How to use the single-chip LM3644 solution to achieve a perfect short-time highlight current waveform

“LM364x can realize fast and high-brightness current driving very well, and can get a good current waveform with IR and Pass mode, and at the same time, it adopts the method of distributing buffer capacitors to effectively reduce the input voltage drop.

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By: Eric Xiong, TI Engineer

In order to achieve a perfect current waveform, the traditional circuit is realized by a discrete circuit scheme (boost circuit + large capacitor buffer + op amp constant current), but this will greatly increase the PCB area, the cost of the circuit and the control method are also very important. not flexible.

This article introduces how to use the single-chip LM3644 solution to achieve a perfect short-time highlight current waveform.

1. Using infrared mode to accelerate the current rising speed

In the traditional flash mode, the current slowly rises to the target value. This time is about 800us/0.5A step and cannot be changed.

Figure 1 Traditional flash mode

With infrared mode, the current is directly changed to the target value. Figure 2 is the target waveform diagram that can be achieved in the IR mode, and Figure 3 is the specific register configuration.

Figure 2 Infrared mode (0.75A in 100us/ 16ms cycle)

Figure 3 Detailed register configuration

2. Use bypass mode to reduce the response time of the boost circuit

Industrial equipment is often powered by battery equipment, and the battery voltage may be used to a lower voltage such as 3.7V (4.2V battery). When VLED=3.4V, VOUT=VLED+VHR may be greater than VIN at this time, and the chip works in boost mode. While in normal 4.2V to 3.7V, the chip is in bypass mode.

Figure 4 Current waveforms under different voltages and modes

Table 1 Rise time of different modes and VIN

Therefore, it is recommended to use the bypassed IR mode to ensure a perfect current waveform when running on battery power, even if it sacrifices a little current accuracy.

3. Use appropriate input and output capacitors to reduce input sag

In traditional circuits, a buffer capacitor of about 300uF is often used to provide a short-time high-brightness current at the moment of flashing. The LM3644 also needs this to reduce the voltage drop at the input and prevent the system from triggering under-voltage protection. However, it is necessary to take into account the stability of the input Inrush and the circuit. Increasing the output capacitor is beneficial to provide LED current more quickly and reduce the drop, but if it is too large, it will cause the chip to start short-circuit protection when it is powered on, and it is not easy to be too large. Increasing the input capacitance can also reduce the drop, but it will increase the inrush current of the power-on, so it needs a comprehensive balance consideration. The experimental test of CIN=220uF; COUT=100uF has a good effect, which can greatly reduce the input voltage drop.

Figure 5 Actual test circuit

Figure 6 VIN drops 0.52V without matching capacitor Figure 7 VIN drops 0.12V with matching capacitor

4. Overview

LM364x can realize fast and high-brightness current driving very well, and can get a good current waveform with IR and Pass mode, and at the same time, it adopts the method of distributing buffer capacitors to effectively reduce the input voltage drop.