TPS6106X is a perfect white LED lights driver series. It uses synchronous rectification topology, does not require the traditional white drive external connected rectifier diodes, which can save PCB board space. It supports digital dimming signal completion of the operation of the dimming, the dimming signal can maintain a constant high, and stored dimming information, this function can solve the EMI problems in the conventional PWM mode dimming . In addition, only one GPIO pins can be controlled TPS6106X dimming and shutdown, start function, thus saving the CPU GPIO resources. This article will discuss how to use TPS6106X converter digital dimming application design.
Digital dimming light operation
First, follow the instructions to write the software code of the digital dimming signal, the corresponding brightness adjustment signal sent by the CPU in the phone. But to note is that the signal must meet the basic requirements of the time step for each level of brightness enhancement for 1us to 75us “and” levels of brightness weakening step 180us to 300us of. Each level “step” is required between the duration of greater than 1.5us (td) high, separated, in order to distinguish the series.
Working mode of application design
Shutdown chip operating the ILED with EN common endpoints exert a constant “low” to turn off the chip.
(2) digital dimming dimming operation is complete, you need to keep the luminance signal is high, and store the brightness information. Accordance with the manual, asked to join a luminance signal and luminance signal ILED pin into the chip to complete dimming operation, since the luminance signal must remain high. This is because the ILED pin only to maintain the “high”, the digital dimming function will be enabled, dimming information will be saved. Otherwise, if the revocation of the high, the chip will shut off the dimming feature and lost the dimming information, enabling the chip to automatically return to the FB pin voltage value of 0.5V the state, whether ILED what level, FB pin voltage of 0.5V, output current 0.5/Rs. The brightness setting is stored in the chip, when again the operation of the dimming, just enter the new digital luminance signal. Please note: each dimming operation, the need to maintain the luminance signal for high-potential, in order to ensure that the digital dimming feature is “enabled” state.
WM dimming
TPS6106X chip also supports PWM dimming operation, but the circuit diagram needs to be done to improve the: ILED direct connection to the Vin side EN pin is directly connected to the PWM signal, the frequency of the PWM signal is not more than 5kHz (marked on the manual is typically 1kHz) and then proceed to pulse width modulation. D = Ton / T, which Ton frequency cycle in the “high” part.
Quick calculation of the power inductor design
Power inductor design is usually plagued by design engineers, following the design method not only applies to the application of TPS6106X, also applies to all series of white drive applications.
The inductance value is usually selected 4.7uH 6.8uH, 10uH, 22uH, can be selected according to the manual of recommended values. Inductor type selected inductor with magnetically shielded to reduce EMI. White drive current is usually relatively small, basic maintenance at 20mA or so, for the design engineer is very easy to overlook the choice of the inductor current, that just any 40 ~ 50mA inductance to meet requirements. In fact, this practice is very dangerous, because it will allow the inductor saturation converter failure, and even lead converter burned. In accordance with the following formula to calculate the minimum inductor current ILmin:
ILmin = (1.5 ~ 2) × Io / (1-D)
Where, D is the duty cycle, D = (Vo-Vin) / Vo,; Vin is equal to a single 4.2V lithium battery voltage typ 3.6V; Vo, the highest output voltage, if the series of four white LED tubes, each just voltage drop of 3.5V, the total pressure drop is equal to 14V, Vo at least 15V.