Tps6106x is a series of white LED drivers with perfect functions. It adopts the topology of synchronous rectification and does not need the rectifier diode connected outside the traditional white light driver, so it can save the space on the PCB board. It supports digital dimming signal. Once the dimming operation is completed, the dimming signal can maintain a high level and store dimming information. This function can solve the EMI problem caused by traditional PWM mode dimming. In addition, only one GPIO pin can control the tps6106x to realize the functions of dimming, turning off and starting, so it can save the GPIO resources of the CPU.

Digital dimming operation

The digital dimming operation can eliminate the EMI interference generated in the dimming process, and the functions shown in Figure 1 can be completed through only one GPIO control line.

First, write the software code of the digital dimming signal according to the description in Figure 1, and the CPU in the mobile phone sends the corresponding brightness adjustment signal. However, it should be noted that the signal must meet the basic requirements of “the time step of brightness enhancement at each stage is 1US to 75us” and “the time step of brightness attenuation at each stage is 180us to 300US”. High level separation with a duration greater than 1.5us (TD) is required between the “steps” of each stage to distinguish the stages.

Working mode application design

1. Turn off the chip by applying a constant “low level” at the common endpoint of iled and en.

2. After the digital dimming operation is completed, it is necessary to keep the brightness signal at high level and store the brightness adjustment information. Add a brightness signal according to the requirements of the manual. The brightness signal will enter the chip through the iled pin to complete the dimming operation. After that, the brightness signal must maintain a high level. This is because only when the iled pin remains “high”, the digital dimming function will be enabled and the dimming information will be saved. Otherwise, if the high level is cancelled, the chip will turn off the dimming function and lose the dimming information, so that the chip will automatically return to the state that the FB pin voltage value is 0.5V. No matter what level the iled is, the voltage on the FB pin is 0.5V and the output current is 0.5/rs. The brightness setting is saved in the chip. When the dimming operation needs to be performed again, only a new digital brightness signal needs to be input. Please note: after each dimming operation, it is necessary to keep the brightness signal high potential to ensure that the “digital dimming” function is in the “enable” state.

How to realize digital dimming using tps6106x white LED driver

 

PWM dimming

Tps6106x chip also supports PWM dimming operation, but the circuit diagram needs to be improved: iled is directly connected to VIN terminal, EN pin is directly connected to PWM signal, the frequency of PWM signal does not exceed 5KHz (the typical value indicated in the manual is 1kHz), and then pulse width modulation is carried out. D = ton / T, where ton is the “high level” part of the frequency cycle. Figs. 3 and 4 show a brightness enhancement adjustment signal and a brightness reduction adjustment signal, respectively.

Fast calculation and design of power inductance

The design of power inductor usually puzzles the design engineers. The following design method is not only applicable to the application of tps6106x, but also applicable to all series white light driver applications.

The inductance value is usually 4.7uh, 6.8uh, 10uh and 22uh, which can be selected according to the recommended values in the manual. Inductors with magnetic shielding function are selected to reduce EMI problems. The current of white light driver is usually small and basically maintained at about 20mA. Therefore, design engineers easily ignore the selection of inductance current and think that any 40 ~ 50mA inductance can meet the requirements. In fact, this method is very dangerous, because it will saturate the inductance, cause the converter to fail, and even cause the converter to burn out. The minimum inductive current ilmin can be calculated according to the following formula:

ILmin=(1.5~2) × Io/(1-D)

Where, D is the duty cycle, d = (VO VIN) / VO; Vin is equal to the typical value of the voltage of a single 4.2V lithium battery, 3.6V; VO is the highest output voltage. If four white LED tubes are connected in series and the voltage drop of each tube is 3.5V, the total voltage drop is equal to 14V, so VO is at least 15V.

Key points of PCB design

For the converter realized by switching power supply, PCB layout and wiring are important steps to ensure the correct operation of the converter. It is directly related to whether the converter can be truly realized, so it must be taken seriously (see the website of this journal for the specific PCB design diagram). The layout and wiring shall comply with the following rules:

1) All power lines (copper foil with large current flow and copper foil connected in series with inductors and capacitors of power devices) shall be widened as much as possible.

2) It is strictly forbidden to lay the ground wire into a ground wire ring. All power grounds must be grounded at a single point, or with the help of a large area of stratum (for example, there is one or more layers in a multilayer board), and then the power ground pin and signal ground pin on the chip are either directly connected to the stratum, or connected to the middle stratum through more than three vias.

3) There are pins for heat dissipation at the bottom of the chip, which should be connected with a large area of ground covered copper. At the same time, the surface copper foil and the middle large area copper foil should be connected with vias. This is not only conducive to the rapid heat dissipation of the chip, but also can greatly reduce the radiation effect of switching noise and improve the anti-interference ability of the chip.

4) All input power filter capacitors should be as close to the power input pin of the chip as possible; The output capacitance is close to the power output pin of the chip.

Source; International LED network

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