LDO chip is an indispensable and important protective device in switching power supply. Especially in the current wave of electronic products, the role of LDO chip is beyond doubt. Its birth is due to the rapid upgrading of electronic products, which leads to the traditional voltage regulator can no longer be suitable for the market. Because for some specific applications, the input and output differential pressure is too low to be used and meet the requirements. At this time, LDO chips were applied, helping us solve this problem. Due to the change of the market, the following six factors should be considered when designing LDO chips to avoid non conformance to the requirements of market products.
1、 Differential pressure: differential pressure is an important parameter of LDO chip, which represents the potential difference between input and output. The smaller the differential pressure of LDO chip, the better. But when the input voltage cannot meet the requirement of "minimum differential pressure", the LDO chip cannot work normally. At this time, the error amplifier will enter the fully on state, making the gain of the loop zero, the voltage stabilizing ability of the load will become very poor, and the power supply rejection ratio will also be greatly reduced.
2、 Maximum output current: The maximum output current is a basic parameter of LDO. Generally, the higher the output current, the higher the price of LDO. LDO must be able to provide enough current for the load under the most unfavorable operating conditions.
3、 Input voltage: it is required that the input voltage must be greater than the sum of the rated output voltage and the input-output differential pressure, i.e. Uin>Uout+△ U. Otherwise, the LDO chip will lose the voltage stabilizing function, and the output voltage will change with the input voltage. At this time, Uout is equal to the product of input voltage minus the on resistance (RON) of regulating tube and load current, i.e. Uout=Uin-RONI0.
4、 Output voltage: The peripheral circuit of the fixed output LDO chip is simple and convenient to use, which can save the cost and space of the external sampling resistance voltage divider. The output voltage value has been consistent (only limited to general voltage) when leaving the factory, and the output voltage accuracy is generally ± 5%, which is sufficient for most applications. The new LDO chip adopts laser correction technology, and the accuracy index can reach ± 1% ～ ± 2%. Special attention should be paid to the applicable conditions of the accuracy indicators given in the product manual, such as at room temperature or within the entire operating temperature range, full load or medium load or no-load conditions. The adjustable output LDO chip 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.
5、 Input power supply type: There are two types of input power supply, one is DC power supply, the other is AC power supply. When AC power is used, it will first be changed into pulsating DC through power transformer and rectifier filter, and then supply input voltage to LDO. At this time, the differential pressure of the LDO chip is no longer a key indicator, because it is easy to increase the input voltage of the LDO chip by increasing the turns of the secondary winding of the power transformer to meet the LDO chip's demand for differential pressure.
6、 Quiescent current: The quiescent current refers to the total current flowing to the ground inside the LDO chip under no-load conditions or when the output is turned off. The smaller the quiescent current is, the lower the power consumption of the regulator is. 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 quiescent current. Recently introduced new LDO chip, static current can be as low as 75~150 μ A. And it has better voltage stabilizing characteristics than ordinary LDO chips. It should be emphasized that the quiescent current of LDO is not a fixed value, which increases with the increase of load current. However, the quiescent current of VLDO chip can be approximately considered as a constant value.