How to design a PCB for a 5 inch round TFT module?
Mechanical constraints and board shape
The first layer of complexity is mechanical. The PCB must match the display’s outer diameter, which is exactly 5 inches (127 mm). But you don’t want a perfect circle—you need a flat edge or a tab for the FPC connector, because the display’s cable exits from one side. In practice, you design the board as a circle with a 127 mm diameter, but you add a small rectangular protrusion (say 10 mm x 15 mm) where the FPC connector sits. This keeps the cable from bending sharply and reduces stress on the solder joints. The PCB thickness should be 1.6 mm standard, but if you’re using a high-brightness backlight that draws more than 200 mA, consider a 2.0 mm board for better thermal dissipation. The copper weight should be 1 oz for the outer layers, but for the power traces carrying the backlight current, bump it to 2 oz. Don’t forget to add four 3.2 mm mounting holes at 90-degree intervals, placed 5 mm from the edge, to align with the display’s bezel or housing.
Power supply design for the TFT and backlight
The TFT itself requires three main voltage rails: VCC (typically 3.3V for the logic), VCI (2.8V for the driver IC), and VDDIO (1.8V for the MIPI interface). The HX8399 driver IC in the 5 inch 1080x1080 round tft display needs a clean 1.8V supply with less than 50 mV ripple. Use a low-dropout regulator (LDO) like the TPS7A91 for the 1.8V rail, with a 10 µF ceramic output capacitor placed within 2 mm of the FPC connector pin. The 3.3V rail can come from a switching regulator (like the TPS62160) set to 3.3V, with a 4.7 µH inductor and 22 µF input/output caps. The backlight is a different beast: it’s a string of white LEDs, typically 6 to 8 LEDs in series, with a forward voltage around 3.0V each, so total voltage is 18V to 24V. You need a boost converter, like the MP3302, that can deliver 200 mA to 400 mA at 20V. The inductor should be 10 µH with a saturation current rating of 1.5A, and the output capacitor should be 10 µF rated at 35V. The PWM dimming pin should be connected to a dedicated GPIO on your microcontroller, with a 10 kΩ pull-up resistor to 3.3V. Place the boost converter at least 10 mm away from the MIPI traces to avoid switching noise coupling into the high-speed data lines.
MIPI DSI signal routing and impedance control
This is where most PCB designs fail. The MIPI DSI interface runs at 500 Mbps per lane for a 1080x1080 resolution at 60 Hz refresh rate. The four data lanes and one clock lane must be routed as differential pairs with a characteristic impedance of 100 ohms differential, 50 ohms single-ended. On a standard 4-layer PCB, you can achieve this with a trace width of 0.15 mm and a spacing of 0.15 mm between the pairs, with a ground plane on layer 2 directly underneath. The stackup should be: layer 1 (signal), layer 2 (ground), layer 3 (power), layer 4 (signal). Keep the MIPI traces as short as possible—ideally under 50 mm from the connector to the display’s FPC pins. Use a 0.5 mm pitch FPC connector, like the FH12-30S-0.5SH, and route the MIPI signals on the top layer with no vias. The clock lane must be at least 0.5 mm away from any data lane to minimize crosstalk. Add a 100 ohm termination resistor between the clock and each data lane pair, placed within 5 mm of the display connector. The HX8399 datasheet specifies a common-mode filter on each lane; use a CMF like the DLW21SN900SQ2, with a 90 ohm impedance at 100 MHz. The MIPI signals should be length-matched within 0.5 mm of each other—use a serpentine trace on the shorter lanes to equalize the delay.
Backlight LED driver circuit details
The backlight in the 5 inch 1080x1080 round tft display typically uses 6 LEDs in series, each with a forward current of 20 mA to 30 mA. The total current is 30 mA maximum, but you can drive it at 20 mA for longer life. The boost converter needs to regulate the current, not the voltage. Use a current-sense resistor of 10 ohms in series with the LED string, with the feedback pin of the boost converter connected to the resistor’s high side. The resistor value sets the current: I = Vref / R, where Vref is typically 0.2V. So for 20 mA, use a 10 ohm resistor (0.2V / 0.02A = 10 ohms). The resistor should be a 1% tolerance, 0805 package, with a power rating of 0.1W. The boost converter’s switching frequency is typically 1.2 MHz, so the inductor and capacitor values are small. Use a 10 µH inductor with a DCR of less than 0.1 ohm, like the XAL6060-103ME. The output diode should be a Schottky with a 40V reverse voltage rating, like the SS34. Place the entire backlight circuit on the bottom layer of the PCB, directly under the display, to keep the high-voltage traces short and away from the MIPI signals.
