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A look at TFT LCD bias supply and white LED driver

29 Sep 2015  | Eddy Wells

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As shown in figure 1, these outputs are sequenced with VOUT, VN, then VH powered, as required by most displays. The outputs are actively discharged when ELCD is brought low, removing voltage from the display. The white LED backlight for the figure 3 circuit consists of two strings with four series LEDs. The LEDs are driven from the high side with the LTC3524, allowing the strings to terminate at ground, reducing the number of wires required to power the display. With RPROG = 100k, each LED is regulated to 20mA. Maximum power for the backlight is approximately 600mW, assuming a forward voltage around 3.6V per element.


Figure 3: Complete TFT and LED Solution.


Dimming is achieved by changing the duty cycle of a 200Hz power signal applied to the LED strings. The frequency is high enough to prevent visually detectable flickering, but low enough to allow a better than 100:1 dimming range. Dimming is implemented by simply connecting a microprocessor controlled port to ELED1 and ELED2. Scope waveforms at 50% duty cycle are shown in figure 4.


Figure 4: Burst Dimming Waveforms.


Efficiency results for this design are given in figure 5 with a 3.6V input. The LCD efficiency curve shows the performance of the synchronous boost converter with VOUT at 5V and varying load current. This curve includes the no load quiescent current of the charge-pumps, which are powered from VOUT. Analog dimming of the LEDs can be implemented by adjusting the current through the PROG pin. Efficiency for analogue dimming is shown in figure 5. Efficiency with PWM dimming would remain close to 78% over a wide dimming range.

Efficiency results for this design are given in figure 5 with a 3.6V input. The LCD efficiency curve shows the performance of the synchronous boost converter with VOUT at 5V and varying load current. This curve includes the no load quiescent current of the charge-pumps, which are powered from VOUT.

Analog dimming of the LEDs can be implemented by adjusting the current through the PROG pin. Efficiency for analogue dimming is shown in figure 5. Efficiency with PWM dimming would remain close to 78% over a wide dimming range.

Figure 5: LCD Bias and LED Efficiency.


Conclusion
The LTC3524 shrinks and simplifies the design of small to medium sized TFT LCDs by combining the LCD supply and LED driver in a single compact package. LCD bias voltages and LED currents are programmable, making it possible to simplify parts stock by using the LTC3524 for a wide variety of displays.


About the author
David Ng is with Linear Technologies Corp.


Excerpted from Section 21 of "Analog Circuit Design: Volume 3—The Design Note Collection". Edited by Bob Dobkin & John Hamburger, Linear Technology Corporation, Milpitas, CA, USA . Published by Newnes, an imprint of Elsevier. 2015 Linear Technology Corporation. Published by Elsevier Inc. All rights reserved.


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