Cold Cathode Electroluminescent Lamp Controller OZ9938. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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The OZ9938 chip from O2Micro is a cold cathode electroluminescent lamp (CCFL) controller, on the basis of which it is possible to implement a power supply for the backlights of LCD TVs and monitors. A circuit based on such a controller converts a constant unregulated voltage into an alternating high-frequency quasi-sinusoidal voltage. The architecture of the OZ9938 chip is shown in fig. 1.

Rice. 1. Architecture of the OZ9938 chip

The controller generates two anti-phase control signals for the converter based on MOSFET transistors. The PWM controller provides a "soft" start of the circuit, stabilization of the output current and voltage of the converter, protection (current and overvoltage), high load capacity of the output drivers and multifunctional backlight brightness control (both by built-in or external PWM signals, and by an external analog signal). Options for the OZ9938 chip are shown in Table 1.

Table 1. Options for the OZ9938 chip

The main electrical parameters of the IC are shown in Table 2.

Table 2. Main electrical parameters of IC OZ9938

The pin assignment of the OZ9938 chip is shown in Table 3.

Table 3. Pin assignment of the OZ9938 chip

A typical switching circuit of the considered controller is shown in fig. 2.

Rice. 2. Schematic diagram of the CCFL power inverter (click to enlarge)

Transistor assemblies U2, U1 (MOS transistors with n-channels) are connected to the outputs of the U15 microcircuit (pin 1, 3), the transistors in each channel are connected in a push-pull circuit. The transistors are loaded by half of the primary windings of the pulse transformers T1, T2, the middle point of the windings is connected to the inverter power supply VIN. The supply voltage of the microcircuit is 5 V (pin 2) is formed by the stabilizer Q1 ZD1.

The inverter is turned on by the ENA signal (pin 6 CN2) generated by the control microcontroller, a high level signal (more than 2 V) is applied to the enable input of the ENA controller (pin 10).

The voltage across the capacitor C13 connected to the pin. 12, gradually increasing. It determines the power transmitted through the transformers in the CCFL and thus prevents inrush currents in the lamps (soft start).

The lamp ignition time is set by the R16 C17 circuit connected to the pin. 3. In this mode, the control PWM frequency is increased relative to the operating mode and is approximately 68 kHz. It is determined by the values ​​of the elements R8, C9. When the lamps are lit and the voltage on the pin. 5 U2 is at least 0,7 V, the circuit enters the operating mode, in which the PWM frequency is reduced to approximately 52 kHz. In this mode, the voltage on the lamps is approximately 450 ... 500 V at a current of 6 ... 7 mA. The CCFL current is controlled by a feedback circuit, which generates a signal on the pin. 5 chips (ISEN). This sets the duty cycle of the output stages that control push-pull MOS transistor circuits in assemblies.

If one of the CCFLs is destroyed or the contact in its connector is broken (disconnected), the voltage at the pin. 12 U2 is growing fast. When its level reaches 2,5 V, the timer (pin 3) is turned on, the current of which charges the capacitor C17, which determines the delay time for turning off the controller (approximately 0,5 s). When it reaches a level of 3 V, the controller outputs turn off. To turn on the controller again, it is necessary to initialize its power supply (pin 2) or the INA signal (pin 10).

The overvoltage protection and current protection circuit as part of U2 monitors the signal to the pin. 6. When the lamps are turned off (destroyed, open circuit), the output voltage on the corresponding secondary winding of the transformer increases, from the dividers (see Fig. 2), the signal is applied to the pin. 6. As soon as its level exceeds a certain one (set by the divider R13 R19 on pin 7, OVP), with the same delay as in the previous case, the controller turns off.

To adjust the brightness, the DIM input is used (pin 4). To select the backlight brightness adjustment mode, pin is used. 11. If the voltage on it is more than 3 V, then the analog dimming mode is selected. The voltage at the DIM input can vary in the range of 0,5 ... 1,25 V, the lower level corresponds to the minimum brightness of CCFL, and the upper one corresponds to the maximum. The brightness is regulated by the internal PWM, if to the pin. 11 connected RC circuit, in this case - R8 C14. The frequency of the internal PWM is hundreds of Hz and is determined by the formula:

F Losc = 3102/RLCT[MOhm]xCLCT[nF] [Hz]

The maximum brightness corresponds to a voltage of 1,5 V per pin. 4U2, and the minimum is 0%.

The mode for adjusting the brightness of the backlight with an external PWM signal is set by a voltage level of 0,5.1 V per pin. 11 using a resistor divider. In this case, the PWM control signal is applied to the DIM input.

Literature

1. O2Micro. OZ9938 datasheet, 2005.

Author: Nikolay Elagin

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