ブログ について What Does a 2.4-Inch High Brightness 1000nits TFT LCD Actually Demand from Your Power and Interface Budget?

Most design reviews for a small high-brightness display start with a block diagram and a list of questions. This one starts with a blank calculation sheet. The panel is the SFTC240H-7171AN from Saef Technology Limited: 240 × 320 IPS, 8-bit MCU interface, 6.2 V / 80 mA backlight, 1000 cd/m² typical. The goal is not to describe the module. It is to work out what the module demands from the power tree, the MCU bus, the thermal path, and the touch interface before the schematic is drawn. Each section below is a calculation with stated assumptions, a result, and the design consequence.

Calculation 1: Backlight boost converter duty cycle and inductor

The backlight is a white-LED string with a typical forward voltage of 6.2 V at 80 mA. The input rail is assumed to be 5 V, which is common in USB-powered and industrial control products.

Assumptions:

• VIN = 5.0 V (minimum 4.75 V)

• VF = 6.2 V typical, 6.6 V at cold start

• ILED = 80 mA

• Switching frequency fSW = 1.0 MHz

• Boost diode forward voltage VD = 0.4 V

• Target ripple current ΔIL = 30% of ILED = 24 mA

Duty cycle at minimum input:

D = 1 − (VIN / (VF + VD)) = 1 − (4.75 / 6.6) = 0.28

Inductor value:

L = (VIN × D) / (fSW × ΔIL) = (4.75 × 0.28) / (1e6 × 0.024) = 55.4 µH

Design consequence: A 47 µH inductor is the nearest standard value and gives slightly higher ripple, which is acceptable. The peak switch current is ILED + ΔIL/2 = 92 mA. A 1 A, 40 V Schottky diode and a boost converter rated for at least 200 mA switch current are sufficient. Do not use a series resistor: at 6.2 V and 80 mA, the backlight dissipates 496 mW, and a resistor would waste additional power and produce inconsistent brightness as the LED forward voltage varies with temperature.

Calculation 2: 8-bit MCU bus frame time and achievable refresh rate

The display uses an 8-bit parallel interface with DB0–DB7. The frame size at 16-bit color is:

Frame size = 240 × 320 × 2 bytes = 153,600 bytes

With an 8-bit bus, each byte requires one write cycle. The datasheet’s minimum write cycle time is 66 ns, but the host MCU’s GPIO toggle rate and bus overhead usually dominate.

Assumptions:

• Host writes one byte per bus cycle

• Effective bus cycle = 200 ns (5 MHz equivalent, realistic for GPIO bit-banging or a slow external bus)

• Command overhead = 5% of frame time

Frame time:

tframe = 153,600 × 200 ns × 1.05 = 32.3 ms

Achievable refresh rate:

frefresh = 1 / 0.0323 = 31 Hz

Design consequence: A 2.4-inch high brightness 1000nits TFT LCD driven by an 8-bit bus at 5 MHz equivalent can sustain about 31 full-frame updates per second. That is adequate for status displays and slow-changing instruments, but not for scrolling text or animated icons. If the UI needs 60 Hz, the host must either use a faster bus (16-bit parallel or SPI at 20 MHz+) or update only the changed region. The controller’s address-window commands make partial updates practical; design the UI around them rather than around full-frame redraws.

Calculation 3: Thermal rise from the backlight in a sealed enclosure

The backlight dissipates approximately 496 mW. In a sealed plastic enclosure, that heat spreads through the display stack and the surrounding air.

