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After the remote control is turned on, the backlight circuit starts to work, and the 24V voltage is raised to the DC output voltage of 33V~50V, which supplies power to the 16-way LED backlight series, and adjusts and balances the backlight series current.
1. Protect the circuit
There is an under-voltage UVLO protection circuit inside the N701 pin 3, and 24V power supply is sent to the N701 pin 3 after partial voltage sampling of R710 and R712. When 24V voltage is normal, N701 works normally; When the 24V voltage is too low and the sampling voltage of pin 3 is below 1.25V, the N701 protection undervoltage protection circuit starts and N701 stops output excitation pulse.
, dimming circuit
The backlight driver circuit adopts PWM pulse digital dimming mode, and the mainboard circuit or external circuit controls and adjusts the N702 internal modulation driver pulse through the clock (SCK), data input\/output (MOSI\/ MISO) and slice selection signal (CSB) of the SPI communication interface circuit of N702. Then control the time ratio of the LED- lamp string to light up or off to adjust the brightness, to achieve the purpose of adjusting the backlight brightness.
Voltage regulation and synchronization circuit
The 33V~50V output voltage output by the boost output circuit is sampled by R729, R730, R738 and R731\/\/R732. The output voltage is not only sent to pin 14 of the boost drive circuit N701 to control the output voltage, but also sent to pin 3 of the regulating drive circuit N702 by R735 and R734. The N702 internal drive circuit is controlled to stabilize the output current and cooperate with the booster circuit to achieve the best matching state.
In addition, the VSYNC-LIKE frame synchronization pulse signal output by the control circuit of the motherboard is sent to pin 51 of the N702 to control the N702 converter circuit to work synchronously with the image, and adjust the LED lamp string current synchronously according to the brightness and darkness of the image to improve the contrast and effect of the image.
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The principle of the LCD TV backlight booster circuit is to convert the low-voltage DC power supply into a high-voltage DC power supply for the use of the backlight module. The specific principle is that the low-voltage DC voltage is amplified into a high-voltage DC voltage by the amplifier, the power supply is stabilized by the capacitor, and the backlight brightness is adjusted by the control circuit.
ILED backlight power supply booster current protection circuit. R922, R941, R921 are the current detection resistance in the boost circuit, when the circuit current is too large, the voltage drop on the detection resistance is also increased correspondingly, this voltage is sent to the ③ pin of N901, when the voltage is greater than 1V, the protection circuit in the chip action, reduce the duty cycle of the output PWM pulse wave, so that the output voltage is lower and the current is reduced.