LDO Output Voltage

Many linear regulators come in fixed output versions as well as adjustable voltage versions, and a few have programmable outputs. For the fixed voltage version, the output voltage is the fixed output voltage. Of course, it is not an absolutely accurate output voltage, but a voltage range, which mainly affects the output accuracy of the power supply.

Generally speaking, the greater the operating current, the more the linear voltage regulation deviates from the rated operating voltage. This is because although the linear voltage regulation works in the linear region of the triode, the "linearity" here is only approximately linear, and due to internal resistance, the greater the current The more energy will be lost in the linear voltage regulation. Although under ideal conditions, the feedback of the output voltage can still stabilize to the rated voltage, but because the current increase will cause the chip temperature to rise, the reference voltage will change slightly, that is, What we call "temperature drift". In addition, if it is a reference voltage provided by a similar Zener tube, it is actually not an ideal Zener tube, and the actual stable voltage is affected by the operating current (input voltage and the resistance to set the operating current). These are the reasons for the deviation between the linear regulated output and the rated voltage. In practical applications, there is no need to consider these issues yourself (if someone uses a triode to build a voltage regulator circuit, the author did not say (111￢ω￢)), Because you can directly read the datasheet to select a voltage regulator chip whose output accuracy meets the requirements.

Of course, in addition to this, the datasheet will also provide output voltage changes due to temperature and load changes, which are generally expressed in percentages. The power supply voltage of most chips is allowed to fluctuate within a certain range, but in some cases special Pay attention to these parameters, such as:

1. The power chip is sensitive to power fluctuations, and undervoltage will cause a protective reset of the chip;

2. Used as a reference voltage, mainly when used as an ADC reference voltage, its fluctuation affects the measurement accuracy of the ADC.

Take the ADC with 12-bit precision as an example, assuming that the power supply is 3.30000V, and the measured external signal is 1.20000V, the ADC reading should be 1.20000V/3.30000V×4096=1489 (the ADC output is an integer, and it may actually be 1490 ), the external signal voltage converted by the user is 1489/4096×3.30000V=1.19963V (or 1.20044V), and the error is 0.0308% (or 0.0367%); if the power supply voltage fluctuation becomes 3.20000V, the external signal is still is 1.20000V, then the ADC reading will become 1.20000V/3.20000V×4096=1536, but since the user still thinks that the power supply is 3.30000V, the calculated external signal voltage at this time is 1536/4096×3.30000V=1.23750V, The error is 3.1250%. It can be seen that the error measured by the ADC has been magnified many times. Of course, the method of eliminating the error is not necessarily to improve the accuracy of the power supply. For example, the STM32F407 integrates a precise reference voltage of 1.2V, and the error can be calibrated by continuously measuring the reference voltage. Refer to the specific method：

stm32 precise voltage measurement method (internal reference voltage)

For the voltage adjustable version, the allowable output range can also be found in the datasheet. It should be noted that the allowable maximum load current may be different under different output voltages, which is limited by the maximum output power.

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