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ADP3604 데이터시트(PDF) 5 Page - Analog Devices |
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ADP3604 데이터시트(HTML) 5 Page - Analog Devices |
5 / 8 page ADP3604 REV. 0 –5– TEMPERATURE – °C 10 1.0 0.01 –50 100 050 0.1 ALUMINUM CERAMIC ORGANIC SEMIC TANTALUM ALUMINUM CERAMIC ORGANIC SEMIC TANTALUM Figure 16. ESR vs. Temperature Table I. Alternative Capacitor Technologies High Type Life Freq Temp Size Cost Aluminum Fair Fair Fair Small Low Electrolytic Capacitor Multilayer Long Good Poor Fair High Ceramic Capacitor Solid Above Avg Avg Avg Avg Tantalum Avg Capacitor OS-CON Above Good Good Good Avg Capacitor Avg The following is a partial list of manufacturers providing low ESR capacitors. Table II. Recommended Capacitor Manufacturers Manufacturer Capacitor Capacitor Type Sprague 672D, 673D, Aluminum Electrolytic 674D, 678D Sprague 675D, 173D, Tantalum 199D Nichicon PF & PL Aluminum Electrolytic Mallory TDC & TDL Tantalum TOKIN MLCC Multilayer Ceramic muRata GRM Multilayer Ceramic EXTERNAL OUTPUT FILTERING In applications requiring very low power supply ripple and noise, the circuit in Figure 18 provides low noise and ripple of less than 2% of the output voltage over the full load current and temperature. Since output current is supplied solely by the output capacitor C3 during one-half of the charge-pump cycle, peak-to-peak output ripple voltage is calculated by using the following formula: VRIPPLE = IOUT 2(FPUMP )(C2) + I OUT ( ESRC2 ) In Figure 15, output ripple voltage vs. capacitance and various ESR are shown. CAPACITANCE – µF 120 0 0 180 20 40 60 80 100 120 140 160 100 80 60 40 20 ESR C 140mA V OUT ADP3604 150m Ω 100m Ω 50m Ω Figure 15. Output Ripple Voltage (mV) vs. Capacitance and ESR Note that as the capacitor value increases beyond the point where the dominant contribution to the output ripple is due to the ESR, no significant reduction in VOUT ripple is achieved by added capacitance. A low ESR capacitor has much greater impact on performance for C2 than C3 since current through C2 is twice the C3 cur- rent. There is a voltage drop across CP’s ESR during the charge as well as during discharges. Therefore, the voltage drop due to C2 is about 4 times C2’s ESR times the load current. The volt- age drop generated by C2’s ESR combined with the voltage drop due to the output source resistance, determines the maxi- mum available VOUT, while C3’s ESR affects the output voltage ripple. When selecting the capacitors, keep in mind that not all manu- facturers guarantee capacitor ESR in the range required by the circuit. In general, the capacitor’s ESR is inversely proportional to its physical size, so larger capacitance values and higher volt- age ratings tend to reduce ESR. ESR is also a function of the operating frequency. When select- ing a capacitor, make sure its value is rated at the circuit’s oper- ating frequency. The other factor affecting the capacitor’s performance is temperature. If the circuit has to operate at tem- peratures significantly different than 25 °C, the capacitance and ESR values must be carefully selected to adequately compen- sate for the change. Various capacitor technologies offer im- proved performance over temperature, for example, certain tantalum capacitors provide good low-temperature ESR but at a higher cost. Figure 16 demonstrates the effect temperature has on various capacitors. ADP3604’s high internal oscillator frequency per- mits the usage of smaller capacitance for both the pump and the output capacitors. |
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