Datasheet Linear Technology LT1613 (Linear Technology) - 6

LT1613
U OPERATIO
Table 1 lists several inductors that will work with the lower ESR will result in lower output ripple. LT1613, although this is not an exhaustive list. There are Ceramic capacitors can be used with the LT1613 provided many magnetics vendors whose components are suitable loop stability is considered. A tantalum capacitor has for use. some ESR and this causes an “ESR zero” in the regulator loop. This zero is beneficial to loop stability. The internally
Diodes
compensated LT1613 does not have an accessible com- A Schottky diode is recommended for use with the LT1613. pensation node, but other circuit techniques can be em- The Motorola MBR0520 is a very good choice. Where the ployed to counteract the loss of the ESR zero, as detailed input to output voltage differential exceeds 20V, use the in the next section. MBR0530 (a 30V diode). If cost is more important than Some capacitor types appropriate for use with the LT1613 efficiency, the 1N4148 can be used, but only at low current are listed in Table 2. loads.
Capacitors OPERATION WITH CERAMIC CAPACITORS
The input bypass capacitor must be placed physically Because the LT1613 is internally compensated, loop sta- close to the input pin. ESR is not critical and in most cases bility must be carefully considered when choosing an an inexpensive tantalum is appropriate. output capacitor. Small, low cost tantalum capacitors have some ESR, which aids stability. However, ceramic The choice of output capacitor is far more important. The capacitors are becoming more popular, having attractive quality of this capacitor is the greatest determinant of the characteristics such as near-zero ESR, small size and output voltage ripple. The output capacitor must have reasonable cost. Simply replacing a tantalum output ca- enough capacitance to satisfy the load under transient pacitor with a ceramic unit will decrease the phase margin, conditions and it must shunt the switched component of in some cases to unacceptable levels. With the addition of current coming through the diode. Output voltage ripple a phase lead capacitor (C results when this switched current passes through the PL) and isolating resistor (R3), the LT1613 can be used successfully with ceramic output finite output impedance of the output capacitor. The capacitors as described in the following figures. capacitor should have low impedance at the 1.4MHz switching frequency of the LT1613. At this frequency, the A boost converter, stepping up 2.5V to 5V, is shown in impedance is usually dominated by the capacitor’s equiva- Figure 5. Tantalum capacitors are used for the input and lent series resistance (ESR). Choosing a capacitor with output (the input capacitor is not critical and has little
Table 1. Inductor Vendors VENDOR PHONE URL PART COMMENT
Sumida (847) 956-0666 www.sumida.com CLS62-22022 22µH Coupled CD43-220 22µH Murata (404) 436-1300 www.murata.com LQH3C-220 22µH, 2mm Height LQH3C-100 10µH LQH3C-4R7 4.7µH Coiltronics (407) 241-7876 www.coiltronics.com CTX20-1 20µH Coupled, Low DCR
Table 2. Capacitor Vendors VENDOR PHONE URL PART COMMENT
Taiyo Yuden (408) 573-4150 www.t-yuden.com Ceramic Caps X5R Dielectric AVX (803) 448-9411 www.avxcorp.com Ceramic Caps Tantalum Caps Murata (404) 436-1300 www.murata.com Ceramic Caps 6