necessary performance for selection. (Table 2)
(Table 2) Product Categories by Operating Voltage (Three-terminal voltage regulators)| Operating Voltage | Product Series | Package | ||||
| USP-3 | SOT-23 | SOT-89 | SOT223 | TO252 | ||
| 1.5V ~ 6V | XC6218 | ○ | ||||
| 1.8V ~ 6V | XC6206 | ○ | ○ | |||
| 2V ~ 10V | XC6201 | ○ | ||||
| 2V ~ 20V | XC6202 | ○ | ○ | ○ | ||
| 2V~28V | XC6216 | ○ | ○ | ○ | ||
CMOS linear regulators are classified into low current consumption, high current, high voltage tolerant, high speed, LDO, and other types. There is no strict definition of each type. Low current consumption refers to products with a consumption current of approximately a few μA, high current refers to products with an output of approximately 500mA or more, high voltage refers to products with an output of 15V to 20V or more, and high speed and ripple rejection are mostly used for products with an output of 60dB@1kHz. LDO also does not have a strict definition, but originally refers to the low-saturation output capability of PNP output or P-channel MOS output for bipolar linear regulators such as NPN emitter amplifier output or NPN composite transistor output, which need to have input-output voltage difference. (Figure 2)
Recently, there is also a representation method that converts the on-resistance to roughly 2Ω@3.3V or less as a reference.
In addition, various types of products such as those with CE terminals to perform on/off as needed, compound regulators such as dual-channel or 3-channel, and built-in voltage detection are also notable features. Since these CMOS processes are applied to each component of a large-scale circuit or IC, when each component stops working, it is easy to make the whole component stop working and reduce the current consumption. Figure 3 shows the block timing diagram of each component of the XC6415 series with dual outputs. The VR1 and VR2 of this product can be turned on or stopped independently of each other.
Figure 3 Block Diagram of 2-Channel Regulator (XC6415 Series)
Internal circuit and basic structure
The internal circuit includes a reference voltage source, an error amplifier, a resistor for pre-adjusting the output voltage, and a P-channel MOS transistor for the output. In addition, a constant current limit or current limit circuit (foldback circuit) and an overheat shutdown function are provided as protection functions.
The IC's internal reference voltage source does not have the bandgap reference circuit used in the bipolar process, and most of the products are unique to the CMOS process, and the temperature characteristics of the output voltage are somewhat inferior to those of bipolar linear regulators.
In addition, low-current consumption types, high-speed types, and low ESR capacitor counterparts can change their internal phase compensation or circuit composition. The low-current type is usually composed of a 2-stage amplifier, while the high-speed type is explored in terms of using a 3-stage amplifier that satisfies both low-current consumption and high-speed transient response. Figure 4 shows the internal circuit diagram of the high-speed type.
A buffer amplifier is added between the primary amplifier and the P-channel MOS transistor for the output, and the gate capacity of the P-channel MOS transistor for the output can be driven at high speed. The output voltage is determined by R1 and R2, and the limit current value is determined by R3 and R4. The output voltage is determined by R1 and R2, and the limit current value is determined by R3 and R4. The accuracy is good because each can be fine-tuned. In particular, the high-speed type is used in radio instruments or portable electronic instruments and has a miniaturization requirement, and most of them can correspond to low ESR capacitors such as ceramic capacitors.
Most of them correspond to low ESR capacitors such as ceramic capacitors.
Figure 4 Basic Circuitry Block Diagram of High-Speed Type Regulator
Lecture 2: Important Characteristics of CMOS Linear Regulators
This is the second lecture in a series to explain the electrical characteristics of linear regulators. The basic characteristics such as output voltage accuracy, current consumption, input stability, load stability, input/output voltage difference, and output voltage temperature characteristics are listed here. These are the essential characteristics of the regulator series. For CMOS linear regulators, there are no items that can be ignored compared to bipolar linear regulators.
Basic Characteristics of Linear Conditioners
Table 3 shows the general electrical characteristics of CMOS regulators.
