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TPS61151DRCR Fiches technique(PDF) 11 Page - Texas Instruments |
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TPS61151DRCR Fiches technique(HTML) 11 Page - Texas Instruments |
11 / 26 page ISET Q1 R1 RISET ON/OFF Logic P IN LIM OUT_MAX IOUT I V I 2 I V § · u u K ¨ ¸ © ¹ P S IOUT F IN IN 1 I 1 1 L F V V V V ª º § · u u « » ¨ ¸ « » © ¹ ¬ ¼ TPS61150, TPS61151 www.ti.com SLVS625E – FEBRUARY 2006 – REVISED NOVEMBER 2015 9.2.2 Detailed Design Procedure 9.2.2.1 Maximum Output Current The overcurrent limit in a boost converter limits the maximum input current (and thus the maximum input power) for a given input voltage. Maximum output power is less than the maximum input power because of power conversion losses. Therefore, the current limit, input voltage, output voltage, and efficiency can all change maximum current output. Because current limit clamps peak inductor current, ripple must be subtracted to derive the maximum DC current. The ripple current is a function of switching frequency, inductor value, and duty cycle. Equation 2 and Equation 3 take all of the above factors into account for maximum output current calculation. Where: • IP = inductor peak to peak ripple • L = inductor value • VF = power diode forward voltage • FS = switching frequency • VIOUT = boost output voltage. It is equal to 330 mV + voltage drop across WLED. (2) Where: • IOUT_MAX = maximum output current of the boost converter • ILIM = overcurrent limit • η = efficiency (3) To keep a tight range on the overcurrent limit, the TPS6115x uses the VIN and IOUT pin voltages to compensate for the overcurrent limit variation caused by the slope compensation. However, the current threshold still has a residual dependency on the VIN and IOUT voltages. Use Figure 1 and Figure 2 to identify the typical overcurrent limit in a specific application, and use a ±25% tolerance to account for temperature dependency and process variations. The maximum output current can also be limited by the current capability of the current-sink circuitry. It is designed to provide a maximum 35-mA current regardless of the current capability of the boost converter. 9.2.2.2 WLED Brightness Dimming There are three ways to change the output current on the fly for WLED dimming. The first method parallels an additional resistor with the ISET pin resistor as shown in Figure 9. The switch (Q1) can change the ISET pin resistance, and therefore modify the output current. This method is very simple, but can provide only limited dimming steps. Figure 9. Switching In or Out With an Additional Resistor to Change Output Current Copyright © 2006–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: TPS61150 TPS61151 |
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