2018-04-19 13:33:16


What is the difference between DC/DC and LDO?


DC/DC converters are generally composed of control chips, pole coils, diodes, transistors, and capacitors. DC/DC converter is a voltage converter that effectively outputs a fixed voltage after switching input voltage. DC/DC converters are divided into three types: boost DC/DC converters, step-down DC/DC converters, and uplift DC/DC converters. Three types of control can be used according to requirements. PWM has high efficiency and good output voltage ripple and noise. The PFM control model has the advantages of low power consumption even when used for a long time, especially for small loads. PWM/PFM switching small load is implemented by PFM control and automatically converted to PWM control during heavy load. Currently, dc-dc converters are widely used in mobile phones, MP3 players, digital cameras, portable media players and other products.


LDO outline principle:


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The main reason that linear regulators can achieve these characteristics is that the internal adjustment tube adopts P channel field effect tube rather than the PNP transistor in normal linear regulators. The field effect tube of P channel does not need the base current drive, so the power current of the device itself is greatly reduced. On the other hand, in the structure of PNP tube, in order to prevent PNP transistor from entering the saturated state to reduce the output capacity, it is necessary to guarantee the large input and output pressure difference. The pressure difference of the P channel field effect tube is approximately equal to the product of the output current and its conduction resistance, and the differential pressure difference of the small conduction resistance is very low. LDO is the best choice when the input voltage and output voltage are close to the system, which can be very efficient. So the lithium ion battery voltage is converted into a "is chosen mostly 3 v voltage applications, although there is no use ten percent of the battery finally discharge energy, but they can still in the low noise structure provides a longer battery life.


Portable electronic devices either by ac mains power supply after rectification (or ac adapter), or by the battery power supply, work process, supply voltage will change in large scope. For example, the voltage of a single lithium ion battery is 4.2V, and the voltage after discharge is 2.3V, and the range is very large. The output voltage of all kinds of rectifier is not only affected by the change of the electric voltage, but also affected by the load change. To ensure that the power supply voltage is stable, almost all electronic devices are powered by a voltage regulator. Small precision electronic equipment also requires very clean power (no ripple, no noise), so as not to affect the normal work of electronic equipment. In order to meet the requirements of the precision electronic equipment, the linear regulator should be added to the input end of the power supply to ensure the constant power supply voltage and realize active noise filtering.


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LDO: is the abbreviation of lowdropoutvoltageregulator, rectifier)


Low pressure differential linear voltage stabilizer, hence the name, is a linear voltage regulator, can only be used in the application of reducing pressure. The output voltage must be less than the input voltage.


Advantages: good stability, quick load response. The output ripple is small.


Disadvantages: low efficiency, the input and output voltage difference cannot be too large. The load cannot be too large, the maximum LDO is 5A (but there are still many restrictions on the output of 5A).


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


There are many kinds of topology, such as BUCK, BOOST, etc.


Advantages: high efficiency, wide input voltage range.


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


I. basic principles of LDO.


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Low dropout linear regulators (they) of the basic circuit as shown in figure 1-1, the circuit is changed from series tube VT (PNP transistor, note: actual applications, the commonly used here is p-channel field-effect tube), sampling resistor R1 and R2, comparative amplifier composed of A.


Figure 1-1 basic circuit of low voltage differential linear regulator.


Uin on sampling voltage comparator of A phase input, and added to the reference voltage at the inverting input Uref (Uout * / R2 (R1 + R2)), compared the difference. After amplified by the amplifier A Uout = (U + - U -) * A note A multiples, to compare the amplifier) control series adjustment of the pressure drop, and stable output voltage.


When the output voltage Uout reduce Uref benchmark voltage and Uin sampling voltage differential value increases, more amplifier output drive current increase, series adjustment of pipe pressure drop is reduced, so that the output voltage increases.


In contrast, if the output voltage Uout exceeds the required set value, the output voltage is reduced by reducing the front drive current of the amplifier output. During the power supply process, the output voltage correction is continuously carried out, and the adjustment time is limited only by the comparison amplifier and output transistor circuit reaction speed.


Should note that the actual linear regulators should also has many other functions, such as load, short circuit protection, over voltage cut-off overheating shut off, reverse connect protection, etc., and also can use MOSFET series adjustment of pipe.



Dc-dc, (brief principle)


In fact, the DC DC power supply is converted to AC power supply. It is usually a self-oscillating circuit, so it requires discrete components such as inductance.

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Then at the output end through the integral filter, back to DC power. Because of the AC power generation, it can be very easy to carry on the lift and reduce pressure. Two transformations will inevitably result in loss, which is the problem that everyone is trying to study to improve the efficiency of dc-dc.




1, DCtoDC including boost (boost), buck (step-down), boost/buck (l/step-down) and reverse phase structure, with high efficiency, high output current, low static current characteristics, with the improvement of integration, the periphery of the many new DC - DC converter circuit only inductor and filter capacitor; However, the output ripple and switching noise of this kind of power controller are large and relatively high.


2. LDO: the outstanding advantage of low voltage differential linear regulators is the lowest cost, lowest noise and lowest static current. Its peripheral devices are also small, usually with only one or two bypass capacitors. The new LDO can achieve the following indexes: 30 mu V output noise, 60dBPSRR, 6 mu A static current and 100mV pressure difference.




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