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Hello everyone, I am Rose. Today I will introduce the differences between LDO and DC to DC Converter to you. LDO stands for low dropout regulator, which is a low dropout linear regulator.A DC/DC converter is a voltage converter that converts an input voltage and efficiently outputs a fixed voltage.
LDO:
Low dropout linear regulators may only be used in step-down applications, which means the output voltage must be smaller than the input voltage, as the name implies.
Advantages: good stability, fast load response, small output ripple.
Disadvantages: Low efficiency, the voltage difference between input and output should not be too large, and the load should not be too large. Currently, the largest LDO is 5A, but there are many restrictions to ensure the output of 5A.
DC/DC:
The voltage is converted from DC to DC. Although LDO is technically a type of DC/DC, today's DC/DC multi-finger switching power supply has a variety of topological structures, such as buck, boost, and so on.
Advantages: High efficiency, wide input voltage range.
Disadvantages: The load response is worse than that of LDO, and the output ripple is larger than that of LDO.
So, what is the difference between DC/DC and LDO?
A control chip, a pole coil, a diode, a triode, and a capacitor are all common components of a DC/DC converter. The DC/DC converter is a voltage converter that, after converting the input voltage, effectively outputs a fixed value. Step-up DC/DC converters, step-down DC/DC converters, and buck-boost DC/DC converters are the three types of DC/DC converters.
Three types of controls can be used according to requirements:
The PWM control method is efficient and produces low output voltage ripple and noise.
Even when utilized for a long time, the PFM control type has the advantage of low power consumption, especially when the load is small.
The PWM /PFM conversion type uses PFM control for small loads and converts it to PWM control for large loads automatically.
Mobile phones, MP3 players, digital cameras, portable media players, and other products all employ DC-DC converters nowadays.
Brief description of DC-DC principle
In fact, discrete components such as inductors are required outside to convert the DC power supply to the alternating current power supply AC, which is commonly a self-excited oscillation circuit. It is then filtered by the integral at the output end before being returned to the DC power supply. The AC power source can be quickly raised and stepped down because it is created. Two conversions will always result in losses, which is why everyone is working so hard to figure out how to increase DC-DC efficiency.
Comparison
With high efficiency, high output current, low quiescent current, and inverting structures, DCtoDC incorporate boost (step-up), buck (step-down), Boost/buck (step-up/step-down), and inverting structures. Many people have benefited from the advancement of integration. The new DC-DC converter's peripheral circuit requires only an inductor and a filter capacitor, but the output ripple and switching noise of this type of power controller are big, and the cost is high.
The lowest cost, lowest noise, and lowest quiescent current are all significant advantages of LDO low dropout linear regulators. It also features a small number of peripheral components, usually only one or two bypass capacitors. 30V output noise, 60dBPSRR, 6A quiescent current, and 100mV dropout are all signs that the new LDO can attain.
A brief description of the principle of LDO
The internal pass transistors in linear regulators use P-channel FETs rather than the typical PNP transistors, which allows them to attain these features. Because P-channel FETs do not require base current driving, the device's power supply current is considerably decreased. In the arrangement using the PNP transistor, on the other hand, a large input-output voltage difference must be guaranteed to avoid the PNP transistor from entering a saturated state and lowering the output capability. The voltage difference of a P-channel FET is generally equal to the product of the output current and the on-resistance, and the voltage drop is very low due to the extremely low on-resistance.
When the system's input and output voltages are close, LDO is the best option for achieving high efficiency. As a result, LDOs are most commonly employed to convert Li-ion battery voltage to 3V voltage. LDOs can give long battery life in a low-noise configuration even if 10% of the battery's ultimate discharge energy is not utilized.
Whether the portable electronic equipment is powered by rectified AC mains (or an AC converter) or by a battery pack, the power supply voltage will fluctuate widely throughout the operation. When a single lithium-ion battery is fully charged, the voltage is 4.2V, and after discharge, the voltage is 2.3V, which fluctuates greatly.
