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TS1520 Folha de dados(PDF) 4 Page - Taiwan Semiconductor Company, Ltd |
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TS1520 Folha de dados(HTML) 4 Page - Taiwan Semiconductor Company, Ltd |
4 / 12 page TS1520 Step-Up DC-DC Converter, White LED Driver 4/12 Version: B07 Application Information Operation The TS1520 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. Inrush Current The maximum switch current is limited to about 900mA by the chip Typically, this large current will occur for about 40us during start up. 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 maximum LED current set initially can be reduced by pulse width modulating the CTRL voltage from 0.4V to 1.8V. A better approach is to adjust the feedback voltage for dimming control. Either a DC level signal or a filtered PWM signal can be used to control the LED current as illustrated in Figure 1 and Figure 2 respectively. Using the above different scheme, the LED current can be controlled from 0% to 100% to its maximum value. Figure 1. Dimming Control Using a DC Voltage Figure 2. Dimming Control Using a Filtered PWM Signal (1kHz, 2Vp-p, duty 20%~80%) Open Circuit Protection The TS1520 has an internal open-circuit protection circuit. When the LEDs are disconnected from the circuit or fail open, the TS1520 will shutdown automatically until input condition changes to bring it out of the shutdown mode. Inductor Selection A 47uH inductor is recommended for most applications to drive more than 10 LEDs in serials and 33Uh inductor is recommended for drive 30 LEDs in serials/parallel. Although small size and high efficiency are major concerns, the inductor should have low core losses at 1MHz and low DCR (copper wire resistance). Diode Selection To maintain high efficiency, the average current rating of the Schottky diode should be large than the peak inductor current, IPK. Schottky diode with a low forward drop and fast switching speeds are ideal for increase efficiency in portable application. Choose a reverse breakdown of the Schottky diode large than the output voltage. |
Nº de peça semelhante - TS1520 |
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Descrição semelhante - TS1520 |
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