Datasheet LTC3423, LTC3424 (Analog Devices) - 8

LTC3423/LTC3424
U U W U APPLICATIO S I FOR ATIO
The inductor current ripple is typically set to 20% to 40% where of the maximum inductor current. IL = Average Inductor Current For high efficiency, choose an inductor with a high fre- IP = Peak Inductor Current quency core material, such as ferrite, to reduce core losses. The inductor should have low ESR (equivalent The ESR is usually the most dominant factor for ripple in series resistance) to reduce the I2R losses and must be most power converters. The ripple due to capacitor ESR is able to handle the peak inductor current at full load without simply given by: saturating. Molded chokes or chip inductors usually do VRCESR = IP • RESR Volts not have enough core to support the peak inductor cur- rents in the 1A to 2A region. To minimize radiated noise, where use a toroid, pot core or shielded bobbin inductor. See RESR = Capacitor Series Resistance Table 1 for a list of component suppliers. Low ESR capacitors should be used to minimize output
Table 1. Inductor Vendor Information
voltage ripple. For surface mount applications, AVX TPS
SUPPLIER PHONE FAX WEBSITE
series tantalum capacitors and Sanyo POSCAP or Taiyo- Coilcraft (847) 639-6400 (847) 639-1469 www.coilcraft.com Yuden ceramic capacitors are recommended. For through- hole applications Sanyo OS-CON capacitors offer low ESR Coiltronics (516) 241-7876 (516) 241-9339 www.coiltronics.com in a small package size. See Table 2 for a list of component Murata (814) 237-1431 (814) 238-0490 www.murata.com (800) 831-9172 suppliers. Sumida In some layouts it may be required to place a 1µF low ESR USA: (847) 956-0666 (847) 956-0702 www.japanlink.com capacitor as close to the V Japan: 81-3-3607-5111 81-3-3607-5144 sumida OUT and GND pins as possible.
Table 2. Capacitor Vendor Information SUPPLIER PHONE FAX WEBSITE
Rt SHDN MODE V AVX (803) 448-9411 (803) 448-1943 www.avxcorp.com C VIN FB Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com SW VOUT VDD IN GND VDD 2.7V Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com TO 5.5V
Input Capacitor Selection
VOUT The input filter capacitor reduces peak currents drawn from the input source and reduces input switching noise. In most 3423/24 F01 applications a 3.3µF is sufficient.
Figure 1. Recommended Component Placement. Traces Carrying High Current Are Direct. Trace Area FB and VC Pins Output Diode Are Kept Low. Lead Length to Battery Should be Kept Short
The Schottky diode across the synchronous PMOS switch
Output Capacitor Selection
is not required, but provides a lower drop during the break- before-make time (typically 20ns) of the NMOS to PMOS The output voltage ripple has several components. The transition. The addition of the Schottky diode will improve bulk value of the capacitor is set to reduce the ripple due peak efficiency (see graph “Efficiency Loss Without to charge into the capacitor each cycle. The max ripple due Schottky vs Frequency”). Use of a Schottky diode such as to charge is given by: a MBRM120T3, 1N5817 or equivalent. Since slow recov- I V V V ery times will compromise efficiency, do not use ordinary VR L IN OUT IN BULK = • ( – ) Volts rectifier diodes. C • V • V • f OUT OUT OUT 34234f 8