Design strategy of linear LED driver circuit for Top1 signal lamp

  Circuit description:This application note introduces a linear LED driving scheme, which is used to drive 6 strings of LED signal lights, each string contains 4 LEDs in series. Each LED load has an independent anode contact, and the cathodes are connected together. The circuit is powered by automobile battery, the minimum voltage is 10V, the maximum voltage is 28V, and can provide 350mA current for each string of LED. Due to the common cathode structure, the current detector must be placed at the positive end of the LED string. The maximum common mode voltage of current detection input terminal of LED driver (max16836) is limited to 4V. Therefore, the voltage at both ends of current detection resistor must undergo level conversion, taking ground as reference, so as to meet the requirements of the driver. A pair of PNP transistors convert the voltage of LED current detection resistor into the voltage with GND as reference, and send it to max16836 current detection pin. The following formula provides the calculation of R1, R2, R3 and R4 (Part U1 in the circuit diagram).

  Design of linear LED driver circuit for signal lamp

Figure 1. Schematic diagram of driver design

When the voltage of the LED string is at the minimum value (7.6v) and the input voltage is at the maximum value (28V), the power consumption of the LED driving circuit is the largest, greater than 7W. It is difficult to dissipate such a large amount of heat only through the circuit board. Therefore, in the case of high input voltage, the dimming signal with low duty cycle (as low as 25%) must be used to drive the universal dim input to reduce the power consumption of the driver.

  Unscramble the circuit of ncv70522 auto adaptive headlamp system

Due to the limitation of mechanical structure, the stepper motor may be locked sometimes in the application of adaptive headlamp system (AFS). Once the motor stalls, the electronic control unit (ECU) will lose the tracking information of the headlamp position and make an inappropriate response, which will breed a very serious safety problem. Therefore, the detection of locked rotor in AFS application is essential. Usually, the back electromotive force (BEMF) of the motor can be used to judge whether the motor is locked or not. BEMF varies with motor speed, load and supply voltage. The traditional stepper motor driver chip has no BEMF output, but has built-in locked rotor detection algorithm. The customer can only set the fixed critical value of locked rotor in the register, which means that under the real road conditions, all the set values must be preset “offline” before operation, and can not adapt to the real working conditions. Ncv70522 micro stepping motor driver provides BEMF output through SLA pin, which means it can carry out real-time stall detection calculation and adjust detection level according to different conditions.

Ncv70522 is a micro stepping motor driver for bipolar stepping motor. The chip is connected to the external microcontroller through I / O pin and SPI interface. The output current of ncv70522 has many choices. It turns the next microstep according to the pulse signal on the “NXT” input pin and the state of the direction register [dirctrl] or “dir” input pin. This device provides 7 stepping modes from full step to 32 microstep, which are selected by SPI register SM [2:0]. Ncv70522 contains the output of SLA, which can be used to detect the locked rotor and adjust the torque and speed calculation according to the motor’s BEMF. Typical application circuit diagram is shown in the figure.

  Unscramble the circuit of ncv70522 auto adaptive headlamp system

When the system is powered on, the microcontroller will be initialized and ncv70522 will be reset. When these actions are completed, the coil current and stepping mode will be set. The motor driver will then be enabled. NXT pulse will be sent to rotate the motor. The motor speed is equal to the NXT pulse frequency multiplied by the value of the step subdivision mode.

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