Digital Camera Power Circuit Design and DC/DC Converter Selection

　　The power circuit of a digital camera includes I/F power, lens drive motor power, LCD power, backlight power, etc. How to optimize the design of these power supplies is related to the key features of the whole machine, such as low power consumption, small size and light weight. This article provides design methods and ideas using practical circuits as examples, and provides comparisons of PWM control, PFM control and PWM/PFM switching control modes, as well as design decisions such as peripheral component selection.

 

　　As digital cameras have gradually become mainstream consumer electronic products in recent years, consumers have increasingly demanded more features, such as high pixels, short video recording, large LCD screens, large storage capacity, small size and light weight. In this case, extending battery life and backup time has become an important challenge for digital camera design engineers. This article will discuss and analyze power management, especially DC/DC converters, and illustrate with examples.

Main power module

　　Now, the system power supply used in digital cameras can be divided into a composite IC to achieve different outputs, and several DC/DC converters are combined to achieve different outputs. In the example of Figure 1, several DC/DC converters are combined to achieve, where the CCD power supply is generated by using a transformer to generate 15V and -5.5V voltages. This circuit uses a VDD/VOUT separation type step-up DC/DC converter (S-8357P50), which is switched in PWM mode and controlled by an external transistor. The output voltage can be generated by changing the number of transformer turns to generate the required voltage.

1. I/F power supply

　　The I/F power supply uses a step-up DC/DC converter (S-8358P33) with a PWM/PFM switching mode, and the common output voltage is 3.3V. Because it is an external FET, it can be used for large currents of more than 500mA. In addition, a small inductor can be used when using high-frequency DC/DC products such as 600kHz. The common output voltage of the CPU core power supply is 1.8V, so a step-down DC/DC converter (such as S-8521E18) is used. It is ideal to use PWM/PFM switching. Even when the current consumption is very low, the efficiency can be improved by working in PFM mode.

2. Power supply for lens drive motor

　　The power supply for the lens drive motor usually uses a 5V voltage. Due to the large operating current, a PWM control mode and an external FET DC/DC (S-8357P50) are used. The characteristic of this circuit is that a single DC/DC converter is used, the wiring is flexible, and a more suitable place can be selected for installation on the substrate. If a composite IC is used to obtain different output voltages, the wiring will be limited, and the space of the substrate cannot be reasonably utilized, which may lead to an increase in the substrate area. Therefore, using a single DC/DC converter can achieve the purpose of miniaturization.

3. LCD driver power supply

　　Figure 2 is an application circuit for generating positive and negative voltages for LCD. It uses a step-up DC/DC converter to generate positive and negative voltages. The circuit uses the VDD/VOUT separation device S-8356Q50. At the Vout terminal interface, the output voltage is divided by using resistors to adjust the output voltage. The positive voltage can be very high through R1 and R2, and the negative voltage can be reversed to a negative voltage through C1, SD3, and SD2.

In this circuit, positive voltage is directly inverted to negative voltage, so there is no unbalanced voltage between positive voltage and negative voltage. When unbalanced voltage is required, a voltage stabilization circuit can be made at the output end using a transistor or the like to stabilize the voltage.

4. Power supply for white light LED

　　The circuit shown in Figure 3 is a power supply for white LEDs. Since white LEDs are current controlled, a stable current is required. In this circuit, a stable current flows through the LEDs. In order to control the brightness deviation of each LED, the LEDs are arranged in series. The DC/DC converter uses the S-8356Q15 VDD terminal and VOUT terminal separated type, and the output voltage is 1.5V.

　　In this circuit, the R1 resistor determines the current flowing through the LED. Even if the Vf of each LED varies, the voltage at point A will be controlled at 1.5V. Therefore, if a current of 20mA flows through the LED, the resistance value is: 1.5V÷20mA=75Ω. The characteristics of this circuit are: because the current flowing through the LED is stable, even if the vf of the LED varies, the brightness of each LED will be the same. In addition, since a general DC/DC converter is used, the required circuit can be easily constructed.

Power Supply Design Decisions

　　When selecting a DC/DC converter, the circuit design should pay attention to output current, high efficiency, miniaturization, and output voltage requirements:

1. If the required output current is small, you can choose a built-in FET type; if the output current needs to be large, choose an external FET type.
2. Regarding efficiency, there are the following considerations: if you need to give priority to ripple voltage and noise elimination under heavy load, you can choose a PWM control type; if you also need to pay attention to efficiency under low load, you can choose a PFM/PWM switching control type.
3. If miniaturization is required, you can choose a high-frequency product that can use a small coil.
4. In terms of output voltage, if the output voltage needs to be above a fixed voltage, or if an unfixed output voltage is required, you can choose a VDD/VOUT separation type product with variable output.

