Due to the continuous improvement of LED technology and the growth and maturity of LED manufacturers, LED applications have become more and more extensive, and have evolved from the original simple small-size LCD backlight application to the current instrumentation indicator lights, lighting lights, miner's lamps, street lamps, and automobiles. Lights and big screen ads, etc. With the demand and development of the market, the application field of LED will penetrate into various industries.
LED applications that illuminate and provide a light source typically require large currents to provide sufficient brightness, thus requiring a large number of LEDs. Traditionally there are two combinations: LED series and LED parallel.
The LED current in the LED series scheme is consistent and the control is simple, but the output voltage must be raised due to the high output voltage required. The boost mode typically uses an inductive boost circuit that converts the low input voltage through a switching power supply to a high output voltage. This method is difficult to design due to switching noise, power inductance and EMI.
Parallel LEDs are driven by a charge pump, which is simpler than a series arrangement and EMI is easier to control. However, the number of LEDs is large, and multiple channels of LED driving are required . However, the existing chip supports up to six channels, so multiple chips are required, which will result in poor current consistency and increased cost.
Considering the advantages and disadvantages of the above two schemes, the series-parallel scheme is usually a better choice. Sipex's SP7616 series is based on this application. It is not a traditional inductive boost chip, nor is it a capacitive charge pump boost chip, but a linear buck sink current constant current source. chip.
The SP7616 is a 4-channel constant-current linear LED driver with a working voltage of 4.5V~30V. Each channel supports a maximum current of 60mA. The built-in current sharing matching circuit makes the current difference between each channel less than 1.5%. In the typical application circuit of SP7616, if VCC>(N×Vf+VDROP), the current flowing through the four channel LEDs is constant current and matched. Among them, RSET is used to set the maximum current of each channel, RSET=1.0V×950/IOUT; N is the number of LEDs per string. Since the input voltage is up to 30V, the maximum number of LEDs per string supports 29/. Vf is; Vf is the conduction voltage drop of the current flowing through the LED; 1.0V is the level of the ISET pin to ground; 950 is the amplification factor of the current, that is, the current flowing through the LED string is 950 times the current flowing through the RSET; VDROP is the cut-off voltage of the chip itself. It also refers to the cathode-to-ground voltage of the LED. It is closely related to the current through the LED. The designer can check the VDROP corresponding to the current according to Figure 1.
Figure 1: LED current vs. Vdrop
As can be seen from Fig. 1, VDROP cannot exceed 0.9V, so the maximum efficiency E = (30-0.9) / 30 = 97%. Due to the linear power scheme, the size of VDROP directly affects the loss, W = VDROP × IOUT. Therefore, when designing the circuit, the Vdrop should be as low as possible while satisfying the ability to drive the LED string, that is, the input voltage should not be too high to reduce the voltage loss on the chip, and solve the heat dissipation problem of the chip by laminating copper on the PCB. To avoid overheating.
Some special applications require more LEDs in series, which requires higher operating voltages. Sipex has also introduced a solution for this type of application, that is, VCC and VIN are powered independently (Figure 2). VCC and VIN use independent power supply mode, VCC power supply allows the chip to work normally, and VIN power supply realizes constant current operation of LED. VIN is determined by the number of LEDs in series and Vf. For example, if 12 LEDs are connected in series, when the current is equal to 40mA and Vf=3V, VIN is greater than 36V. Considering VDROP and heat dissipation, it is recommended that VIN be powered by 37V.
Figure 2: VCC, VIN use independent power supply to meet the application requirements of more LEDs in series
In addition, if some systems are unable to provide two independent power supplies, 12V can be converted to 5V through a Zener and a triode to power the chip (Figure 3).
Figure 3: If the system cannot provide two independent power supplies, convert 12V to 5V through the Zener and Transistor to power the chip.
If the system is unable to provide a high input voltage, then the boost circuit is required to boost VIN to the voltage required by the LED string. The circuit in Figure 4 uses SP6136 to boost the 9~12V voltage to drive 8 LEDs. This solution can be applied to handheld devices or battery-backed backlights, such as mobile DVDs and digital photo frames.
Figure 4: If the system fails to provide a high input voltage, boost the VIN to the voltage required by the LED string through the boost circuit
In short, the SP7616 is an ultra-small constant-current driver chip in the industry. It is packaged in a 2×3mm DFN-8 package. It has no inductance, no capacitance, no switching noise, no switching frequency, no EMI, built-in current sharing circuit, and simple design. The advantages of high efficiency and low cost can quickly meet the needs of current high current LED applications. (Text / Sipex company Huang Weigang)
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The transmitting power of the WIFI module is generally about 18dBm, and the transmitting power of the high-power WIFI module is about 28dBm.
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