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LM2788MMX-1.5 データシート(PDF) 10 Page - National Semiconductor (TI) |
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LM2788MMX-1.5 データシート(HTML) 10 Page - National Semiconductor (TI) |
10 / 12 page Application Information (Continued) low-ESR ceramic capacitor is recommended on the output to keep output voltage ripple low. Placing multiple capacitors in 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 capaci- tance 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 pri- mary output capacitor, the net output ESR can be signifi- cantly reduced by placing a low-ESR ceramic capacitor in parallel with this primary output capacitor. Ripple is increased when the LM2788 is gain hopping. Thus, in the presence of high currents, ripple is likely to vary significantly over the input voltage, depending on wether or not the part is gain hopping. CAPACITORS The LM2788 requires 4 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 (ESR, ≤15mΩ typ.). Tantalum capacitors, OS-CON capacitors, and alumi- num electrolytic capacitors generally are not recommended for use with the LM2788 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 LM2788. These capacitors have tight capacitance tolerance (as good as ±10%), hold their value over temperature (X7R: ±15% over -55oCto125oC; X5R: ±15% over -55oCto85oC), and typically have little voltage coefficient. Capacitors with Y5V and/or Z5U temperature characteristic are generally not recommended for use with the LM2788. These types of capacitors typically have wide capacitance tolerance (+80%, -20%), vary significantly over temperature (Y5V: +22%, -82% over -30oCto+85oC range; Z5U: +22%, -56% over +10oC to +85oC range), and have poor voltage coefficients. Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a capacitance of only 0.1µF. Such detrimental deviation is likely to cause these Y5V and Z5U of capacitors to fail to meet the minimum capacitance require- ments of the LM2788. The table below lists some leading ceramic capacitor manu- facturers. Manufacturer Contact Information AVX www.avx.com Murata www.murata.com Taiyo-Yuden www.t-yuden.com TDK www.component.tdk.com Vishay-Vitramon www.vishay.com OUTPUT CAPACITOR The output capacitor of the LM2788 plays an important part in LM2788 performance. In typical high-current applications, a 10µF low-ESR (ESR = equivalent series resistance) ce- ramic capacitor is recommended for use. For lighter loads, the output capacitance may be reduced (capacitance as low as 1µF for output currents ≤ 60mA is usually acceptable). The performance of the part should be evaluated with spe- cial attention paid to efficiency and output ripple to ensure the capacitance chosen on the output yields performance suitable for the application. In extreme cases, excessive ripple could cause control loop instability, severely affecting the performance of the part. If excessive ripple is present, the output capacitance should be increased. The ESR of the output capacitor affects charge pump output resistance, which plays a role in determining output current capability. Both output capacitance and ESR affect output voltage ripple (See Output Voltage Ripple section, above). For these reasons, a low-ESR X7R/X5R ceramic capacitor is the capacitor of choice for the LM2788 output. FLYING CAPACITORS The flying capacitors (C 1 and C2) transfer charge from the input to the output, and thus are like the engine of the charge pump. Low-ESR ceramic capacitors with X7R or X5R tem- perature characteristic are strongly recommended for use here. The flying capacitors C1 and C2 should be identical. As a general rule, the capacitance value of each flying capacitor should be 1/10th that of the output capacitor. Polarized capacitor (tantalum, aluminum electrolytic, etc.) must not be used for the flying capacitors, as they could become reverse- biased upon start-up of the LM2788. The flying capacitance determines the strength of the charge pump-the larger the capacitance, the bigger the engine. ESR in the flying capacitors negatively affects the strength of the charge pump and should be minimized, as ESR contributes to undesired output resistance. If capacitors are too small the LM2788 could spend excessive amount of time gain hopping: decreasing efficiency, increasing output voltage ripple, and possibly impeding the ability of the part to regu- late. On the other hand, if the flying capacitors are too large they could potentially overwhelm the output capacitor, result- ing in increased output voltage ripple. INPUT CAPACITOR If the flying capacitors are the charge pump engine, the input capacitor (CIN) is the fuel tank: a reservoir of charge that aids a quick transfer of charge from the supply to the flying capacitors during the charge phase of operation. The input capacitor helps to keep the input voltage from drooping at the start of the charge phase, when the flying capacitor is first connected 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. As a general recommendation, the input capacitor should be cho- sen to match the output capacitor. POWER DISSIPATION LM2788 power dissipation will, typically, not be much of a concern in most applications. Derating to accommodate self- heating will rarely be required due to the high efficiency of the part. When operating within specified operating ratings, the peak power dissipation (PD) of all LM2788 voltage op- tions occurs with the LM2788-1.5 operating at the maximum rated operating output current of 120mA. With an input volt- age of 5.5V, the power efficiency (E) of the LM2788-1.5 bottoms out at 54%. Assuming a typical junction-to-ambient thermal resistance ( θJA) for the MSOP package of 220˚C/ Watt, the junction temperature (T J) of the part is calculated below for a part operating at the maximum rated ambient temperature (T A) of 85˚C. www.national.com 10 |
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