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TS19452 Scheda tecnica(PDF) 3 Page - Taiwan Semiconductor Company, Ltd |
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TS19452 Scheda tecnica(HTML) 3 Page - Taiwan Semiconductor Company, Ltd |
3 / 7 page TS19452 AC/DC WLED Driver with Internal MOSFET Universal Switch Mode 3/7 Version: A12 Function Description The TS19452 is a PWM peak current controller for controlling a buck converter topology in continuous conduction mode (CCM). The output current is internally preset at 100mA When the input voltage of 20~400V appears at the Drain pin, the internal high-voltage linear regulator seeks to maintain a voltage of 7.5VDC at the VDD pin. Until this voltage exceeds the internally programmed under-voltage threshold, the output switching MOSFET is non-conductive. When the threshold is exceeded, the MOSFET turns on. The input current begins to flow into the Drain pin. Hysteresis in provided in the under-voltage comparator to prevent oscillation. When the input current exceeds the internal preset level, a current sense comparator resets an RS flip-flop, and the MOSFE turns off, At the same time, a one-shot circuit is activated that determines the duration of the off-state (12µS typ.) As soon as this time is over, the flip-flop sets again. The new switching cycle begins. A “blanking” delay of 300nS is provided that presents false triggering of the current sense comparator due to the leading edge spike caused by circuit parasitic. Application Information The TS19452 is specifically designed for driving multi LED strings. It can be operated from either universal AC line range of 85~264VAC or 20~400VDC, and drives up to tens of high brightness LEDs. All LEDs can be run in series and this device regulates at constant current, yielding uniform illumination. Selecting L1 and D1 There is a certain trade-off to be considered between optimal sizing of the output inductor L1 and the tolerated output current ripple. The required value of L1 is inversely proportional to the ripple current ∆IO in it. L1 = (VO x TOFF) / ∆IO VO is the forward voltage of the LED string. TOFF is the off-time of the TS19452. The output current in the LED string (IO) is calculated as: IO = IH – (∆IO / 2) Where ITH is the current sense comparator threshold. The ripple current introduces a peak-to-average error in the output current setting that needs to be accounted for. Due to constant off-time control technique used in the TS19452, the ripple current is independent of the input AC or DC line voltage variation. Therefore, the output current will remain unaffected by the varying input voltage. According a filter capacitor across the LED string can reduce the output current ripple even further, thus permitting a reduced value of L1. However, one must keep in mind that the peak-to-average current error is affected by the variation of TOFF. Therefore, the initial output current accuracy might be sacrificed at large ripple current in L1. Another important aspect of designing an LED driver with TS19452 is related to certain parasitic elements of the circuit, including distributed coil capacitance of L1, junction capacitance and reverse recover of the rectifier diode D1, capacitance of the PCB traces CPCB and output capacitance CDRAIN of the controller itself. These parasitic elements affect the efficiency of the switching converter and could potentially cause false triggering of the current sense comparator if not properly managed. Minimizing these parasitic is essential for efficient and reliable operation of the TS19452. Coil capacitance of inductors is typically provided in the manufacturer’s data books either directly or in terms of the self-resonant frequency (SRF). SRF = 1 / (2π · √ (L · CL)) |
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