Introduction
Hey friends, in this article, I'll try to give you a pretty good, repair-friendly understanding of the LED TV's Logic Level Shifter circuit. So, if you want to learn, you've got to read! Please read this completely with patience, and you can watch the video below for learning purposes. Previously, through my videos and articles, I've given you some basic information about the internal structure of an LED TV's TFT panel. From that, many of you have learned that to display an image on an LCD panel, it contains millions of TFTs (Thin Film Transistors), three times that amount of Liquid Crystal cells, and an RGB filter composed of millions of RGB particles. This Level Shifter circuit controls the intensity and timing of light and shadow in those bright, dim, and shaded RGB pixels to bring images to life on the screen.
The "no image" or "black screen" problem in LCD TVs is a common challenge for technicians. Often, to solve this problem, many rush to cut CKV (Clock Vertical) tracks. However, many times the root cause of this problem lies in a crucial component: the Logic Level Shifter circuit. In today's post, we'll learn in detail what a Logic Level Shifter is, what its function is in an LCD panel, its common problems, and how it can solve the "no image" problem without cutting CKV (CLK) tracks.
What is a Logic Level Shifter Circuit?
In simple terms, a Logic Level Shifter is a sophisticated electronic circuit that helps establish successful communication between electronic signals or data at two different voltage levels. You can think of it as a "voltage translator."
In an LCD TV, digital control signals coming from the main processor or T-Con board are typically at low voltages (e.g., 3.3V). The LCD panel's gate driver ICs, which turn the display's pixels on and off, require much higher positive (VGH) and negative (VGL) voltage pulses to perform this task. The Logic Level Shifter bridges these two different voltage domains.
Input Voltages of This Logic Level Shifter IC and its Schematic Circuit Diagram (Practical Insight):
Please note the schematic circuit diagram below. It uses a widely common Logic Level Shifter SMD IC in BOE LCD panels, with the number 1H=5J. Pins 2 and 25 of this IC are designated as VDD1 and VDD2, respectively, where 3.3 volts are input as the operating voltage. Pin 7 supplies voltage to the IC's internal MOSFET source terminals, and its Pin 9 is for the ground (negative) connection.
Pay special attention: Pins 10 and 11 of this Logic Level Shifter IC directly receive VGL (-5V) voltage as input from the DC to DC IC 6861AAQ Circuit. This negative voltage is used to generate the "off" command pulse for the display's pixels.
Furthermore, 26 volts are input to Pin 8 of this IC from VGH. This high positive voltage is used to generate the "on" command pulse for the gate drivers' pixels.
Using these input voltages, the Logic Level Shifter converts the low-voltage timing signals coming from the T-Con into high-swing pulses required by the panel's gate drivers.
The schematic circuit diagram of the 1H=5J Logic Level Shifter IC is published below, which can fulfill the complete requirement for the 1H=5J IC's pinout and datasheet.
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| Figure 1: The 1H=5J Logic Level Shifter IC schematic circuit diagram. This diagram, meticulously reverse-engineered from a Samsung LED TV board, provides crucial pinout and connection details, serving as a comprehensive alternative to an official datasheet. |
Level shifter voltage details of BOE panel HV236WHB-N41
CLK1: 5.7V
CLK2: 5.7V
CLK3: 5.7V
CLK4: 5.7V
STV: -8V
VMID: 7.8V
VSS: -8V
VDD1- 26V and -8V (Sequential)
VDD2: -8V and 26V (Sequential)
VMID: 7.4V
VGH: 26V
VGL: -8V
What is the Function of a Logic Level Shifter in LCD/LED TV Panels?
The main functions of the Logic Level Shifter in an LCD TV panel are:
Voltage Signal Conversion: The low-voltage digital timing and control signals (such as CLK, STV pulses generated by the T-Con IC) coming from the T-Con IC cannot be directly applied to the panel's gate driver ICs. The Logic Level Shifter converts these signal voltage levels into high-swing pulses by swinging them within or between the VGH (Gate High) and VGL (Gate Low) voltage ranges, as required by the gate drivers.
Precise Timing Pulse Delivery: These converted pulses (such as CLK1, CLK2, CLK3, CLK4, STV1, STV2) directly enter the gate driver ICs via the COF (Chip On Film) on the panel. These pulses maintain extremely precise timing, helping to turn the pixels on or off at the correct moment. According to our observation, these CLK voltages can range from +3V to +8V or -3V to -8V in different TV models, depending on the specific requirements of the panel.
Pixel Control and Image Generation: The gate drivers use these high-swing pulses to activate each pixel on the panel row by row. This process creates a coherent and clear image on the screen.
Common Faults and "No Image" Solution
If the Logic Level Shifter IC itself is faulty, or more commonly, if its associated critical filter capacitors (especially those connected from the input voltage pins to ground) become shorted, the IC will not receive the necessary VGH, VGL, or operating voltages to function correctly. As a result:
* The Logic Level Shifter will fail to generate the required CLK, STV, or other output pulses.
* The gate driver ICs will not receive proper timing signals.
* Consequently, no image will be displayed on the TV screen; even if the backlight is on, the screen will appear black or blank.
A Simpler Solution Without Cutting CKV in Some Cases:
Often, the signals output from the Level Shifter in BOE panels are expressed as "CLK," which is analogous to CKV. When the backlight is working but there's no picture on the screen or a black screen issue, many technicians fail to repair the display by cutting the CKV (CLK) lines without checking other potential circuit faults.
Since this problem often occurs due to shorted filter capacitors in the path of the Logic Level Shifter's input voltages, these capacitors should be checked before resorting to risky steps like cutting CKV tracks. If any capacitor is found to be shorted, simply replacing it can often restore the picture on the TV screen, which is a much easier and safer solution than cutting CKV tracks.
I've brilliantly demonstrated what needs to be done in the Level Shifter circuit before cutting CKV or CLK tracks to solve black screen or no display issues in the video tutorial below. The video is [Link to your video here, if applicable].
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