Table 3 General Characteristics of CMOS Regulators by Types|
Type |
Low Supply Current |
High Voltage |
Large Current |
Super High Speed |
High speed |
Ultra High speed |
Unit |
| XC6201 | XC6216 | XC6220 | XC6204/05 | XC6221 | XC6222 | ||
| Output Voltage Range | 1.3~6 | 2.0~23 | 0.8~5 | 0.9~6 | 0.8~5 | 0.8~5 | V |
|
Output Voltage Accuracy |
±2 | ±2 | ±1 | ±2 | ±2 | ±1 | % |
|
Output Voltage Temp.Stability |
±100 | ±100 | ±100 | ±100 | ±100 | ±100 | ppm/℃ |
| Max Output Current | 200 | 150 | 1000 | 300 | 200 | 700 | mA |
| Dropout Voltage@100mA | 0.16 | 1.3 | 0.02 | 0.2 | 0.08 | 0.04 | V |
| Supply Current | 2 | 5 | 8(PS mode) | 70 | 25 | 100 | μA |
| Standby Current | - | 0.1 | 0.01 | 0.01 | 0.1 | 0.01 | μA |
| Line Regulation | 0.2 | 0.3 | 0.2 | 0.2 | 0.2 | 0.1 | %/V |
| Input Voltage | 2~10 | 2~28 | 1.6~6 | 2~10 | 1.6~6 | 1.7~6 | V |
| Output Noise | - | 30 | μVrms | ||||
| PSRR@1 kHz | 30 | 30 | 50 | 70 | 70 | 65 | dB |
Ripple Suppression
CMOS linear regulators are classified by various applications, but by performance, they are broadly classified into regulators that emphasize low current consumption and high-speed LDOs that emphasize transient response, and these differences are difficult to express using only general DC characteristics because they vary with input voltage or output current. Therefore, recently, ripple rejection has been included to express the basic characteristics of CMOS linear regulators.
The ripple rejection ratio is explained by the following formula
Ripple rejection = 20 × Log (output voltage change/input voltage change)
Figure 5 shows the ripple rejection characteristics of the XC6223 Series high-speed regulator. In addition, the actual waveform is shown in Figure 6. The ripple characteristics of the output voltage can be read when the frequency is changed by using a sine wave to show the peak-to-peak value of the input voltage of 1V.
According to the ripple rejection rate of 78dB at the frequency of 1kHz in Fig. 5, the change of the output voltage is about 0.1mV corresponding to the change of the input voltage of 1V; although this result cannot be confirmed with the oscilloscope in Fig. 6, it can be confirmed from the oscilloscope that the ripple rejection rate is about 40dB at the frequency of 100kHz and the output ripple voltage is about several mV.
Input and output voltage difference
Next, we introduce the input-output voltage difference, which is one of the basic properties of linear regulators. In particular, CMOS linear regulators are basically LDO types with a very small input-output voltage difference. This is because the purpose is to characterize the battery exhaustion. Figure 7 shows the relationship between input voltage and output voltage.
The relationship between the input voltage and the output voltage is shown in Figure 7. It is clear that the input and output voltage difference is very small.
Figure 7 The Relationship between Input Voltage and Output Voltage (XC6209B302: Output Current=30mA)
In addition, the input-output voltage difference is literally the voltage difference between the input voltage and the output voltage, which means that [as long as there is a certain voltage difference between the input voltage and the output voltage, the corresponding current can be drawn]. For reference, Figure 8 shows an example of the input and output potential difference characteristics of the XC6209B302. For example, for a regulator with the output voltage set to 3V, in order to get 150mA of output current, there must be 300mV of input-output potential difference, which means an input voltage of 3.3V is required.
With recent LDOs, the drive performance of P-channel MOS drivers has been improved so that the output current can reach the current limit without any voltage drop, as long as there is a certain degree of input-output voltage difference.
Transient Response Characteristics
Transient response characteristics are the following characteristics when the input voltage or load current changes in a step shape.
With the adoption of trigger mode in digital signal processing in electronic instruments, the variation of load current in LSI or memory ICs has become greater. Therefore, the transient response characteristics of the regulator that can follow the changes are required to be continuously improved.
Transient response characteristics include input transient response characteristics and load transient response characteristics, and the transient response characteristics of a linear regulator are dependent on the current consumption of the circuit.
Here, we focus on the gate capacity of the error amplifier and the output P-channel MOS transistor as shown in the basic internal circuit block diagram (Figure 4 in the first lecture). The output impedance of the error amplifier and the gate capacity of the MOS transistor required to drive the P-channel MOS transistor basically determine the response speed. The output impedance of this error amplifier is determined by the current consumption in the circuit. The higher the current consumption, the lower the impedance and the faster the response.
A buffer is inserted in the high-speed product to increase the drive capability, and the buffer also acts as an amplifier, resulting in a three-stage amplification of primary (error amplifier: 40dB) + (buffer 20dB) + (P-channel MOS transistor for output: 20dB). Therefore, the high-speed type product
The feedback system with 80dB or more sensitivity is formed by the open loop gain. It responds to changes in output voltage and can respond at high speed.
In fact, if we look at the load transient response waveform of the high-speed type in Figure 9, we can see that the output voltage change caused by the change of load current starts to recover after a few μs.
