Ldo and dcdc difference, principle and application

LDO: LOW DROPOUT VOLTAGE LDO (short for low dropout voltage regulator, rectifier)

Low-dropout linear regulators, as the name suggests, are linear regulators that can only be used in buck applications. That is, the output voltage must be less than the input voltage.

Advantages: good stability and fast load response. The output ripple is small.

Disadvantages: low efficiency, the input and output voltage difference can not be too large. The load can't be too big, the current maximum LDO is 5A (but there are still many restrictions to ensure the output of 5A)

DC/DC: DC voltage to DC voltage. Strictly speaking, LDO is also a kind of DC/DC, but currently DC/DC multi-finger switching power supply.

Has a variety of topological structures, such as BUCK, BOOST, and so on.

Advantages: high efficiency and wide input voltage range.

Disadvantages: The load response is worse than LDO, and the output ripple is larger than LDO.

What is the difference between DC/DC and LDO?

The DC/DC converter is generally composed of a control chip, a pole coil, a diode, a triode, and a capacitor. The DC/DC converter is a voltage converter that effectively outputs a fixed voltage after the input voltage is converted. There are three types of DC/DC converters: step-up DC/DC converters, step-down DC/DC converters, and buck-boost DC/DC converters. Three types of controls are available depending on the requirements. The PWM control type is highly efficient and has good output voltage ripple and noise. The PFM control type has the advantage of low power consumption even when used for a long time, especially at small loads. PFM control is implemented during PWM/PFM conversion type small load, and is automatically switched to PWM control at heavy load. At present, DC-DC converters are widely used in mobile phones, MP3 players, digital cameras, portable media players and other products.

DC-DC, (simplified principle)

In fact, the internal is to first convert the DC power supply to AC power AC. Usually it is a self-excited oscillating circuit, so there are discrete components such as inductors on the outside.

Then, through the integral filtering at the output, it returns to the DC power supply. Since the AC power is generated, it is easy to boost and step down. Two conversions will inevitably lead to loss, which is how everyone is trying to improve the efficiency of DC-DC.


1. DCtoDC includes boost (buck), buck (buck), Boost/buck (l/b) and inverting structure. It features high efficiency, high output current, low quiescent current, etc. Many peripheral circuits of new DC-DC converters only require inductors and filter capacitors; however, the output controllers of this type of power controller have large output ripple and switching noise and relatively high cost.

2. LDO: The outstanding advantages of low dropout linear regulators are the lowest cost, lowest noise and lowest quiescent current. It also has few peripheral components, usually with only one or two bypass capacitors. The new LDO achieves the following specifications: 30μV output noise, 60dBPSRR, 6μA quiescent current, and 100mV dropout.

LDO brief principle:

The main reason why linear regulators can achieve these characteristics is that the internal trim tube uses a P-channel FET instead of a PNP transistor in a typical linear regulator. The P-channel field effect transistor does not need the base current drive, so the power supply current of the device itself is greatly reduced. On the other hand, in the structure using the PNP tube, in order to prevent the PNP transistor from entering the saturation state and reducing the output capability, it is necessary to ensure The large input-output voltage difference; the P-channel FET voltage difference is roughly equal to the product of the output current and its on-resistance, and the small on-resistance makes the voltage difference very low. LDO is the best choice when the input voltage and output voltage are close in the system, achieving high efficiency. Therefore, LDOs are mostly used in applications that convert Lithium-ion Battery voltages to 3V. Although 10% of the final discharge energy of the battery is not used, LDOs still provide longer battery life in low-noise structures.

Whether the portable electronic equipment is powered by the AC mains via rectification (or AC adapter) or by the battery pack, the power supply voltage will vary within a wide range during operation. For example, when the single-cell lithium-ion battery is fully charged, the voltage is 4.2V, and the voltage after discharging is 2.3V, which varies greatly. The output voltage of various rectifiers is not only affected by changes in the mains voltage, but also by load changes. In order to ensure that the supply voltage is stable, almost all electronic devices are powered by a voltage regulator. Small precision electronic equipment also requires a very clean power supply (no ripple, no noise), so as not to affect the normal operation of electronic equipment. In order to meet the requirements of precision electronic equipment, a linear regulator should be added to the input of the power supply to ensure constant supply voltage and active noise filtering.

First, the basic principle of LDO

The basic circuit of the low dropout linear regulator (LDO) is shown in Figure 1-1. This circuit consists of a series adjustment tube VT (PNP transistor, Note: In practical applications, P-channel FET is commonly used here), sampling The resistors R1 and R2 and the comparison amplifier A are composed.

Figure 1-1 Basic circuit of low dropout linear regulator

The sampling voltage Uin is applied to the non-inverting input of the comparator A, compared with the reference voltage Uref (Uout*R2/(R1+R2)) applied to the inverting input terminal, and the difference between the two is amplified by the amplifier A. Uout= (U+-U-)*A Note A is a multiple of the comparison amplifier.) Controls the voltage drop across the series regulator to stabilize the output voltage.

When the output voltage Uout decreases, the difference between the reference voltage Uref and the sampling voltage Uin increases, the drive current of the comparison amplifier output increases, and the series regulator tube voltage drop decreases, thereby increasing the output voltage.

Conversely, if the output voltage Uout exceeds the desired set value, the pre-drive current of the comparator output is reduced, thereby reducing the output voltage. During the power supply process, the output voltage correction is continuously performed, and the adjustment time is limited only by the reaction speed of the comparison amplifier and the output transistor loop.

It should be noted that the actual linear regulator should also have many other functions, such as load short circuit protection, over voltage shutdown, thermal shutdown, reverse connection protection, etc., and the series regulator can also use MOSFET.

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