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LM2825 Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM2825 Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 12 page Application Information (Continued) It is especially useful in situations where the input power source is limited in the amount of current it can deliver, since you avoid loading down this type of power supply. Under some operating conditions, a Soft-start capacitor is required for proper operation. Figure 5 indicates the input voltage and ambient temperature conditions for which a Soft-start capacitor may be required. This curve is typical for full guaranteed output current and can be used as a guideline. As the output current decreases, the operating area requiring a Soft-start capacitor de- creases. Capacitor values between 0.1 µF and 1 µF are rec- ommended. Tantalum or ceramic capacitors are appropriate for this application. INPUT CAPACITOR C IN: An optional input capacitor is required if the package is more than 6" away from the main filter or bypass capacitor. A low ESR aluminum or tantalum bypass capacitor is recom- mended between the input pin and ground to prevent large voltage transients from appearing at the input. In addition, to be conservative, the RMS current rating of the input capaci- tor should be selected to be at least 1⁄2 the DC load current. With a 1A load, a capacitor with a RMS current rating of at least 500 mA is recommended. The voltage rating should be approximately 1.25 times the maximum input voltage. With a nominal input voltage of 12V, an aluminum electrolytic capacitor (Panasonic HFQ series or Nichicon PL series or equivalent) with a voltage rating greater than 15V (1.25 x V IN) would be needed. Solid tantalum input capacitors should only be used where the input source is impedance current limited. High dV/dt ap- plied at the input can cause excessive charge current through low ESR tantalum capacitors. This high charge cur- rent can result in shorting within the capacitor. It is recom- mended that they be surge current tested by the manufactur- er.The TPS series available from AVX, and the 593D series from Sprague are both surge current tested. Use caution when using ceramic capacitors for input bypass- ing, because it may cause ringing at the V IN pin. LOWERING OUTPUT RIPPLE When using the adjustable parts, one can achieve lower out- put ripple voltage by shorting a resistor internal to the LM2825. However, if this resistor is shorted, a feed forward capacitor must be used to keep the regulator stable. For this reason, this resistor must be left open on all of the fixed out- put voltage versions or instability will result. See the feed for- ward capacitor selection below. Shorting the internal resistor is accomplished by shorting pins 8 and 9 on the LM2825, and will typically reduce output ripple by 25 to 33%. FEED FORWARD CAPACITOR SELECTION (C FF) When using an adjustable part and pins 8 and 9 are shorted to reduce output ripple, a feed forward capacitor is required. This capacitor is typically between 680 pF and 2700 pF. The table of Figure 4 shows the value for C FF for a given output voltage and feedback resistor R 2 (R1 = 1kΩ). SHUTDOWN The circuit shown in Figure 10 shows 2 circuits for the Shutdown/Soft-start feature using different logic signals for shutdown and using a 0.1 µF Soft-start capacitor. THERMAL CONSIDERATIONS The LM2825 is available in a 24-pin through hole DIP. The package is molded plastic with a copper lead frame. When the package is soldered to the PC board, the copper and the board are the heat sink for the LM2825. V OUT R2 C FF LM2825-ADJ 2 630 N/A 3 1.43k N/A 4 2.26k 2700 pF 5 3.09k 2700 pF 6 3.92k 2200 pF 7 4.75k 1800 pF 8 5.49k 1500 pF LM2825H-ADJ 7 4.75k 2700 pF 8 5.49k 2200 pF 9 6.34k 1800 pF 10 7.15k 1500 pF 11 8.06k 1000 pF 12 8.87k 820 pF 13 9.53k 680 pF 14 10.5k 680 pF 15 11.3k 680 pF FIGURE 4. C FF Selection Table DS012661-22 FIGURE 5. Usage of the Soft-start Capacitor www.national.com 8 |
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