Application of Cp2122b1 in Infrared Lighting Fill Light System

This paper introduces an infrared lighting system using a low-cost and high-efficiency boost power management chip cp2122b1 to drive the infrared emitting diode. According to the characteristics of infrared emitting diode and combined with the working characteristics of cp2122b1, the application of cp2122b1 in infrared lighting fill system is discussed in detail, and a variety of typical application schemes are given. The scheme can be widely used in various monitoring systems to provide efficient and stable night infrared lighting.

1、 Driving requirements of infrared emitting diodes

At present, infrared camera technology is mostly used in covert night vision monitoring system. Infrared camera technology can be divided into passive infrared camera technology and active infrared camera technology. The so-called passive infrared camera technology makes use of the characteristics that substances will emit infrared light above absolute zero (- 273oc), and the infrared can be captured and imaged by using a special camera; Active infrared imaging technology uses infrared radiation "illumination" to generate infrared light invisible to human eyes, and ordinary cameras are used to capture the infrared light reflected from the surrounding environment to realize imaging. The use of passive imaging technology requires the use of expensive special cameras, and the passive imaging technology can not completely reflect the surrounding environmental conditions. Therefore, most of the night vision systems use active imaging technology to achieve night covert shooting. The reliability of the infrared lighting supplement system will greatly affect the effect of the monitoring system at night.

According to its infrared radiation mechanism, infrared lamps are divided into semiconductor solid-state light-emitting (infrared emitting diode) infrared lamps and thermal radiation infrared lamps. The infrared emitting diode is made of a PN junction made of a material with high infrared radiation efficiency (commonly gallium arsenide GaAs), and a forward bias is applied to inject a current into the PN junction to excite infrared light. The spectral power distribution is the central wavelength of 830-950nm, and the half peak bandwidth is about 40nm. It is a narrow-band distribution, which can be felt by ordinary CCD black-and-white cameras. Its biggest advantage is that it can completely have no red storm (940 950nm wavelength infrared tube) or only have weak Red Storm (red storm has visible red light), and its service life is long; The thermal radiation infrared lamp uses the thermal radiation effect of the object to maintain its temperature by heating the filament in the thermal radiation light source for continuous radiation. Its biggest disadvantage is that it contains visible light components, that is, there are red storms, and its service life is short. Due to the above advantages of infrared emitting diode, it will be more widely used in infrared lighting fill system.

The infrared radiation power of the infrared LED is directly proportional to the forward working current, but when it is close to the maximum rated value of the forward current, the temperature of the device increases due to the heat consumption of the current, resulting in the decrease of the light emission power. If the current of infrared diode is too small, it will affect its radiation power, but if the working current is too large, it will affect its service life and even burn the infrared diode. The volt ampere characteristics of infrared light emitting diodes are very similar to those of ordinary silicon diodes. When the voltage exceeds the forward threshold voltage (about 0.8V), the current begins to flow, and it is a very steep curve, indicating that its working current is very sensitive to the working voltage. Therefore, the working voltage is required to be accurate and stable, otherwise it will affect the exertion and reliability of radiation power. The radiation power of infrared LED will decrease with the increase of ambient temperature (including the increase of ambient temperature caused by its own heating). Due to the above characteristics of infrared LED, the design of its driving circuit will largely determine its application effect in infrared lighting system. In short, in the design of its power supply system, the current flowing through the infrared emitting diode should be controlled to give consideration to brightness and efficiency.

2、 Traditional solutions

In the traditional solution, a constant voltage source is used to drive the infrared emitting diode. In this way, the requirements can be fully met when the power supply voltage is stable. However, in practical application, the voltage of the constant voltage power supply fluctuates greatly due to the fluctuation of the power grid, resulting in the failure of the system to work normally, as shown below:

Power supply voltage rise: the current flowing through the infrared emitting diode increases. When the current rises close to the maximum rated value of the forward current, the temperature of the device rises due to the heat consumption of the current, resulting in the decrease of the optical emission power. If the current continues to rise, the infrared emitting diode will burn out.

Power supply voltage drop: the current flowing through the infrared emitting diode decreases. Because the infrared radiation power of infrared LED is directly proportional to the forward working current, its brightness will be greatly reduced and can not meet the demand of lighting supplementary brightness.

Obviously, using constant voltage output to drive the infrared emitting diode has obvious shortcomings in practical application, which can not solve the demand of the driving circuit in the night lighting system. Therefore, in the design of infrared lighting fill light driving circuit, it is necessary to realize constant current driving in a wide voltage input range.

