Digital photographic electronics are becoming more portable and integrate high quality solutions. The high performance and small size of camera applications are often affected by low dropout (LDO) regulators in cameras that supply complementary metal oxide semiconductor (CMOS) image sensors. Figure 1 shows an example of a camera circuit in a smartphone.
Figure 1: Camera circuit in a smartphone
We have all heard of the word "pixel", but what does pixel mean for consumers? The pixel size can help determine the size of the photodetector in a CMOS image sensor. Knowing the size of the photodetector is necessary because it is the photodetector that captures the light to record the perfect photo. Figure 2 is a photographic process.
Figure 2: The process of producing high quality photos
So what you need is a larger image sensor to capture high quality photos, right? Not exactly that. Since CMOS image sensors are sensitive to noise, they need to be powered in a special way. The components of the CMOS image sensor are particularly susceptible to damage from transient overcurrent caused by the power supply. Power supply noise can affect the ability of a pixel to properly capture light, resulting in poor photo quality. 3 is an exemplary diagram of an advanced tree power module that powers a CMOS image sensor.
Figure 3: CMOS image sensor advanced tree power module diagram
The LDO regulator filters out noise in the power supply. In order to power the image sensor, you need to check the noise specifications of the LDO to ensure that the LDO meets the specifications of the terminal device. Let us take the high performance LDO LP5907 as an example.
Figure 4 shows the power supply rejection ratio (PSRR) between 10 Hz and 10 MHz. The PSRR feature allows the LDO to block the noise generated by the power supply. In short, the higher the PSRR, the more power supply noise the LDO can block.
Figure 4: LP5907 PSRR
But this is only at the input of the LDO, at the output of the LDO? This involves the noise spectral density. Figure 5 shows the output voltage noise between 10 Hz and 10 MHz. The lower the output voltage noise, the less noise is transmitted to the CMOS image sensor.
Figure 5: LP5907 noise spectral density
Table 1: LP5907 PSRR vs. Output Voltage Noise
At full load currents of 1 kHz and 6.5 μV RMS, the LP5907 PSRR is as high as 82 dB. These parameters of the LP5907 indicate that the LP5907 can be selected as the core component when designing the CMOS image sensor power supply. Because the LP5907 is extremely compact, it takes up very little board space. Table 2 lists the package information for the LP5907.
Table 2: LP5907 Package Information
For camera applications, LDO regulators can be used to create more convenient, higher quality digital camera solutions. LDO offers high performance elements while maintaining a high degree of compactness. Figure 6 shows an example of a 3-MHz 50-mVpk-pk input signal that is suppressed and filtered by the LP5907 at the output.
Figure 6: Input voltage ripple vs. LP5907 regulated output voltage
Check out TI's complete portfolio of LDO solutions.
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