Datasheet LT1610 (Analog Devices) - 8

LT1610
U U W U APPLICATIONS INFORMATION COMPONENT SELECTION
impedance of the output capacitor. The capacitor should have low impedance at the 1.7MHz switching frequency of
Inductors
the LT1610. At this frequency, the impedance is usually Inductors used with the LT1610 should have a saturation dominated by the capacitor’s equivalent series resistance current rating (–30% of zero current inductance) of ap- (ESR). Choosing a capacitor with lower ESR will result in proximately 0.5A or greater. DCR should be 0.5Ω or less. lower output ripple. The value of the inductor should be matched to the power Perhaps the best way to decrease ripple is to add a 1µF requirements and operating voltages of the application. In ceramic capacitor in parallel with the bulk output capaci- most cases a value of 4.7µH or 10µH is suitable. The Murata tor. Ceramic capacitors have very low ESR and 1µF is LQH3C inductors specified throughout the data sheet are enough capacitance to result in low impedance at the small and inexpensive, and are a good fit for the LT1610. switching frequency. The low impedance can have a Alternatives are the CD43 series from Sumida and the dramatic effect on output ripple voltage. To illustrate, DO1608 series from Coilcraft. These inductors are slightly examine Figure 6’s circuit, a 4-cell to 5V/100mA SEPIC larger but will result in slightly higher circuit efficiency. DC/DC converter. This design uses inexpensive aluminum Chip inductors, although tempting to use because of their electrolytic capacitors at input and output to keep cost small size and low cost, generally do not have enough down. Figure 7 details converter operation at a 100mA energy storage capacity or low enough DCR to be used load, without ceramic capacitor C5. Note the 400mV successfully with the LT1610. spikes on VOUT. After C5 is installed, output ripple decreases by a factor of
Diodes
8 to about 50mVP-P. The addition of C5 also improves The Motorola MBR0520 is a 0.5 amp, 20V Schottky diode. efficiency by 1 to 2 percent. This is a good choice for nearly any LT1610 application, Low ESR and the required bulk output capacitance can be unless the output voltage or the circuit topology require a obtained using a single larger output capacitor. Larger diode rated for higher reverse voltages. Motorola also tantalum capacitors, newer capacitor technologies (for offers the MBR0530 (30V) and MBR0540 (40V) versions. example the POSCAP from Sanyo and SPCAP from Most one-half amp and one amp Schottky diodes are Panasonic) or large value ceramic capacitors will reduce suitable; these are available from many manufacturers. If the output ripple. Note, however, that the stability of the you use a silicon diode, it must be an ultrafast recovery circuit depends on both the value of the output capacitor type. Efficiency will be lower due to the silicon diode’s and its ESR. When using low value capacitors or capaci- higher forward voltage drop. tors with very low ESR, circuit stability should be evalu- ated carefully, as described below.
Capacitors
The input capacitor must be placed physically close to the
Loop Compensation
LT1610. ESR is not critical for the input. In most cases The LT1610 is a current mode PWM switching regulator inexpensive tantalum can be used. that achieves regulation with a linear control loop. The The choice of output capacitor is far more important. The LT1610 provides the designer with two methods of com- quality of this capacitor is the greatest determinant of the pensating this loop. First, you can use an internal compen- output voltage ripple. The output capacitor performs two sation network by tying the COMP pin to the VC pin. This major functions. It must have enough capacitance to results in a very small solution and reduces the circuit’s satisfy the load under transient conditions and it must total part count. The second option is to tie a resistor RC shunt the AC component of the current coming through and a capacitor CC in series from the VC pin to ground. This the diode from the inductor. The ripple on the output allows optimization of the transient response for a wide results when this AC current passes through the finite variety of operating conditions and power components. 8