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TPS61040 Datasheet(PDF) 12 Page - Texas Instruments |
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TPS61040 Datasheet(HTML) 12 Page - Texas Instruments |
12 / 22 page ![]() www.ti.com LINE AND LOAD REGULATION OUTPUT CAPACITOR SELECTION DVout + Iout Cout 1 fS(Iout) – I P L Vout ) Vd–Vin ) IP ESR (7) TPS61040 TPS61041 SLVS413C – OCTOBER 2002 – REVISED AUGUST 2007 The larger the feedforward capacitor the worse the line regulation of the device. Therefore, when concern for line regulation is paramount, the selected feedforward capacitor should be as small as possible. See the LINE AND LOAD REGULATION section for more information about line and load regulation. The line regulation of the TPS61040/41 depends on the voltage ripple on the feedback pin. Usually a 50 mV peak-to-peak voltage ripple on the feedback pin FB gives good results. Some applications require a very tight line regulation and can only allow a small change in output voltage over a certain input voltage range. If no feedforward capacitor CFF is used across the upper resistor of the voltage feedback divider, the device has the best line regulation. Without the feedforward capacitor the output voltage ripple is higher because the TPS61040/41 shows output voltage bursts instead of single pulses on the switch pin (SW), increasing the output voltage ripple. Increasing the output capacitor value reduces the output voltage ripple. If a larger output capacitor value is not an option, a feedforward capacitor CFF can be used as described in the previous section. The use of a feedforward capacitor increases the amount of voltage ripple present on the feedback pin (FB). The greater the voltage ripple on the feedback pin ( ≥50 mV), the worse the line regulation. There are two ways to improve the line regulation further: 1. Use a smaller inductor value to increase the switching frequency which will lower the output voltage ripple, as well as the voltage ripple on the feedback pin. 2. Add a small capacitor from the feedback pin (FB) to ground to reduce the voltage ripple on the feedback pin down to 50 mV again. As a starting point, the same capacitor value as selected for the feedforward capacitor CFF can be used. For best output voltage filtering, a low ESR output capacitor is recommended. Ceramic capacitors have a low ESR value but tantalum capacitors can be used as well, depending on the application. Assuming the converter does not show double pulses or pulse bursts on the switch node (SW), the output voltage ripple can be calculated as: where: IP = Peak current as described in the Peak Current Control section L = Selected inductor value Iout = Nominal load current fS (Iout) = Switching frequency at the nominal load current as calculated previously Vd = Rectifier diode forward voltage (typically 0.3 V) Cout = Selected output capacitor ESR = Output capacitor ESR value See Table 4 and the typical applications section for choosing the output capacitor. Table 4. Recommended Input and Output Capacitors DEVICE CAPACITOR VOLTAGE RATING COMPONENT SUPPLIER COMMENTS 4.7 μF/X5R/0805 6.3 V Tayo Yuden JMK212BY475MG CIN/COUT 10 μF/X5R/0805 6.3 V Tayo Yuden JMK212BJ106MG CIN/COUT TPS61040/41 1 μF/X7R/1206 25 V Tayo Yuden TMK316BJ105KL COUT 1 μF/X5R/1206 35 V Tayo Yuden GMK316BJ105KL COUT 4.7 μF/X5R/1210 25 V Tayo Yuden TMK325BJ475MG COUT 12 Submit Documentation Feedback Copyright © 2002–2007, Texas Instruments Incorporated Product Folder Link(s): TPS61040 TPS61041 |
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