Several factors need to be considered in LDO design

Traditional voltage regulators are obviously not suitable for the market, because for some specific applications, the dropout voltage between input and output is too low to be used.

At this time, the LDO-type power conversion chip was born, which helped us solve this problem well, but here we remind everyone that the following issues should be considered when designing an LDO:

1

The voltage drop is an important parameter of the LDO, which represents the potential difference between the input and the output, and the smaller the LDO voltage drop, the better.

However, when the input voltage cannot meet the minimum dropout voltage requirement, the LDO cannot work normally. At this time, the error amplifier will enter a fully turned-on state, making the gain of the loop zero, and the ability to regulate the voltage of the load will become very poor. , the power supply rejection ratio is also greatly reduced, the following points need to be noted: 

First: There can be multiple or even multiple sets of differential pressure data in the LDO parameter table, such as the minimum, typical, and maximum values ​​of differential pressure under light load, medium load, and full load conditions.

Among them, the typical value is only for design reference, the most practical value should be the maximum value of the pressure difference under the full load condition, and the parameter value is measured under the most unfavorable condition.

This should be used as a basis for design to allow enough margin to ensure that the LDO will work properly even in the worst case.

Second: For the sake of reliability, sometimes the lowest input voltage value can be selected according to the relational formula of Uin=Uout+△U+lV, and it is a bit wasteful to choose the power by more than 1.5 times.

But it is not too much to add a margin of 20%-30%. The self-destructive power consumption of a general LDO is Pd_max=(Uin-Uout)*Iout.

Third: The input-output pressure difference is not a fixed value, it increases with the increase of the output current, and increases with the increase of temperature. 

2

Maximum output current The maximum output current is a basic parameter of the LDO. Generally, the larger the output current, the higher the price of the LDO. The LDO must be able to provide sufficient current to the load under the most unfavorable working conditions. 

3

The input voltage requires that the input voltage must be greater than the sum of the rated output voltage and the input-output voltage difference, that is, Uin>Uout+△U, otherwise the LDO will lose its voltage regulation function, and the output voltage will change with the input voltage. At this time, Uout is equal to the input voltage Subtract the product of the on-resistance (RON) of the adjustment tube and the load current, that is, Uout=Uin-RONI0.

4

The peripheral circuit of the fixed output voltage LDO is simple, easy to use, and can save the cost and space of the external sampling resistor divider.

Its output voltage value has tended to be the same at the factory (only for universal voltage), and the output voltage accuracy is generally ±5%, which is sufficient for most applications.

The new LDO uses laser correction technology, and the accuracy index can reach ±1% to ±2%.

Special attention should be paid to the applicable conditions of the accuracy index given in the product specification, for example, whether it is at room temperature or within the entire operating temperature range, whether it is under full load conditions or under medium load or no-load conditions.

The adjustable output LDO allows continuous adjustment of the output voltage within the specified range. If the output terminal is connected to the feedback terminal to make the output voltage equal to the internal reference voltage, the minimum output voltage is generally about 1.2V. 

There are two types of input power, one is DC power and the other is AC power.

When using alternating current, it must first pass through the power transformer and rectifier filter to become pulsating direct current, and then provide input voltage to the LDO. At this time, the voltage difference of the LDO is no longer a key indicator, because by increasing the number of turns of the secondary winding of the power transformer, It is easy to increase the input voltage of the LDO to meet the LDO's need for differential voltage.

6

Quiescent current is the total current that flows inside the LDO to ground under no-load conditions or when the output is turned off.

The smaller the quiescent current, the lower the power consumption of the voltage regulator. In some applications, the standby mode is often selected to turn off the output. At this time, the service life of the battery depends on the size of the quiescent current.

Recently introduced new LDO, quiescent current can be as low as 75 ~ 150μA, and better than ordinary LDO voltage regulation characteristics.

What needs to be emphasized is that the quiescent current of the LDO is not a fixed value, it increases with the increase of the load current. But the quiescent current of VLDO can be approximately considered as a constant value.

7

Additional functions of LDO:

• The on/off control function allows the use of mechanical switches, gate circuits or microcontrollers to turn off the output of the LDO to enter a low-power standby mode (also known as standby mode).

• The input voltage reverse polarity protection function is used to prevent damage to the LDO when the input voltage polarity is reversed.

• Fault flag output function, when the output voltage (or input voltage) is lower than the specified threshold voltage, the LDO can output a fault flag signal, and the microprocessor can complete data storage and other tasks in time after receiving this signal.

• Transient voltage protection function. When LDO is used in automotive electronic equipment, it is necessary to protect the transient change of the load (such as sudden unloading). Once the transient voltage appears at the output terminal, the output will be turned off immediately. After the transient voltage passes , and quickly resumed normal operation.

• Tracking capability Some multi-output LDOs need to have tracking capability, in which one or several auxiliary output voltages can automatically track changes in the main output voltage, and adjust their own output voltage values ​​in time to reduce the relative amount of change.

• Sorting, the so-called sorting, is to make the output of each regulated power supply turn on or off in a prescribed order in a power system composed of multiple regulated power supplies.

When designing an LDO, if you can take the above seven factors into account, then the LDO you design is undoubtedly the best. The output ripple control can be achieved by improving the value of C61 to achieve 220uf, 100uf, etc.