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LC5202D データシート(PDF) 11 Page - Sanken electric |
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LC5202D データシート(HTML) 11 Page - Sanken electric |
11 / 16 page 11 SANKEN ELECTRIC CO., LTD. LC5200-DS, Rev. 6.0 Off-Line LED Driver ICs LC5200 Series Comp2 compares VSEN and VREF, and when VREF < VSEN, Comp2 inverts its output (see point A in figure 4). This resets the output Q of the RS flip-flop. After a turn-off signal is transmitted from the AND gate to the logic, to the gate driver, and finally to the internal power MOSFET, the power MOSFET turns off. At the same time, an internal MOS switch, connected to the PWM pin, turns on, and CPWM is discharged. After a certain period, when the PWM pin voltage, VPWM , decreases to less than 2 V, the comparator for the off-time, Comp1, inverts and the Q output of the RS flip-flop is set. Then the MOS switch for discharging CPWM turns off, and CPWM is charged by the REF pin voltage through RPWM (connected between the REF and PWM pins). When VPWM increases to more than 3 V, the fixed off-time expires and the power MOSFET turns on. After that, the opera- tion reverts to the initial state, shown as point B in figure 4. VSEN is detected in the on-time period, except during the PWM blanking time, tBLK , in order to prevent malfunction. The fixed off-time is determined by RPWM and CPWM. Figure 5 shows PWM off-time curves based on various values of RPWM and CPWM . The recommended value for RPWM is 560 kΩ, and that for CPWM is 220 pF. In addition, the proper values for RPWM and CPWM are changeable according the load conditions of the LEDs used, and the value of RPWM also affects the losses in the internal regulator directly. It is necessary to pay attention to these factors. Current Value Setting for Dimming Control The LC5200 series allows constant current control using the internal PWM control, an external PWM signal, or a combination of both of them. Using Internal PWM Dimming The LC5200 series has a built-in PWM constant current control circuit, and thus can achieve a constant current drive system for the LED string, while requiring few external components. The peak output current, IPEAK , for driving the LED string is calcu- lated as follows: VREF RS IPEAK = VREG × R2 RS × ( R1 + R2 ) = (1) The LC5200 series allows external adjustment of the current flowing through the LED string, using either of the following methods: • Adjusting the analog voltage on the REF pin (figure 6A) • Inputting the analog voltage integrated PWM signal through a low pass filter, LPF, to the REF pin (figure 6B) However, the dimming control by adjusting the REF pin voltage can not set the output current to zero. When controlling to zero current, an external signal and external circuit are needed to set the PWM pin voltage to low. R1 R2 CPWM RS C2 C1 RPWM SEN GND REG PWM REF SEN GND REG PWM REF CPWM RS CLPF LC5200 RPWM PWM Signal LPF RLPF LC5200 50 200 R PWM Value for Setting Off-Time (Ω) 400 600 800 1000 20 10 5 2 1 C PWM = 470 pF C PWM = 220 pF C PWM = 100 pF C PWM = 47 pF C PWM = 22 pF C PWM = 10 pF Figure 5. PWM Off-Time versus RPWM Value, for Various Values of CPWM Figure 6. Dimming Application Circuits with Internal PWM Control (A) Analog Control (B) Integrated PWM Control |
同様の部品番号 - LC5202D |
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同様の説明 - LC5202D |
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