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ZibBee control based on high dynamic range analog dimming LED device design

In Electronic Infomation Category: Z | on February 25,2011

Abstract: The LED used in machine vision for recognition and TC7W08F datasheet and other special occasions, this paper presents a control based on high dynamic range ZibBee LED analog dimming device, provides a different from the high-frequency PWM dimming method. Linear power supply on the original design improvements, an increase of variable step-down circuit improves the efficiency of their work and TC7W08F price and achieve a 500: 1, high dynamic range analog dimming. Using both circuit current control CC2430 chip and TC7W08F suppliers and wireless remote control functions, can be combined with the ZigBee wireless gateway for remote dimming control.

1 Introduction

The light emitting diode (LED) technology continues to evolve, LED because of its high light efficiency, fast response, wide range of brightness adjustment, is gradually applied to the signal, display, lighting and machine vision to identify other areas . The LED brightness control method commonly used is to simulate dimming and digital dimming (PWM). Compared to the existing analog dimming, digital dimming light dimming to get a higher accuracy ratio and the current applied more widely. In general lighting, PWM dimming switching frequency is generally between a few hundred to several thousand hertz, the human eye can effectively prevent visible flicker. However, identification and industrial inspection, machine vision and other fields, due to the high rate of speed cameras and sensors to respond much faster than the eye, so PWM dimming using these areas must be increased to tens of thousands of switching frequency is even higher, to achieve more complex , and analog dimming is no such problem.

This paper and the linear variable step-down two-level dimming circuit of the efficient, accurate and high dynamic range analog dimming output and use TIs C2430 chip to achieve the output brightness adjustment and wireless control features, particularly suitable for the identification of machine vision applications where high response rate.

2 high dynamic range analog dimming circuit

2. 1 Overview

Common LED constant current circuit has the following two: linear constant current circuit and switching constant current circuit. Linear constant current circuit by monitoring the voltage on the sampling resistor to dynamically adjust the turn on the transistor level, the control current and voltage of the input voltage is higher than the LED string part of commitment. The constant current circuit switching topologies in their different, adjust the switch on the duty cycle to regulate the output, the same effect by constant current. In contrast, if the input voltage and the voltage difference between light strings is large, the high current linear voltage drop across the transistor circuit will cause a large power loss, resulting in lower efficiency.

Figure 1 linear constant current circuit

Figure 2 switching constant current circuit

2. 2 specific circuit design

Existing switching power supply controller chip also offers analog dimming feature, but dimmer than the very small, generally in the dozens or so, as a complementary PWM dimming, the dimming ratio and the aforementioned machine vision to identify the requirements were significantly different. For the above, this re-linear constant current circuit has been improved, and in this part of the circuit an increase of variable step-down circuit used to match the input voltage and LED light string voltage, efficiency; the same time using high-precision D / A to control the current output, get a higher analog dimming ratio. Structure of the circuit shown in Figure 3, in AC / DC power supply output can be mounted on the bus all the way more than the adjustable constant current circuit, through the ZigBee module output current control, to ensure that each output current and accurate, adjustable.

circuit in Figure 3 Schematic

Use of variable input buck AC / DC power supply for 48V bus, this part of the circuit followed by the LED according to the number of how many pieces the size and output current, dynamic adjust the output so that the output voltage and voltage LED light string for the minor difference between the levels, thereby reducing the loss of high current transistor. This article uses the companys LM5010 buck NationalSemiconductor chip to build a variable step-down circuit, the schematic diagram shown in Figure 4:

Figure 4 variable built using LM5010 step-down circuit

LM5010 is a constant on-time Buck controller. R1 and R2 form a voltage feedback circuit, the output voltage divider and input to FB pin. When FB pin voltage is below 2. 5V when the switch chip will be fixed on-time, on-time on the input voltage and Ron, then switch off until the FB pin 265ns or voltage drop to 2 . 5V or less. Circuit by (R1 + R2) / R2 VFB to set the maximum output voltage. On the other hand, in order to reduce the power loss in the transistor, we also monitor the collection and sampling transistor voltage drop and resistance, and amplified using the LM358 for a positive input to the FB pin through D2. Therefore, the sampling resistor in the transistor and the voltage drop on the sum of not more than Vdrop = (VFB + VD2) R3 / (R3 + R4). So when the voltage on the LED light string LM5010 is less than the maximum output voltage, the excess voltage will be sampled by the transistor and the resistance of commitment, when the voltage is amplified through the FB pin is greater than the threshold, LM5010 switch off extended time so that the output voltage drops, so the final Vout = Vled + Vdrop. Thus the number of LED stars or less than the design value of the dimming of the LED, the front-end output voltage can be more reasonable to match the voltage string lights, concrete shown in Table 1 and Table 2.

