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TPS92074D Datasheet(PDF) 11 Page - Texas Instruments |
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TPS92074D Datasheet(HTML) 11 Page - Texas Instruments |
11 / 24 page ( ) IN BULK L LED LED LED ripple P C 4 f R V I ³ p ´ ´ ´ ´ TPS92074 www.ti.com SLUSBO7 – AUGUST 2013 For example, a resistor combination creating a dominant effect when the thermistor reaches the desired temperature and resistance can be incorporated by paralleling a thermistor and another resistor like R10 with the suggested On-Time clamp (see Figure 12. ). This circuit option creates a shorter on-time as the temperature increases, reducing the output current. The use of a thermistor (NTC or PTC) in these types of circuit implementations is simple and saves costly added circuitry and additional device pins. Overvoltage Protection (OVP) The implementation of overvoltage protection is simple and built-in if using a two-coil magnetic (coupled inductor) to derive VCC. If the LED string is opened the auxiliary VCC rises and reaches the VCC(OVP) trip point. This action disables and grounds the gate pin, preventing the converter from switching. The converter remains disabled until VCC drops 0.5 V after a 1 second time-out. If an inductor is used, implement other discrete circuits to disable the converter. Output Bulk Capacitor The required output bulk capacitor, CBULK, stores energy during the input voltage zero crossing interval and limits twice the line frequency ripple component flowing through the LEDs. Equation 1 describes the calculation of the of output capacitor value. where • RLED is the dynamic resistance of LED string • ILED(ripple) is the peak to peak LED ripple current • and fL is line frequency (1) Compute RLED as the difference in LED forward voltage divided by the difference in LED current for a given LED using the manufacturer’s VF vs. IF curve. For an initial estimate, a typical value of 0.25 Ω per LED can be used. More detail can be found in the Application Report Design Challenges of Switching LED Drivers (AN-1656). In typical applications, the solution size becomes a limiting factor and dictates the maximum dimensions of the bulk capacitor. When selecting an electrolytic capacitor, manufacturer recommended de-rating factors should be applied based on the worst case capacitor ripple current, output voltage and operating temperature to achieve the desired operating lifetime. It should also be a consideration to provide a minimum load at the output of the driver to discharge the capacitor after the power is switched off or during LED open circuit failures. Copyright © 2013, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: TPS92074 |
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