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LM26001 Datenblatt(PDF) 11 Page - National Semiconductor (TI) |
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LM26001 Datenblatt(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page Design Information (Continued) SETTING OUTPUT VOLTAGE The output voltage is set by the ratio of a voltage divider at the FB pin as shown in the typical application. The resistor values can be determined by the following equation: Where Vfb = 1.234V typically. A maximum value of 150k Ω is recommended for the sum of R1 and R2. As input voltage decreases towards the nominal output volt- age, the LM26001 can skip up to seven off-pulses as de- scribed in the Low Vin Operation section. In low output voltage applications, if the on-time reaches Ton MIN, the de- vice will skip on-pulses to maintain regulation. There is no limit to the number of pulses that are skipped. In this mode of operation, however, output ripple voltage may increase slightly. INDUCTOR The output inductor should be selected based on inductor ripple current. The amount of inductor ripple current com- pared to load current, or ripple content, is defined as Iripple/ Iload. Ripple content should be less than 40%. Inductor ripple current, Iripple, can be calculated as shown below: Larger ripple content increases losses in the inductor and reduces the effective current limit. Larger inductance values result in lower output ripple voltage and higher efficiency, but a slightly degraded transient re- sponse. Lower inductance values allow for smaller case size, but the increased ripple lowers the effective current limit threshold. Remember that inductor value also affects the sleep mode threshold as shown in Figure 3. When choosing the inductor, the saturation current rating must be higher than the maximum peak inductor current and the RMS current rating should be higher than the maximum load current. Peak inductor current, Ipeak, is calculated as: For example, at a maximum load of 1.5A and a ripple content of 40%, peak inductor current is equal to 1.8A which is safely below the minimum current limit of 1.85A. By increasing the inductor size, ripple content and peak inductor current are lowered, which increases the current limit margin. The size of the output inductor can also be determined using the desired output ripple voltage, Vrip. The equation to de- termine the minimum inductance value based on Vrip is as follows: Where Re is the ESR of the output capacitors, and Vrip is a peak-to-peak value. This equation assumes that the output capacitors have some amount of ESR. It does not apply to ceramic output capacitors. If this method is used, ripple content should still be verified to be less than 40%. OUTPUT CAPACITOR The primary criterion for selecting an output capacitor is equivalent series resistance, or ESR. ESR (Re) can be selected based on the requirements for output ripple voltage and transient response. Once an induc- tor value has been selected, ripple voltage can be calculated for a given Re using the equation above for Lmin. Lower ESR values result in lower output ripple. 20179430 FIGURE 7. Example Circuit 1.5A Max, 305 kHz www.national.com 11 |
Ähnliche Teilenummer - LM26001 |
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Ähnliche Beschreibung - LM26001 |
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