1. Comparison between PFM and PWM

　　The three control modes of PWM control, PFM control and PWM/PFM switching control mode each have their own advantages and disadvantages: The DC/DC converter performs voltage boost or voltage reduction by switching synchronously with the internal frequency, and controls by changing the number of switches to obtain an output voltage that is the same as the set voltage. In PFM control, when the output voltage reaches above the set voltage, the switching will stop, and the DC/DC converter will not perform any operation before it drops to the set voltage. However, if the output voltage drops below the set voltage, the DC/DC converter will start switching again to make the output voltage reach the set voltage. PWM control also switches synchronously with the frequency, but when the boost setting value is reached, it will minimize the current flowing into the coil and adjust the boost to keep it consistent with the set voltage.

　　Compared with PWM, the output current of PFM is small, but because the DC/DC converter controlled by PFM stops operating when the voltage reaches above the set voltage, the current consumption becomes very small. Therefore, the reduction of current consumption can improve the efficiency at low load. Although PWM is less efficient at low load, it is easier to design a noise filter and eliminate noise because of its small ripple voltage and fixed switching frequency.

If you need to have the advantages of both PFM and PWM, you can choose a PWM/PFM switching control DC/DC converter. This function is controlled by PWM when the load is heavy, and automatically switches to PFM control when the load is light, that is, a product has the advantages of both PWM and PFM. In a system with a standby mode, products using PFM/PWM switching control can achieve higher efficiency.

2. Advantages of high frequency

　　By actually testing the efficiency of PWM and PFM/PWM, it can be found that the efficiency of PWM/PFM switching products is higher at low loads. As for high frequency, by increasing the frequency of the DC/DC converter, large current, miniaturization and high efficiency can be achieved. However, it must be noted that efficiency can only be improved by matching the characteristics of the coil. Because when the DC/DC converter is high-frequency, the switching loss will increase due to the increase in the number of switches, resulting in a decrease in efficiency. Therefore, efficiency is determined by a compromise between the improvement of coil performance and the increase in switching losses.

　　By using high-efficiency products, coils with relatively low inductance values ​​can be used, and small coils can be used. Even with small coils, the same efficiency and output current can be obtained.

　　The comparison of the charge characteristics of the coils using the 600kHz S-8357N33 and the 300kHz S-8358F33 shows that the two have the same characteristics in terms of efficiency, and both can obtain a current of 200mA in terms of output current. However, if the small coil used in the 600kHz DC/DC converter is used at 300kHz, only an output current of about 100mA can be obtained, because the Ipk has exceeded the rated current of 600mA of the small coil, which proves that high frequency can indeed achieve the goal of product miniaturization.

3. How to use VDD/VOUT separation

　　Figure 4 illustrates the principle and method of using a VDD/VOUT separated DC/DC converter. When adjusting the output voltage or setting the output voltage above the absolute maximum rating of the IC, use a VDD/VOUT separated product. Figure 4 is an application circuit diagram of VDD and VOUT separation. When the output exceeds the rated withstand voltage of the IC, VDD is connected to the output voltage terminal. VOUT is the voltage after the voltage is divided by Ra and Rb. The voltage can be adjusted by changing the resistance values ​​of Ra and Rb.

4. External device selection

　　In addition to the characteristics of the DC/DC converter itself, the selection of external components cannot be ignored. The coil, capacitor and FET in the external components have a great influence on the characteristics of the switching power supply. The so-called characteristics here refer to the output current, output ripple voltage and efficiency.

Coil: If you need to pursue high efficiency, it is best to choose a coil with a small DC resistance and inductance value. However, if a coil with a small inductance value is used for a DC/DC with a low frequency, it will exceed the rated current of the coil, and the coil will produce magnetic saturation, causing efficiency deterioration or damage to the coil. And if the inductance value is too small, it will also cause the ripple voltage to increase. So when selecting a coil, please pay attention to the current flowing to the coil not exceeding the rated current of the coil. When selecting a coil, it is necessary to make a comprehensive decision based on conditions such as output current, DC/DC frequency, coil inductance value, coil rated current and ripple voltage.

Capacitor: The larger the output capacitor, the smaller the ripple voltage. However, a larger capacity also means a larger capacitor volume, so please choose the most suitable capacity.

Transistor: As an external transistor, compared with bipolar transistors, FET has a faster switching speed, so the switching loss will be smaller and the efficiency will be higher.

Conclusion

The following points should be noted when selecting components for digital camera power supply design:

1. 选择设计灵活性較大的DC/DC变换器，扩大电路设计的范围。

2. Low current consumption and high efficiency can extend the battery life.

3. Small external components can be used to achieve product miniaturization.

4. Powerful technical support tools.

　　The last point is not mentioned before. Technical support is also an important part in the DC/DC design process. After all, DC/DC is a semi-custom product to some extent. DC/DC is standard, but when equipped with different external components, different efficiencies and output voltages can be obtained. Therefore, the specifications of DC/DC itself are important, but the technical support of the supplier cannot be ignored. The technical support can be divided into two parts: one part is hardware, including providing evaluation circuit boards and external component support; the other part is providing some simulation software so that evaluation can be made before actual testing to save design time.