Figure 9 Load Transient Response of High-Speed LDO (XC6209B302)
In addition, the load transient response is improving day by day. Comparing the load transient response of the high-speed XC6221 and the low-current XC6219 in Figure 10, it is clear that the XC6221 has a voltage drop of 60mV compared to the XC6219's voltage drop of roughly 110mV, an improvement of roughly 50%. Although the size of the respective P-channel MOS transistors remains the same for each model, the waveforms can be observed to be significantly different.
Figure 10 Ripple Rejection Rate: Actual Input Voltage and Output Voltage Waveform （IOUT=30mA）
There are two types of silicon chassis used in CMOS processes, P-type and N-type.
In general, P-type silicon boards have improved input transient response time or ripple rejection characteristics. This is because in the P-type silicon circuit board, the VSS of the silicon circuit board is grounded, and the circuit formed on the silicon circuit board has a structure that is less susceptible to power supply effects. Figure 11 shows the inverter circuit formed on a P-type silicon board. In particular, there are products that are less susceptible to external noise, such as the reference power supply inside the IC, that take advantage of this structural feature.
Nowadays, almost all models use P-type circuit boards.
Figure 11 Inverter Formed on P-Silicon Substrate
Although recent LDO products have a very high speed transient response and good follow-through of load transient changes, the power supply line will be disturbed by the impedance caused by the connection part of the connector in the power supply line, the coiling of the wiring, etc. because of the fast response. Not only will the performance of the high speed regulator not be fully utilized, but it will also affect the output of other linear regulators. In order not to cause impedance on the power supply line, special attention must be paid to the winding of the wiring on the PC board.
Output Noise Characteristics
The noise of the output voltage includes the white noise output by the error amplifier amplified by the thermal noise generated by the resistor inside the IC for pre-adjusting the output voltage.
The noise of the output voltage includes the white noise output by the error amplifier. Thermal noise is more likely to be generated when the impedance (for pre-adjusting the output voltage inside the IC) is high, so there is an ultra-high-speed/low-voltage amplifier that adjusts the consumption current (inside the IC) to 70 μA.
An ultra-high-speed/low-noise CMOS regulator with a consumption current of 70 μA is available. Figure 12 shows the noise characteristics.
Figure 12 Output Noise Density (XC6204B302)
Product Description XC6601 Series
Combined with DC/DC converters to improve efficiency
The XC6601 series is a combination of step-down DC/DC converters from the XC9235/36/37 series to improve efficiency and is best suited for high-speed regulators for portable instruments.
For example, when a 1.2V output voltage is obtained from a 3.6V input voltage as a core power supply for an LSI, a DC/DC converter is used to step it down to 1.5V with high efficiency, and then the 1.5V output is shunted and the XC6601 series, which can operate at low input and output voltage differences with low on-resistance, can output high current with high efficiency, which is 35% more efficient than direct 1.2V reduction with a high-speed regulator. The efficiency of the XC6601 series can be improved by 35% compared to the direct 1.2V reduction with the high-speed regulator.
The circuit structure is divided into VBIAS terminal (power supply voltage terminal) and VIN terminal, and the N-channel MOS transistor is used as the driver transistor to achieve low ON-resistance operation. (Figure)
The conventional high-speed regulator uses P-channel MOS transistors, which have a tendency to increase on-resistance when VIN is low. Comparing the graph with our previous high-speed regulator XC6210 series, we can confirm that the XC6601 series can reduce the on-resistance in the low voltage range by approximately 50% even when the output voltage is as low as 1.2V. (Figure ①) indicates that a circuit capable of providing VBIAS power can use a lower on-resistance.
In addition, the load transient response is good, and stable output can be obtained when the load changes due to trigger mode, etc. (Fig. 2).
The XC6601 series can be used in parallel with the DC/DC converter XC9235/36/37 series to achieve high efficiency operation, and is best suited for portable instrumentation products.
Lecture 3: The Protection Function of CMOS Linear Regulator
Have you understood the basic characteristics in the previous lecture? The electrical characteristics are the most important point of inspiration for understanding linear regulators and must be kept in mind. In general, linear regulators as power supply ICs have some protection functions. Here, we explain the main protection functions.
Limit current and overheat protection
Generally speaking, there is a stable current limiting circuit and a suppression circuit for overcurrent protection, and an overheat shutdown circuit for overheat protection.
Figure 13 shows an example of the operating characteristics of the stabilized current limiting and current suppression. When the output current is about to reach 250mA, the current limit circuit first operates to stabilize the current. From there, the current limit circuit is gradually reduced to the point where the suppression circuit begins to operate, reducing the output current as the voltage drops. The output current is finally reduced to about 25mA when the output terminal is short-circuited, and the heat loss of 4V × 25mA is suppressed to about 100mW when the input voltage is 4.0V, for example.