The output voltage of different rectifiers is impacted not only by changes in the mains voltage but also by changes in the load. Almost all electronic gadgets are powered by a voltage stabilizer to ensure that the power supply voltage remains stable. Small and accurate electronic equipment also necessitates a highly clean power supply, free of ripples and noise, in order to avoid interfering with the equipment's normal operation. A linear regulator should be added to the input end of the power supply to provide a consistent power supply voltage and achieve active noise filtering in order to meet the requirements of precision electronic equipment.
Figure 1-1 depicts the basic circuit of a low dropout linear regulator (LDO). The circuit is made up of a series regulator VT (PNP transistor, Note: P-channel field effect transistors are often employed in real applications), sampling resistors R1 and R2, and the comparator amplifier A.
Figure. 1-1 Basic Circuit of Low Dropout Linear Regulator
The non-inverting input terminal of the comparator A receives the sampling voltage Uin, which is compared to the inverting input terminal's reference voltage Uref (Uout*R2/(R1+R2)). Uout =(U+-U-)*A when the difference between the two is amplified by the amplifier A The comparator amplifier, which controls the voltage drop of the series adjustment tube to stabilize the output voltage, is multiplied by A. When the output voltage Uout falls, the difference between the reference voltage Uref and the sampling voltage Uin rises, the comparator's drive current rises, and the series regulator's voltage drop falls, increasing the output voltage.
In contrast, if the output voltage Uout exceeds the required set value, the comparator amplifier's pre-drive current drops, lowering the output voltage. The output voltage correction is conducted constantly during the power supply process, and the adjustment time is only limited by the comparator amplifier's and output transistor loop's reaction speeds. The actual linear regulator should also include many other features, such as a load short-circuit protection, overvoltage shutdown, thermal shutdown, reverse connection protection, and so on, and the series pass transistor can be a MOSFET.
1. The output voltage
The output voltage of a low dropout linear regulator is the most crucial characteristic, and it's also the first thing that electronic equipment designers should think about when choosing a regulator. Fixed output voltage and adjustable output voltage are the two types of low-dropout linear regulators. Fixed output voltage regulators are more convenient to use, and the regulator has great precision since the output voltage is carefully controlled by the manufacturer. The established output voltage values, on the other hand, are all common voltage values that cannot fulfill all application requirements; however, changes in the value of external components will affect the stability accuracy.
2. Maximum output current
The voltage stabilizer's maximum current need is varies depending on the electrical equipment's power. The voltage stabilizer with a higher output current usually costs more. In a power supply system with several voltage stabilizers, the appropriate voltage stabilizer should be selected based on the current value required by each part to save money.
3. Input and output voltage difference
The difference between the input and output voltages is the most significant parameter of a low dropout linear regulator. The lower the voltage difference, the better the linear regulator's performance under the constraint of ensuring output voltage stability. A 5.0V low dropout linear regulator, for example, can maintain a stable output voltage of 5.0V as long as the input voltage is 5.5V.
4. Ground current
When the output current of the series regulator is zero, the grounding circuit IGND relates to the working current of the regulator given by the input power supply. This current is frequently referred to as quiescent current, however, this term is erroneous when a PNP transistor is used as a series regulator element, since the ground current of an ideal low-dropout regulator is typically quite tiny.
5. Load Regulation
Figure 2-1 and Equation 2-1 can be used to calculate the load regulation rate. The LDO's ability to reduce load interference increases as the load regulation rate of the LDO decreases.
Figure. 2-1 Output Voltage & Output Current
(2-1)
in the formula:
△Vload—load regulation rate;
Imax—LDO maximum output current;
Vt—the output voltage of the LDO when the output current is Imax ;
Vo—When the output current is 0.1mA, the output voltage of the LDO;
△V—The difference between the output voltages when the load current is 0.1mA and Imax respectively.
6. Linear adjustment rate
Figure 2-2 and Equation 2-2 can be used to get the linear adjustment rate. The lower the LDO's linear adjustment rate, the less influence the input voltage change has on the output voltage, and the better the LDO's performance.
Figure. 2-2 Output Voltage & Input Voltage
(2-2)
in the formula:
△Vline—LDO linear adjustment rate;
Vo—LDO nominal output voltage;
Vmax—LDO maximum input voltage;
△V—The difference between the maximum value and the minimum value of the LDO input Vo to Vmax output voltage.
