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LM2760M5X Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM2760M5X Datasheet(HTML) 8 Page - National Semiconductor (TI) |
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8 / 10 page  Application Information (Continued) is illustrated in Figure 1, which graphs of output voltage ripple versus input voltage for a few different output capaci- tor values. In addition to the amount of output capacitance present, the ESR of the output capacitor also affects output voltage ripple. A low-ESR ceramic capacitor is recommended on the output to keep output voltage ripple low. Placing multiple capacitors is parallel can reduce ripple significantly, both by increasing capacitance and reducing ESR. When capacitors are in parallel, ESR is in parallel as well. The effective net ESR is determined according to the properties of parallel resistance. Two identical capacitors in parallel have twice the capacitance and half the ESR as compared to a single capacitor of the same make. On a similar note, if a large- value, high-ESR capacitor (tantalum, for example) is to be used as the primary output capacitor, the net output ESR can be significantly reduced by placing a low-ESR ceramic ca- pacitor in parallel with this primary output capacitor. CAPACITORS The LM2760 requires 3 external capacitors for proper opera- tion. Surface-mount multi-layer ceramic capacitors are rec- ommended. These capacitors are small, inexpensive and have very low equivalent series resistance ( ≤10mΩ typ.). Tantalum capacitors, OS-CON capacitors, and aluminum electrolytic capacitors generally are not recommended for use with the LM2760 due to their high ESR, as compared to ceramic capacitors. For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with the LM2760. These capacitors have tight capacitance tolerance (as good as +/-10%), hold their value over temperature (X7R: +/-15% over -55oC to +125oC; X5R: +/-15% over -55oCto+85oC), and typically have little voltage coefficient. Capacitors with Y5V and/or Z5U temperature characteristic are generally not recommended. These types of capacitors typically have wide capacitance tolerance (+80%, -20%), vary significantly over temperature (Y5V: +22%, -82% over -35oCto+85oC; Z5U: +22%, -56% over -10oCto+85oC), and have poor voltage coefficients. Under some conditions, a nominally 1µF Y5V or Z5U capacitor could have a capaci- tance of only 0.1µF. Such detrimental deviation is likely to cause these Y5V and Z5U of capacitors to fail to meet the minimum capacitance requirements of the LM2760. The table below lists some leading ceramic capacitor manu- facturers. Manufacturer Contact Information TDK www.component.tdk.com AVX www.avx.com Murata www.murata.com Taiyo-Yuden www.t-yuden.com Vishay-Vitramon www.vishay.com INPUT CAPACITORS The input capacitor (C IN) is used as a reservoir of charge, helping to quickly transfer charge to the flying capacitor during the charge phase (ø1) of operation. The input capaci- tor helps to keep the input voltage from drooping at the start of the charge phase, when the flying capacitor is first con- nected to the input, and helps to filter noise on the input pin that could adversely affect sensitive internal analog circuitry biased off the input line. As mentioned above, an X7R/X5R ceramic capacitor is recommended for use. An input capaci- tance of 4.7µF is recommended. It may be possible to re- duce this value in applications where the maximum output current is significantly less than 20mA. A smaller input ca- pacitance typically results in higher input voltage ripple, which may also show up as higher output voltage ripple as well. FLYING CAPACITORS The flying capacitor (C FLY) transfers charge from the input to the output, providing the voltage boost of the doubler. A polarized capacitor (tantalum, aluminum electrolytic, etc.) must not be used here, as the capacitor will be reverse- biased upon start-up of the LM2760. For most applications, a 0.1µF X7R/X5R ceramic capacitor is recommended for C FLY. Smaller flying capacitance typically will reduce the output current capability of the part. The ESR also affects the output current capability of the LM2760. For optimal performance, a low-ESR ceramic capacitor (< 10m Ω typ.) should be used. Larger flying capacitance may increase output current capa- bility, but it comes with the drawback of typically increasing the magnitude of the output voltage ripple. OUTPUT CAPACITOR Both output capacitance and ESR of the LM2760 output capacitor affect output voltage ripple. A low-ESR X7R/X5R ceramic capacitor is the capacitor of choice for the LM2760 output. An output capacitance of 10µF is recommended for most applications. Reducing output capacitance typically in- creases output voltage ripple. In applications where the maximum output current is significantly less than 20mA, output voltage ripple is less of a concern and it may be desirable to reduce the output capacitance. POWER EFFICIENCY Efficiency of the LM2760 mirrors that of an unregulated switched capacitor converter cascaded with a linear regula- tor when more-than nominal output current is present ( ≥ 20044306 FIGURE 1. Output Voltage Ripple vs. Input Voltage (T A =25 oC) I OUT = 10mA, TA =25 oC Top: C OUT = 4.7µF Mid: C OUT = 10µF Bottom: C OUT = 22µF www.national.com 8 |
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