Grounding and decoupling strategies
A solid ground plane is non-negotiable. On a 4-layer board, use layer 2 as a continuous ground plane with no splits. The MIPI signals, power traces, and backlight circuits all return through this plane. For the TFT’s VCC and VCI pins, place a 100 nF ceramic capacitor (X7R, 0402) within 1 mm of each pin, plus a 10 µF bulk capacitor (0805, 6.3V) for each rail. The 1.8V rail needs a 1 µF capacitor in parallel with a 100 nF cap, placed right at the FPC connector. The backlight output capacitor (10 µF, 35V) should be within 5 mm of the boost converter’s output pin. Avoid running any ground traces in a star configuration; use a single ground plane and connect all component grounds with vias directly to it. For the FPC connector, add a ground pin on the PCB that connects to the display’s ground through the cable—this reduces EMI. The total decoupling capacitance for the TFT should be at least 22 µF per voltage rail, distributed across the board.
Thermal management and layout considerations
The round shape of the PCB means you have less area for heat dissipation compared to a rectangular board. The backlight boost converter and the LDOs generate heat, especially if the ambient temperature is above 40°C. The boost converter’s power dissipation is roughly (Vout - Vin) * Iout, which for a 20V output at 30 mA is about 0.5W. This is manageable, but you should add a thermal pad on the bottom layer under the boost converter IC, with a 2x2 array of 0.3 mm vias connecting to the ground plane. The LDO for the 1.8V rail dissipates (3.3V - 1.8V) * 50 mA = 0.075W, which is fine without a heatsink. The TFT itself generates heat from the backlight, but the display’s metal frame acts as a heatsink. On the PCB, keep the boost converter at least 15 mm away from the FPC connector to avoid heating the cable. If you’re using a plastic housing, add a 1 mm thick thermal pad between the boost converter and the housing to transfer heat out.
Connector selection and cable routing
The FPC connector is the most critical mechanical interface. Use a 0.5 mm pitch, 30-pin connector with a locking mechanism, like the Hirose FH12-30S-0.5SH. The connector should be positioned on the PCB so that the display’s cable exits straight out, not at an angle. The cable length from the display to the PCB is typically 30 mm to 50 mm—keep it as short as possible to reduce parasitic capacitance on the MIPI lines. The FPC cable’s impedance is not controlled, so you must rely on the PCB’s routing to maintain signal integrity. Avoid routing the cable near any switching power supplies or high-current traces. If you’re using a custom cable, specify a 0.5 mm pitch, 30-pin, 0.3 mm thick FPC with a 0.1 mm copper layer. The cable’s bend radius should be at least 5 mm to prevent cracking of the copper traces.
Testing and validation points
After the PCB is fabricated, you need to verify the MIPI signal quality. Use a differential probe and an oscilloscope with at least 1 GHz bandwidth. Check the eye diagram at the display connector: the eye opening should be at least 200 mV and the jitter less than 100 ps. The clock lane’s duty cycle should be 50% ±5%. For the backlight, measure the LED current with a precision resistor and a multimeter—it should be within 5% of the target. The boost converter’s output voltage ripple should be less than 50 mV peak-to-peak. If the ripple is higher, increase the output capacitor to 22 µF or add a small ferrite bead in series with the output. Also, measure the temperature of the boost converter after 30 minutes of operation at 25°C ambient—it should not exceed 60°C. If it does, increase the copper area around the IC or add a small heatsink.
Common pitfalls and how to avoid them
One frequent mistake is routing the MIPI signals through vias. Vias add inductance and capacitance, which degrade the signal. If you must use a via, keep it to one per lane and use a 0.3 mm via with a 0.2 mm drill. Another issue is placing the boost converter too close to the MIPI traces—the switching noise can couple into the data lines and cause display flickering. Keep a 10 mm clearance. Also, don’t forget to add a pull-up resistor on the MIPI’s TE (tearing effect) pin if the display uses it for synchronization. The HX8399 driver typically uses the TE pin to indicate when the display is ready for new data; connect it to a GPIO with a 10 kΩ pull-up to 1.8V. Finally, the PCB’s circular shape makes it tricky to panelize for manufacturing. Use a V-cut or tab routing with mouse bites to separate the boards. The panel should have at least 5 mm of spacing between each circular board to allow for the routing tool.
Bill of materials (BOM) considerations
For the 5 inch 1080x1080 round tft display, the BOM should include the following key components: the FPC connector (FH12-30S-0.5SH), the MIPI common-mode filters (DLW21SN900SQ2, 4 pieces), the boost converter (MP3302), the LDO for 1.8V (TPS7A91), the switching regulator for 3.3V (TPS62160), the inductor (10 µH, XAL6060-103ME), the Schottky diode (SS34), the current-sense resistor (10 ohms, 1%), and the decoupling capacitors (100 nF, 10 µF, 1 µF, 22 µF). The total component cost for the PCB is around $3 to $5 in volume, not including the display itself. The PCB fabrication cost for a 4-layer, 127 mm diameter board with ENIG finish is about $10 to $15 per board in low quantities. Use a 0.2 mm minimum trace width and 0.2 mm minimum spacing for the MIPI traces to keep the cost down.