Assumptions:

• Backlight power PBL = 0.496 W

• Display module area A = 42.72 mm × 60.26 mm = 2,574 mm² = 0.002574 m²

• Effective thermal resistance from LED junction to ambient, Rth = 80°C/W (typical for a small edge-lit module in a sealed plastic enclosure with no thermal pad)

• Ambient temperature TA = 50°C (inside a cabinet)

Junction temperature rise:

ΔT = PBL × Rth = 0.496 × 80 = 39.7°C

Estimated LED junction temperature:

TJ = 50 + 39.7 = 89.7°C

Design consequence: The datasheet specifies a 20,000-hour lifetime to 50% brightness at 25°C. At an estimated junction temperature near 90°C, the LED lifetime drops substantially. The practical fixes are: add a thermal pad or metal bracket from the LED bar to the enclosure, reduce maximum brightness when the ambient temperature exceeds 40°C, or both. A 2.4-inch high brightness 1000nits TFT LCD running at full brightness in a sealed 50°C cabinet is not a 20,000-hour design without a thermal path.

Calculation 4: Touch ADC resolution and coordinate error

The module includes 4-wire resistive touch pins (XR, YD, XL, YU). The touch position is read by driving one pair of lines and measuring the voltage on the other pair with the host ADC.

Assumptions:

• ADC reference = 3.3 V

• ADC resolution = 12 bits (4,096 counts)

• Touch panel X-axis resistance RX = 500 Ω typical (datasheet range 150–500 Ω)

• Touch panel Y-axis resistance RY = 800 Ω typical (datasheet range 200–800 Ω)

• ADC input impedance >> panel resistance

Voltage per ADC count:

VLSB = 3.3 V / 4,096 = 0.806 mV

X-axis position error from one ADC count:

ΔX = (screen width / ADC counts) = 36.72 mm / 4,096 = 0.009 mm

Design consequence: A 12-bit ADC gives sub-0.01 mm theoretical resolution, which is far finer than the touch panel’s ±1.5% linearity. The limiting factor is not ADC resolution but noise, contact resistance, and mechanical stack. Use a median filter of 5–7 samples and a debounce time of 20 ms (the datasheet’s maximum chattering time). Calibrate the touch at multiple points with the final cover film, because the panel’s linearity changes when the bezel applies pressure to the touch film.

Calculation 5: Total power budget and supply sizing

Assumptions:

• Logic supply VDD = 3.3 V

• Logic current IDD = 8 mA (estimated; the datasheet lists TBD, so measure the first prototype)

• IOVCC current = 2 mA (estimated)

• Backlight power = 496 mW

• Touch panel current = negligible (resistive, no active controller)

• Boost converter efficiency η = 85%

Logic power:

Plogic = 3.3 × (8 mA + 2 mA) = 33 mW

Backlight input power from 5 V rail:

PBL,in = 496 mW / 0.85 = 583 mW

Total input power:

Ptotal = 33 mW + 583 mW = 616 mW

5 V rail current:

I5V = 616 mW / 5 V = 123 mA

Design consequence: A 5 V rail rated for at least 250 mA gives 2× margin for startup and brightness transients. The 3.3 V rail needs at least 30 mA. If the product is battery-powered, the backlight dominates the budget: dimming to 50% brightness reduces the backlight power to roughly 250 mW and the total to about 310 mW, nearly doubling runtime. A 2.4-inch high brightness 1000nits TFT LCD should have a firmware brightness policy, not a fixed 100% setting.

What the calculations change in the schematic

The calculation sheet produces four concrete schematic decisions. First, the backlight driver is a 1 MHz boost with a 47 µH inductor and a 1 A Schottky diode, not a series resistor. Second, the host interface budget must support partial updates; if the MCU cannot sustain 31 Hz full-frame, the UI must be designed around address-window writes. Third, the thermal path from the LED bar to the enclosure is a requirement, not an option, and the firmware should derate brightness above 40°C ambient. Fourth, the touch ADC needs a median filter and a calibration routine, and the touch stack should be defined before the bezel drawing is released.

The full pin definitions, timing table, and backlight specifications are in the SFTC240H-7171AN Datasheet 2.4 inch high brightness 1000nits TFT LCD.pdf. Place this link in the power-budget section of the product page, with the anchor text above. For a design review, bring the calculation sheet to the engineering team at Saef Technology Limited along with the enclosure thermal data and the host interface plan. The team can confirm the module configuration and discuss the custom touch or cover glass options before the bezel is frozen.