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YB1520 Datasheet(PDF) 7 Page - YOBON TECHNOLOGIES,INC. |
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YB1520 Datasheet(HTML) 7 Page - YOBON TECHNOLOGIES,INC. |
7 / 10 page YB1520 Step-up DC-DC Converter, White LED Driver YB1520 Rev.1.4 www.yobon.com.tw 7 Function Block Figure 3: YB1520 Block Diagram Operation The YB1520 uses a constant frequency, current mode control scheme to regulate the output LED current. Its operation can be understood by referring to the block diagram in Figure 3. At the start of each oscillator cycle, a voltage proportional to the switch current is added to a ramp output and the resulting sum is fed into the positive terminal of the PWM comparator. When this voltage exceeds the level of the comparator negative input, the peak current has been reached, and the SR latch is reset and turns off the power switch. The voltage at the negative input of the comparator comes from the output of the error amplifier. The error amplifier sets the correct peak current level to keep the output in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. Application Information Inrush Current The maximum switch current is limited to about 450mA by the chip. Typically, a larger COUT will increase the duration of high current. However, when VIN is first connected, an even larger current can flow into COUT. This occurs when the SW pin is open circuit and the COUT voltage changes from 0V to VIN in a few microseconds. Because the duration of this large current is short, it will usually not cause problems. If this current is not desirable, an external soft-start circuit can be added to significantly reduce this inrush current. LED Current and Dimming Control The LED’s current is set by a resistor connected at FB pin to GND using: The maximum LED current set initially can LED LED R mV I 200 = |
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