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LT1610 Datasheet(PDF) 8 Page - Linear Technology |
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LT1610 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1610 APPLICATIONS INFORMATION COMPONENT SELECTION Inductors Inductors used with the LT1610 should have a saturation current rating (–30% of zero current inductance) of ap- proximately 0.5A or greater. DCR should be 0.5 Ω or less. The value of the inductor should be matched to the power requirements and operating voltages of the application. In most cases a value of 4.7 µHor10µHissuitable.TheMurata LQH3C inductors specified throughout the data sheet are small and inexpensive, and are a good fit for the LT1610. Alternatives are the CD43 series from Sumida and the DO1608 series from Coilcraft. These inductors are slightly larger but will result in slightly higher circuit efficiency. Chip inductors, although tempting to use because of their small size and low cost, generally do not have enough energy storage capacity or low enough DCR to be used successfully with the LT1610. Diodes The Motorola MBR0520 is a 0.5 amp, 20V Schottky diode. This is a good choice for nearly any LT1610 application, unless the output voltage or the circuit topology require a diode rated for higher reverse voltages. Motorola also offers the MBR0530 (30V) and MBR0540 (40V) versions. Most one-half amp and one amp Schottky diodes are suitable; these are available from many manufacturers. If you use a silicon diode, it must be an ultrafast recovery type. Efficiency will be lower due to the silicon diode’s higher forward voltage drop. Capacitors The input capacitor must be placed physically close to the LT1610. ESR is not critical for the input. In most cases inexpensive tantalum can be used. The choice of output capacitor is far more important. The quality of this capacitor is the greatest determinant of the output voltage ripple. The output capacitor performs two major functions. It must have enough capacitance to satisfy the load under transient conditions and it must shunt the AC component of the current coming through the diode from the inductor. The ripple on the output results when this AC current passes through the finite impedance of the output capacitor. The capacitor should have low impedance at the 1.7MHz switching frequency of the LT1610. At this frequency, the impedance is usually dominated by the capacitor’s equivalent series resistance (ESR). Choosing a capacitor with lower ESR will result in lower output ripple. Perhaps the best way to decrease ripple is to add a 1 µF ceramic capacitor in parallel with the bulk output capaci- tor. Ceramic capacitors have very low ESR and 1 µF is enough capacitance to result in low impedance at the switching frequency. The low impedance can have a dramatic effect on output ripple voltage. To illustrate, examine Figure 6’s circuit, a 4-cell to 5V/100mA SEPIC DC/DC converter. This design uses inexpensive aluminum electrolytic capacitors at input and output to keep cost down. Figure 7 details converter operation at a 100mA load, without ceramic capacitor C5. Note the 400mV spikes on VOUT. After C5 is installed, output ripple decreases by a factor of 8 to about 50mVP-P. The addition of C5 also improves efficiency by 1 to 2 percent. Low ESR and the required bulk output capacitance can be obtained using a single larger output capacitor. Larger tantalum capacitors, newer capacitor technologies (for example the POSCAP from Sanyo and SPCAP from Panasonic) or large value ceramic capacitors will reduce the output ripple. Note, however, that the stability of the circuit depends on both the value of the output capacitor and its ESR. When using low value capacitors or capaci- tors with very low ESR, circuit stability should be evalu- ated carefully, as described below. Loop Compensation The LT1610 is a current mode PWM switching regulator that achieves regulation with a linear control loop. The LT1610 provides the designer with two methods of com- pensating this loop. First, you can use an internal compen- sation network by tying the COMP pin to the VC pin. This results in a very small solution and reduces the circuit’s total part count. The second option is to tie a resistor RC and a capacitor CC in series from the VC pin to ground. This allows optimization of the transient response for a wide variety of operating conditions and power components. |
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