7. power supply rejection ratio
The input source of the LDO often has many interference signals, and the PSRR reflects the LDO's ability to suppress these interference signals.
Figure 3-1 depicts a typical use of a low-dropout linear regulator. One of the most typical AC/DC power supplies is shown in Figure 3-1(a). The required voltage is converted from the AC power source voltage, which is rectified into a DC voltage. The low dropout linear regulator's job in this circuit is to stabilize the output voltage when the AC power supply voltage or load changes, suppress the ripple voltage and eliminate the power supply's AC noise.
The voltage at which various batteries work varies within a particular range. A low-dropout linear voltage regulator should normally be attached to the output end of the battery pack to provide a constant output voltage, as shown in Figure 3-1. (b).
Because the low-dropout linear regulator's power is low, it can help the battery last longer. Moreover, because the low-dropout linear regulator's output voltage is close to the input voltage, the output voltage can be guaranteed to be steady even when the battery is nearly depleted. As we all know, switching regulated power supplies have a high efficiency, but the output ripple voltage is high, the noise is high, and the voltage regulation rate and other performance are also bad, which will have a significant influence when the analog circuit is powered.
Active filtering can be achieved by connecting a low-dropout linear regulator to the output end of the switching regulator, as shown in Figure 3-1(c), and can also dramatically increase the voltage regulation precision of the output voltage while maintaining power system efficiency.
There is normally just one battery to supply power in some applications, such as radio communication equipment, but each section of the circuit utilizes distinct voltages that are isolated from each other, necessitating the usage of numerous voltage regulators. The low dropout linear regulator is supposed to work in the sleep state to save the power of the common battery while the device is not in use. As a result, an enable control terminal is required on the linear regulator. Figure 3-1(d) depicts a power supply system with numerous outputs and an on-off control feature that is powered by a single battery.
Figure. 3-1 Low Dropout Linear Regulator (LDO) Typical Application
DC-DC refers to the conversion of distinct DC power values from one to the other. It can be called a DCDC converter if it meets this definition, which includes LDO. The device that switches DC to (to) DC is known as DCDC, according to popular belief. Step-up, step-down, step-up/step-down, and inverse equalization circuits are all examples of DC-DC converters. High efficiency, high output current, and low quiescent current are all advantages of DC-DC converters. Many contemporary DC-DC converters require only a few external inductors and filter capacitors due to enhanced integration.
This form of power controller, on the other hand, has a huge output ripple and switching noise, as well as a high cost. Surface-mounted inductors, capacitors, and highly integrated power control chips have all become more common in recent years, thanks to advances in semiconductor technology. Costs are continuing to fall, while volumes are shrinking. An external high-power FET is not necessary because a MOSFET with a tiny on-resistance may output a huge amount of power. For example, utilizing the NFET on the device, an output of 5V/2A can be obtained for a 3V input voltage. Second, a low-cost compact package can be employed for low- and medium-power applications. Furthermore, increasing the switching frequency to 1MHz lowers the cost and allows for the use of smaller inductors and capacitors. Many additional features, including as soft start, current limit, and PFM or PWM mode selection, are available on some new devices.
In general, for boost, DCDC should be chosen, and for step-down, DCDC or LDO should be compared in terms of cost, efficiency, noise, and performance.
To begin with, in terms of efficiency, DC/DC is generally significantly more efficient than LDO, which is defined by its operating principle. Second, there are Boost, Buck, and Boost/Buck in DC/DC, Some people lump ChargePump into this category as well, despite the fact that LDO only has a step-down kind.
It is, once again, an extremely essential point. Because of the high switching frequency of DC/DC. its power supply noise is substantially higher than that of LDO. This PSRR parameter is worth paying attention to. As a result, when dealing with more sensitive analog circuits, it may be necessary to forgo efficiency in order to assure power supply purity and opt for LDO.
Inductors, diodes, huge capacitors, and some MOSFETs are typically required by LDO, whereas DC/DC typically requires inductors, diodes, large capacitors, and some MOS FETs, Because the Boost circuit must take into account the inductor's maximum working current, the diode's reverse recovery time, the ESR of the big capacitor, and other factors, the selection of peripheral devices is more involved than the LDO, and the area required is much larger.
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