3、 Driving solution of infrared emitting diode using cp2122b1

For infrared emitting diodes, when the voltage exceeds the forward threshold voltage (about 0.8V), the current begins to flow, and it is a very steep curve, indicating that its working current requirements are very sensitive. Therefore, the working current is required to be accurate and stable, otherwise it will affect the exertion and reliability of radiation power. The radiation power will decrease with the increase of ambient temperature (including the increase of ambient temperature caused by its own heating). Therefore, in the driving circuit of infrared LED, the control of current and voltage needs special attention.

Cp2122b1 is a high-efficiency, high-power inductive boost DC / DC Driver developed by Qipan microelectronics. The chip has current feedback function and can realize accurate control of current flowing through external consumers, i.e. constant current control. Its main indicators are as follows:

1. Working voltage range: 2.7-5.5v;

2. The efficiency can be as high as 88%;

3. 300mV feedback voltage;

4. The maximum driving capacity of the switch tube is 900mA;

5. Built in overcurrent and overheating protection.

Therefore, cp2122b1 can fully meet the requirements of power supply design in infrared lighting fill system. Combined with the characteristics of infrared emitting diodes and cp2122b1, two typical circuits using cp2122b1 to drive 40 infrared emitting diodes are given below. Of course, cp2122b1 can drive more or fewer infrared emitting diodes according to different practical applications.

(1) Scheme 1: cp2122b1 is used to drive 2 series X20 PCs

This scheme is designed according to the infrared LED edir9405b1 of Rixin technology, and its main technical indexes are as follows: forward conduction voltage vftypical = 1.7V @ if = 60mA.

Cp2122 drives 40 infrared LED lamps (2 strings & tips; 20)

In the above scheme, R1 is used to realize current setting, which is used to set the total current IR after two series of infrared LED lamps are connected in parallel, R1 = VFB / IR. Where, the voltage of FB terminal VFB = 300mV.

For example, if the current of a single LED lamp is 60mA, the current passing through R1 is 120mA. At this time, R1 = 300mV / 120mA = 2.5 ohms. When the voltage drop of a single LED lamp is 1.7V, Vout = 1.7V & times; 20=34V。 At this time, it should be noted that the parameter selection of Zener diode D1 must meet the minimum requirements of load, that is, the current through D1 must be greater than 120mA, and the withstand voltage of D1 must be greater than 34V; It is also necessary to reasonably select capacitors CIN and cout. (2) Scheme 2: cp2122b1 is used to drive 4 series X10 PCs

This scheme is designed according to the infrared LED edir9405b1 of Rixin technology, and its main technical indexes are as follows: forward conduction voltage vftypical = 1.7V @ if = 60mA.

In this scheme, a current setting resistor R1 is used to set the total current IR after four series of infrared LED lamps are connected in parallel, R1 = VFB / IR. Where, the voltage of FB terminal VFB = 300mV.

For example, if the current of a single LED lamp is 60mA, the current passing through R1 is 240mA. At this time, R1 = 300mV / 240mA = 1.25 ohm. When the voltage drop of a single LED lamp is 1.7V, Vout = 1.7V & times; 1017V。 At this time, pay attention to the parameter selection of Zener diode D1, which must meet the minimum requirements of load, that is, the current passing through D1 must be greater than 240mA, and the withstand voltage of D1 must be greater than 17V; It is also necessary to reasonably select capacitors CIN and cout.

4、 Practical application

The above two schemes have been verified in practical application. The scheme given above can accurately control the current flowing through the led to keep the infrared LED emitting normally and continuously. The technical characteristics of the two schemes are as follows:

The overall driving current of scheme 1 is small, and the thermal power loss on the feedback resistance is small. However, the working voltage required is also relatively high. Therefore, in practical use, when the input voltage is too low (VIN The overall driving current of scheme 2 is large, and the thermal power loss on the feedback resistance is large, but the required working voltage is also relatively low. Therefore, in practical use, scheme 2 is applicable to the case where the input voltage is relatively low.

In addition, because the technical indicators of infrared emitting diodes from different manufacturers are different, in practical application, it is necessary to increase or decrease the number of single string LEDs and adjust the R1 resistance value according to the characteristics of the selected infrared LEDs.

Application of Cp2122b1 in Infrared Lighting Fill Light System 1

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