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Backlight PWM control circuit with 4 current levels Disclosure Number: IPCOM000010466D
Publication Date: 2002-Dec-05
Document File: 1 page(s) / 37K

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Backlight PWM control circuit with 4 current levels

New generation monitors and TV sets will be flat panel displays using LCD technology preferably lit by LEDs.

The described circuit is a suitable control circuit for controlling a switch mode power supply operating LEDs of such display.

Description of the circuit.

The combined circuit of Block I and Block II in figure 1 shows an oscillator circuit that can be used as a building block in a control loop for e.g., current, voltage, light intensity. Block I shows a saw‑tooth waveform generator.

Block II receives two inputs from e.g., a digital controller. The amplitude of these inputs is e.g. 5V. The pulse widths of these two signals (in this application) are identical. The two voltages are added up at the comparator stage U1B. The resulting voltage is used as set-point for e.g., an LED current amplitude reference. By choosing the resistor values like in figure 1, it is possible to generate four values for the amplitude, as shown in the table of figure 1. The output “gate" then generates a drive signal for an HF switch in a Switch Mode Power Supply. Simultaneously the pulse width of this transistor drive circuit is modulated by the pulse width of the signal PWM1 and PWM2.

Thus a very cost efficient way for a combined 2‑bit AM/P\VM control Is realized, The PWM resolution is determined externally by the resolution of the PWM source. The choice of the resistors R7 and R8 and the timing of the signals PWM1 and PWM2 determine t...