500mA output in Table 1 the number of stars of different variable buck LED output and LED light string circuit voltage comparator

Table 2 13 LED output current under different variable step-down circuit voltage output and the LED light string comparison

Figure 4, the base of the transistor is in the box next to the current control circuit, the specific structure shown below 5. Current is mainly controlled by AnalogDevice the AD5611, which is a 10-bit digital-analog converter chip, the use of reference direct the output of the power supply, PC CC2430 SPI interface using the output voltage programming. Chip output and the voltage on the sampling resistor, respectively, 5 and 6 received the LM358 pin operational amplifier used as an open-loop amplifier. Amplifier to amplify the two input bias control transistor turn-on level, then control the LED string current, and ultimately the two open-loop input voltage is equal, then meet the following formula: Rsen ILED = VA / D R6 / (R5 + R6). R5 and R6 in the circuit mainly to the A / D converter output voltage divider in order to use smaller sample resistance and improve efficiency. Taking into account the D / A chip median and overall accuracy, this paper linear current control circuit can be 500: 1 output current ratio.

Figure 5, built using AD5611 analog dimming circuits

3 ZigBee control and monitoring

3. 1 hardware design

Wireless communication control module uses the CC2430-F128 Texas Instruments chip that specifically for IEEE802. 15. 4 and ZigBee applications. CC2420 chip combines a radio core, enhanced 8051MCU, 128KB of system programmable Flash memory, 8KB of SRAM, 8 Road 8 to 14-bit ADC, 4 timers, two serial communication interface module, AES co-processor , watchdog timer, power-on reset circuit, brownout detection circuit and 21 general purpose IO ports, shown in Figure 6.

Figure 6 CC2430 chip block diagram of the internal structure

Shown in Figure 3, CC2430 chip by AC / DC power supply, and assume the output current control and circuit condition monitoring functions. Current control is through the SPI interface, the AD5611 and the aforementioned communication, according to the desired output current to adjust the corresponding D / A output voltage. Note that when the output current is very low, there will be input AD5611 digital value and the final output of the current situation not a linear scale, which is due at this time D / A output voltage is too small, by the D / A conversion error, a greater impact line drop, etc., need to be amended in the program.

The monitoring circuit shown in Figure 7, paper, chip ADC respectively by two AC / DC output voltage and output current monitoring. Voltage monitoring is carried out through the R11 and R12 resistor divider, followed by P0. 7 feet sampled. Current monitoring is through the MAX4080 chip, which is a fixed gain of the high-side current detection chips that can take power directly in the high end, assuming a gain of A, then P0. 6 feet for ADC conversion by monitoring the voltage and current between see the relationship type:

Through the conversion of internal procedures can be the actual current. In this paper, while AC / DC circuit and the back end of the constant current drive circuit by adding a relay between the U2, the CC2430 the P0. 5 feet for switch control, when necessary, cut off in the constant current part of the power supply.

Figure 7, voltage, current monitoring circuit

3. 2 Software Design

Device designed in this paper is mainly distributed in different distances from the main controller, several locations, each location vary the number of devices, the devices at each location are not far, there is no information exchange between devices Therefore, directly using star network topology. Topology shown in Figure 8, in the center of the ZigBee network is ZigBee wireless gateway, wireless gateway plays with the main control computer communications and ZigBee network coordinator functions. The device described in this article is dimming as the ZigBee end-node, end node is responsible for dimming and receive wireless gateway query command to the corresponding state feedback dimming.

Dimming device terminal node within the workflow is as follows: First, wait for the wireless terminal nodes in a network gateway, and then apply to join the domain net, waiting for the coordinator assigned 16-bit short address, then enter the application through the binding process, terminal nodes and the coordinator will be binding. Because each network address starting with a random distribution of short, so the device within the other curing the individual identification address, which is used to represent the geographical and serial number. After the terminal nodes into a wait state, and regularly monitors its own voltage and current parameters and possible exceptions to report. When the host computer you want on a single terminal node dimming, it will for a specific ZigBee wireless gateway to send the command packet contains dimming types, ZigBee individual identification address and the dimming value, then be able to receive a particular terminal node to the wireless gateway through the coordinator to convey the command data, through the SPI interface to A / D converter to convey, and ultimately regulate the current to a specific value.

Figure 8 dimming system topology

4 Summary

Speed sensors for certain applications require dimming of the occasion, this paper, linear LED driver circuit has been redesigned, adding a variable step-down circuit improves the efficiency of the linear drive circuit, and achieved 500: 1 analog dimming. At the same time using the ZigBee chip dimming, condition monitoring and wireless control, remote monitoring and control as a node to provide a strong support.

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