Digital imaging electronics are becoming more portable and integrated into high-quality solutions. The cohesiveness of high
-performance and small -sizes in camera applications is often influenced by the low-dropout (LDO) voltage regulators powering the complementary metal-oxide semiconductor
Figure 1: Camera circuitry in a smartphone
We’ve all heard the term “pixel,” but what does that mean to consumers? Pixel size can help determine the size of each discrete photodetector in a CMOS image sensor. It is imperative to know the size of these photodetectors because that is what captures light to record the perfect image. Figure 2 illustrates the process.
Figure 2: Production of high-quality images
So all you need is a large image sensor to capture good quality images, right? Not quite. These CMOS image sensors need to be powered in a specific way given their sensitivity to noise. The components that make up a CMOS image sensor are extremely susceptible to transients caused by the power supply. A noisy power supply affects a pixel’s ability to properly capture light, which results in a poor-quality image. Figure 3 is an example of a high-level power-tree block diagram for powering a CMOS image sensor.
Figure 3: High-level power-tree block diagram for a CMOS image sensor
An LDO voltage regulator filters out the unwanted noise from the power supply. To power the image sensor, you will want to look at the noise specs of the LDO to make sure it meets the specifications of the end equipment. Let’s use the LP5907, a high-performance LDO, as an example.
Figure 4 shows the power-supply rejection ratio (PSRR) swept from frequencies between 10Hz to 10MHz. The PSRR characterization allows the LDO to block noise produced by the power supply. Simply put, the higher the PSRR, the more noise the LDO can block from the power supply.
Figure 4: LP5907 PSRR
But this is only at the input of the LDO; what about the output? That is where spectral-noise density comes in. Figure 5 shows the output-voltage noise swept between 10Hz to 10MHz. The lower the output-voltage noise, the less noise transferred into the CMOS image sensor.
Table 1 lists the LP5907’s PSRR and output-voltage noise specifications.
Table 1: LP5907 PSRR and output-voltage noise
The LP5907 has an extremely high PSRR of 82
Table 2: LP5907 package information
When it comes to camera applications, using an LDO voltage regulator produces more accessible and improved digital
Figure 6: Comparison between input voltage ripple and LP5907 regulated output
Check out TI’s complete portfolio of LDO solutions.
- Learn more about these TI Designs reference designs:
- Read the blog post, “How LDOs meet power requirements for automobile camera modules.”