●CE/CL Discharge
The recent CMOS linear regulator comes with a function that automatically discharges the charge remaining in the output capacitor synchronized with the turn-on/stop of the regulator. This is a power management function that takes into account the battery efficiency of portable electronic devices, and discharges the residual charge on the capacitor at the same time as the power is cut off in each section, shortening the time to wait for the capacitor to discharge and making it easy to arrange the turn-on/stop sequence of each section. Figure 14 shows the output voltage waveform of the XC6221 series CE terminal with the high-speed discharge function when the circuit is stopped. It can be observed that the residual charge of the CL capacitor is discharged at high speed when the CE terminal voltage reaches 0V (low level signal input).
Introduction of XC6217 Series ~GO(Green Operation)
The XC6217 Series are CMOS linear regulators with GO (Green Operation) function. The GO function is a function that automatically switches the internal consumption current of the IC between HS (High Speed) mode and PS (Power Saving) mode in response to the output current. Under the conditions of XC6217 series, when the output current is more than 8mA, the consumption current reaches 25μA in the high-speed transient response state. When the output current is below 0.5mA, the consumption current is 4.5μA, and it automatically switches to the low consumption current state. The automatic switching of the consumption current corresponding to the output current can reduce the waste of consumption current while operating with high efficiency and high speed transient response. If the GO terminal is fixed at the high level, the operation can be fixed in HS (High Speed) mode. The XC6217B/D type is generally considered to have the function of high speed discharge of output capacitor, which is a convenient function for portable electronic devices such as cell phones that require detailed power management.
It is a very convenient function for portable electronic devices such as mobile phones that require detailed power management.
[Figure ②] Supply current in of Green Operation.
[Figure ③] Example of Load transient response using GO pin
[Figure ①] Operational Circuit of XC6217
As the last lecture of [Basics of LDO voltage regulator], we introduce the technology related to how to use linear regulator more conveniently.
●Drilling into the packaging process to increase the allowable loss
The thermal loss of a linear regulator is determined by the relationship between the input voltage and output voltage and output current.
Thermal loss (Pd) = (input voltage - output voltage) × output current
In fact, it is extremely important to improve the heat dissipation of the package when manufacturing the instrument. Among the packages that dissipate heat efficiently is the USP package. The metal chip on the back of the package, where the IC silicon is mounted, is exposed to dissipate heat from the chip to the PCB substrate. (Photo 2) Heat dissipation is determined by the metal area of the PCB substrate, and Figure 15 shows an example of the heat dissipation characteristics of the USP-6C package. For the USP-6C itself, there is an allowable loss of 120mW, and the loss is 1W for a thin copper sheet area of 400mm2 , and increasing the area of the thin copper sheet results in a larger allowable loss. Figure 16 shows the board used as the evaluation loss.
The board used to evaluate the losses is presented in Figure 16.
● Laser Trimming
The output voltage of the CMOS linear regulator is pre-set so that it is almost impossible to adjust the output voltage with an external resistor, and is compensated by the output voltage at 0.1V or 0.05V intervals. This is due to the use of laser trimming technology that makes it easy to set arbitrary high precision voltages. Since it is difficult to make a stable reference voltage source similar to the bipolar transistor bandgap reference voltage as shown in (Note 1) in CMOS process, changing the resistor for pre-setting the output voltage to laser trimming can set the irregularly scattered value of the internal reference voltage to an arbitrary voltage value, and is also a general way to ensure the accuracy of the output voltage. (Figure 17)
The output voltage accuracy is ±2% for general products and ±1% for high precision products. Depending on the product, the output voltage accuracy is specified for different operating temperature ranges.
(Note 1) Bandgap Reference Voltage
A circuit that uses the power band gap and resistance of a bipolar transistor to obtain a stable voltage with respect to temperature by using the opposite of the voltage temperature coefficient proportional to the absolute temperature.
Which voltage regulators are easy to purchase when buying in small quantities?
Since almost all output voltages cannot be adjusted externally, it is important to check the stock status when purchasing. In general, the output voltages that are easy to get are 5V, 3.3V, 3V, 2.8V, 2.5V, 1.8V, 1.2V.
Future development trends
The microfabrication of the CMOS process LSI is progressing every year, and mass production of the product called 90nm specification has been started. For power supply ICs such as linear regulators, the input power must be voltage tolerant. Although extreme microfabrication is not always considered beneficial, it is possible to use processes such as 0.35 μm or 0.5 μm to provide an output voltage of 1.2 V and a current of 1A with an input voltage of 1.5 V, taking advantage of the microfabrication technology available in CMOS. In addition, a variety of new technologies and advantages are being introduced using the CMOS process at the top of the range, such as voltage tolerance.
The CMOS process has accumulated a large amount of process technology for the development of LSI and memory. It is expected that CMOS regulators will be used in a wider range of applications than mobile devices.
