Basic Knowledge of Voltage Regulator（2/4）
Basic Electrical Characteristics
Table 3 shows general electrical characteristics of a CMOS regulator.
|Input Voltage Range||1.4~6||2~28||1.6~5.5||1.6~6||0.5~3||1.7~6||V|
|Output Voltage Range||1.1~5.0||2.0~23||2.0~4.0||0.8~5||0.5~1.8||1.2~5||V|
|Output Voltage Accuracy||±1||±1||±1||±1||±20mV||±1||%|
|Max Output Current||150||150||300||1000||1000||2000||mA|
|Supply Current||0.6||5||100||8(PS mode)
|CE PIN||Yes||Yes / No||Yes||Yes||Yes||Yes|
Note: The values shown are typical values.
Ripple Rejection Rate
The basic performances of CMOS linear regulators include output voltage accuracy, supply current, line regulation, load regulation, dropout voltage, and output voltage temperature characteristics. Because these parameters are fundamental characteristics of series regulators, there is no major difference between CMOS regulators and bipolar linear regulators.
There are various types of CMOS linear regulators depending on applications, but they can be roughly subdivided into two categories according to their performance; regulators that feature low supply current and high-speed LDO regulators that focus on transient response characteristics. This categorization is based on the differences of the following capability to the changes of input voltage or output current, and therefore this feature is hard to be indicated by conventional DC characteristics. Hence, these days ripple rejection rate is included in the basic characteristics in order to indicate the basic performance of CMOS linear regulators.
The formula of Ripple Rejection Rate is as shown below.
Ripple Rejection Rate=20*Log (change in input voltage / change in output voltage)
Graph 5 indicates the ripple rejection rate of XC6223 series, high-speed regulators. Also, the actual waveform is shown in Graph 6. Using the input voltage with the peak-to-peak voltage of 1V and changing frequency, you can see the changes in the ripple value of output voltage. In Graph 1, the ripple rejection rate is -80dB when frequency is 1 kHz. So the output voltage will change approximately 0.1mV as input voltage changes 1V, and therefore we cannot identify major change in the oscilloscope in Figure 5. But when frequency is 100 kHz the ripple rejection rate will be 50dB, so the output ripple becomes a few mV and the changes become visible on the oscilloscope.
[Graph 5] Ripple Rejection Rate Characteristics (XC6223)
[Graph 6] Ripple Rejection Rate: Actual Input Voltage and Output Voltage Waveform (IOUT=30mA)
Another basic characteristic of linear regulators is dropout voltage, but most CMOS regulators are low dropout types that have very small dropout voltage. This trait derives from the fact that the regulators have been developed to achieve long-time use of battery life． Graph 7 shows the relationship between input voltage and output voltage. You can tell that the dropout voltage is remarkably small.
[Graph 7] The Relationship between Input Voltage and Output Voltage
(XC6209B302: Output Current=30mA)
“Input-output dropout voltage” literally means the voltage difference between input and output voltage, but it also suggests the amount of available current. The dropout voltage characteristics of XC6222x281 are shown in Graph 8 for your reference. For example, if you need the output current of 600mA using a regulator with the output voltage of 2.8V, then needed dropout voltage is 300mV, and therefore needed input voltage is 3.1V.
[Graph 8] Dropout Voltage vs. Output Current (XC6222x281)
Recent LDO has the improved drivability of a P-ch MOSFET driver, so you are likely able to get output current up to the current limit almost without dropout, if there is certain dropout voltage.
Transient Response Characteristics: Compliance Capability When Input Voltage or Load Current Changes Stepwise
These days burst mode is commonly used for the digital signal processing of electronics devices, and therefore the change of load current on LSI and memory is getting larger than ever. Hence, transient response characteristics that can be compliant with the changes are now an essential quality of regulators. Transient response characteristics can be categorized as line-transient response and load transient response, and these characteristics are completely depending on the supply current of a circuit. Let us focus attention on the error amplifier and gate capacitance value of the P-ch MOSFET load switching transistor shown in Figure 2, a basic internal circuit block diagram. A CMOS linear regulator contains a large P-ch MOSFET transistor as a load switch, and the response speed is determined almost entirely by the output impedance of the error amplifier to drive the MOSFET transistor and the MOSFET transistor gate capacitance value. The factor that determines the output impedance of the error amplifier is the supply current of a circuit; as supply current increases, the impedance becomes lower and response becomes higher speed.
As previously described, the drivability of a high-speed regulator is improved by adding a buffer. Because the buffer functions as an amplifier as well, there will be 3 sets of amplification: a preamplifier error amplifier (40dB), a buffer amplifier (20dB), and a load P-ch MOSFET transistor (20dB). They form a feedback circuit with the open-loop gain of 80dB or more sensitivity and can respond to the change of output voltage in sensitive and high-speed manner. Observing the actual waveform of the load transient response in Graph 9, the voltage recovery starts within a few micro seconds after the output voltage changes, which are caused by the change of the load current.
[Graph 9] Load Transient Response of High-Speed LDO (XC6209B302)
Graph 10 compares the load transient responses between XC6221 series and XC6219 series, a low supply current type and a high-speed type. The size of P-ch MOSFET transistors in both series is just about the same because both ICs are as small as to be assembled in SOT-25 package, but the waveform is apparently different.
[Graph 10] Load Transient Response Comparison
Generally speaking, P-silicon substrate can improve line-transient response time and ripple rejection rate. This is because P- silicon substrate is grounded to the VSS, and therefore circuits on the silicon substrate are less affected by input power source. Figure 5 shows the inverter circuit on P-silicon substrate. Reference voltage sources inside of ICs are often designed using this characteristic.
[Figure 5] Inverter Formed on P-Silicon Substrate
Recent LDO regulators response in extremely high-speed, and have good compliance capability to load transient response. Yet at the same time this high-speed response can make the power line unstable, and it may not only worsen the performance as a high-speed regulator but also can affect output of other linear regulators when there is impedance from connecters and wires at a power line. Wiring on PC board must be designed carefully in order to avoid impedance from the power line.
Output Noise Characteristics
White noise is one of the output noises, which occurs when thermal noise arising in an output-voltage preset resistor is amplified by an error amplifier. Thermal noise can get larger when impedance is high, so there is ultra high-speed, low noise CMOS regulators of which supply current is 70μA. The noise characteristic of the regulator is shown in Graph 11.
[Graph 11] Output Noise Density (XC6204B302)