POWER IC REVIEWS
Power IC uses the input voltage of the system to generate 5 working voltages. The general external voltage is 3.3V for NB, 5V for Monitor, and 12V for TV
; Differential linear regulator (Low Dropout Voltage Regualtor, LDO circuit);
②AVDD: Gamma main voltage for pixel voltage on the data line, about 5~16V, usually boosted by Boost converter;
③VGH: used to open The maximum on-state voltage of TFT is about 20V, and the maximum can reach 40V. Generally, a positive charge pump is used for voltage conversion (Positive Charge Bump); ④VGL: The
maximum on-state voltage used to turn off the TFT is about -5V, and the maximum can reach -40V, generally use a negative charge pump for voltage conversion (Negative Charge Bump);
⑤VCOM: Common electrode voltage on the TFT substrate or CF substrate, between 5~6V, generally generated by a resistor divider circuit, divided from AVDD get.
VDD generation: LDO circuit
Working principle : According to the increase or decrease of the voltage difference between the PN poles of the differential amplifier, the increase or decrease of the current output by A is controlled, and the voltage drop of the NMOS is controlled to stabilize the output VDD.
Note : NMOS is equivalent to a variable resistor, filter, and capacitor filter at this time, blocking DC and traffic, which means that DC signals cannot pass, but AC signals can pass. Due to the unidirectional conductivity of the diode, the signal can only flow from the positive pole to the negative pole. The voltage divider resistor is 100K ohms. Use resistors R1 and R2 to control the maximum output current of the LDO:
AVDD Generation: Boost Converter
Basic working principle: In the open state, the NMOS is turned on, the inductor blocks the circuit current, and the internal current rises, which is similar to the battery storing electric energy; in the off state, the NMOS is disconnected, the inductor generates power, and the current flows to the RC load through the diode.
V_out/V_in=1/1−D, D represents the duty cycle of the switching state in one conversion cycle
IC working principle: IC adjusts the pulse width (PWM) to turn on the NMOS on the rising edge, and uses resistors R1 and R2 to control the LDO Maximum output current:
Note : NMOS is equivalent to a switch at this time, and the voltage dividing resistor R2 is generally 10K ohm
Boost converter basic circuit
NMOS on-state equivalent circuit NMOS
off-state equivalent circuit
Boost converter integrated IC
VGH and VGL generation: Charge Pump (now improved)
Basic working principle diagram of positive charge pump
NMOS and PMOS tube schematic diagram
Basic working principle : During a cycle, in the first half cycle, the input voltage Vin charges the flying capacitor Cf, the voltage rises to Vin, and the diode D1 conducts in the second half cycle, so that the voltage of Cf is changed from Vin rises to Vin+VA, and before D2 is turned off, the charge is transferred from Cf to the storage capacitor Co.
IC working principle : control by PMOS and NMOS, positive charge pump, NMOS is turned on in the first half cycle, NMOS is turned off in the second half cycle, PMOS is turned on, negative charge pump is opposite, to control the charge and discharge of Cf in the circuit. Generally, the reference voltage VREF is set to 1.25V, VGH is 20V, and the voltage dividing resistor R2 is generally 10K ohms.
VCOM Generation: Adjustable Resistor Divider Circuit
Basic working principle : Obtain from the voltage division of the AVDD circuit, respectively connect the fixed resistors R1 and R2 in series, and the adjustable resistor RV, and realize the acquisition and adjustment of the VCOM voltage by controlling RV.
Function : In TFT-LCD, in order to adjust the flicker of the product, you can try to adjust the voltage value of VCOM during the detection process.
