Datasheet LT3048 (Analog Devices) - 11

LT3048 Series
APPLICATIONS INFORMATION
45 L = 5.6µH 40 DCR = 120mΩ VIN = 2.7V 35 VIN = 3.3V C1 C2 V 30 IN = 3.6V GUARANTEED 25 20 15 LOAD CURRENT (mA) L1 10 5 0 0 2 4 6 8 10 12 14 16 18 20 22 C4 C3 LDOOUT VOLTAGE (V) 3048 F02
Figure 2. LT3048 Typical and Guaranteed Load Current vs. Output Voltage Reducing Output Noise With A Bypass Capacitor
The LT3048 relies on the power supply rejection of the 3048 F03 linear regulator to reduce switching regulator noise at
Figure 3. Recommended PCB Layout
LDOOUT. The linear regulator also contributes thermal noise to the output. The thermal noise can be reduced, deviations and provide improved transient response for and transient response improved, by adding a capacitor larger load current changes. Bypass capacitors, used to between LDOOUT and BYP. A typical value is 1nF. This decouple individual components powered by the LT3048, capacitor increases start-up time of the regulator. increase the effective output capacitor value.
Recommended PCB Layout
Give consideration to the use of ceramic capacitors as they are manufactured with a variety of dielectrics, each Figure 3 shows the recommended layout for LT3048 cir- with different behavior across temperature and applied cuits. Most important is careful placement of the BSTOUT voltage. The most common dielectrics used are specified bypass capacitor C2. High frequency AC current flows in a with EIA codes of Z5U, Y5V, X5R and X7R. Typical volt- loop formed by C2, internal power transistor Q1 and boost age and temperature coefficients are shown in Figures 4 diode D2. Keep this loop small. Also be sure to place an and 5. The X5R and X7R dielectrics have more stable unbroken ground plane below this loop, on the highest characteristics and are most suitable for use as the output copper layer below the surface. This prevents the AC loop capacitor. The X7R type has better stability across tem- from coupling to LDOOUT and other nearby circuitry. Keep perature, while the X5R is less expensive and is available the SW node as small as possible. in higher values. Care still must be exercised when using X5R and X7R capacitors; the codes only specify operating
LDO Stability and Output Capacitance
temperature range and maximum capacitance change The LT3048 linear regulator requires an output capacitor over temperature. Capacitance change due to DC bias for stability. It is designed to be stable with most low with X5R and X7R capacitors can be significant enough ESR capacitors (typically ceramic, tantalum or low ESR to drop capacitor values below appropriate levels. Capaci- electrolytic). A minimum output capacitor of 1μF with an tor DC bias characteristics tend to improve as case size ESR of 1Ω or less is recommended to prevent oscilla- increases, but expected capacitance at operating voltage tions. Larger values of output capacitance decrease peak should be verified. Rev B For more information www.analog.com 11 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions LT3048 LT3048-xx LT3048 Block Diagrams Operation Applications Information Package Description Revision History Related Parts