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1956IFE-5 Datasheet(PDF) 9 Page - Linear Technology |
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1956IFE-5 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 28 page  9 LT1956/LT1956-5 1956f Figure 2. Frequency and Current Limit Foldback with high input voltage. High frequency pickup will in- crease and the protection accorded by frequency and current foldback will decrease. CHOOSING THE INDUCTOR For most applications, the output inductor will fall into the range of 5 µH to 30µH. Lower values are chosen to reduce physical size of the inductor. Higher values allow more output current because they reduce peak current seen by the LT1956 switch, which has a 1.5A limit. Higher values also reduce output ripple voltage. When choosing an inductor you will need to consider output ripple voltage, maximum load current, peak induc- tor current and fault current in the inductor. In addition, other factors such as core and copper losses, allowable component height, EMI, saturation and cost should also be considered. The following procedure is suggested as a way of handling these somewhat complicated and con- flicting requirements. Output Ripple Voltage Figure 3 shows a comparison of output ripple voltage for the LT1956 using either a tantalum or ceramic output capacitor. It can be seen from Figure 3 that output ripple voltage can be significantly reduced by using the ceramic output capacitor; the significant decrease in output ripple voltage is due to the very low ESR of ceramic capacitors. – + 1.2V BUFFER VSW L1 VC GND TO SYNC CIRCUIT 1956 F02 TO FREQUENCY SHIFTING R3 1k R4 2k R1 C1 R2 OUTPUT 5V ERROR AMPLIFIER FB 1.4V Q1 LT1956 Q2 + APPLICATIO S I FOR ATIO Output ripple voltage is determined by ripple current (ILP-P) through the inductor and the high frequency impedance of the output capacitor. At high frequencies, the impedance of the tantalum capacitor is dominated by its effective series resistance (ESR). Tantalum Output Capacitor The typical method for reducing output ripple voltage when using a tantalum output capacitor is to increase the inductor value (to reduce the ripple current in the induc- tor). The following equations will help in choosing the required inductor value to achieve a desirable output ripple voltage level. If output ripple voltage is of less importance, the subsequent suggestions in Peak Inductor and Fault Current and EMI will additionally help in the selection of the inductor value. Figure 3. LT1956 Output Ripple Voltage Waveforms. Ceramic vs Tantalum Output Capacitors 1 µs/DIV 10mV/DIV VOUT USING 22 µF CERAMIC OUTPUT CAPACITOR VOUT USING 100 µF, 0.08Ω TANTALUM OUTPUT CAPACITOR 10mV/DIV VIN = 12V VOUT = 5V L = 15 µH 1956 F03 |
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