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PTH08T255WAZ Datasheet(PDF) 11 Page - Texas Instruments |
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PTH08T255WAZ Datasheet(HTML) 11 Page - Texas Instruments |
11 / 35 page Designing for Fast Load Transients Capacitor Table PTH08T255W www.ti.com ........................................................................................................................................................................................... SLTS290 – NOVEMBER 2008 The transient response of the dc/dc converter has been characterized using a load transient with a di/dt of 2.5A/µs. The typical voltage deviation for this load transient is given in the Electrical Characteristics table using the minimum required value of output capacitance. As the di/dt of a transient is increased, the response of a converter’s regulation circuit ultimately depends on its output capacitor decoupling network. This is an inherent limitation with any dc/dc converter once the speed of the transient exceeds its bandwidth capability. If the target application specifies a higher di/dt or lower voltage deviation, the requirement can only be met with additional low ESR ceramic capacitor decoupling. Generally, with load steps greater than 100A/µs, adding multiple 10µF ceramic capacitors plus 10×1µF, and numerous high frequency ceramics ( ≤0.1µF) is all that is required to soften the transient higher frequency edges. The PCB location of these capacitors in relation to the load is critical. DSP, FPGA and ASIC vendors identify types, location and amount of capacitance required for optimum performance. Low impedance buses, unbroken PCB copper planes, and components located as close as possible to the high frequency devices are essential for optimizing transient performance. Table 3 identifies the characteristics of acceptable capacitors from a number of vendors. The recommended number of capacitors required at both the input and output buses is identified for each capacitor. This is not an extensive capacitor list. Capacitors from other vendors are available with comparable specifications. Those listed are for guidance. The RMS ripple current rating and ESR (at 100 kHz) are critical parameters necessary to ensure both optimum regulator performance and long capacitor life. Table 3. Input/Output Capacitors(1) Capacitor Characteristics Quantity Max Output Bus Max. Ripple Capacitor Vendor, Working ESR at Value Current Physical Input Type Series (Style) No Voltage 100 Vendor Part No. TurboTrans (µF) at 85°C Size (mm) Bus Turbo (V) kHz (Cap Type)(2) (Irms) Trans ( Ω) (mA) Panasonic 25 1000 0.043 1690 16 × 15 1 ≥ 2(3) N/R(4) EEUFC1E102S FC (Radial) 25 1800 0.029 2205 16 × 20 1 ≥ 1(3) N/R(4) EEUFC1E182 FC (SMD) 25 2200 0.028 2490 18 × 21,5 1 ≥ 1(3) N/R(4) EEVFC1E222N FK (SMD) 25 1000 0.060 1100 12,5×13,5 1 ≥ 2(5) N/R(4) EEVFK1V102Q United Chemi-Con PTB Poly-Tant (SMD) 6.3 330 0.025 2600 7,3x4,3x 2,8 N/R(6) 3 - 4(3) (C) ≥ 2(2) 4PTB337MD6TER LXZ, Aluminum (Radial) 25 680 0.068 1050 10 × 16 1 2 - 3(3) N/R(4) LXZ25VB681M10X20LL PS, Poly-Alum (Radial) 16 330 0.014 5060 10 × 12,5 2 3 (B) ≥ 2(2) 16PS330MJ12 PXA, Poly-Alum (SMD) 16 330 0.014 5050 10 × 12,2 2 3 (B) ≥ 2(2) PXA16VC331MJ12TP PS, Poly-Alum (Radial) 6.3 680 0.010 5500 10 × 12,5 N/R(6) 2 (C) ≥ 1(2) 6PS680MJ12 PXA, Poly-Alum (Radial) 6.3 680 0.010 5500 10 × 12,2 N/R(6) 2 (C) ≥ 1(2) PXA6.3VC681MJ12TP (1) Capacitor Supplier Verification Please verify availability of capacitors identified in this table. Capacitor suppliers may recommend alternative part numbers because of limited availability or obsolete products. In some instances, the capacitor product life cycle may be in decline and have short-term consideration for obsolescence. RoHS, Lead-free and Material Details See the capacitor suppliers regarding material composition, RoHS status, lead-free status, and manufacturing process requirements. Component designators or part number deviations can occur when material composition or soldering requirements are updated. (2) Required capacitors with TurboTrans. See the TransTrans Application information for Capacitor Selection Capacitor Type Groups by ESR (Equivalent Series Resistance) : a. Type A = (100 < capacitance × ESR ≤ 1000) (use Type A capacitors in addition to Type B or C) b. Type B = (1,000 < capacitance × ESR ≤ 5,000) c. Type C = (5,000 < capacitance × ESR ≤ 10,000) (3) Total bulk nonceramic capacitors on the output bus with ESR of ≥ 15mΩ to ≤ 30mΩ requires an additional ≥ 200 µF of ceramic capacitor. (4) Aluminum Electrolytic capacitor not recommended for the TurboTrans due to higher ESR × capacitance products. Aluminum and higher ESR capacitors can be used in conjunction with lower ESR capacitance. (5) Output bulk capacitor's maximum ESR is ≥ 30 mΩ. Additional ceramic capacitance of ≥ 200 µF is required. (6) N/R – Not recommended. The voltage rating does not meet the minimum operating limits. Copyright © 2008, Texas Instruments Incorporated 11 Product Folder Link(s): PTH08T255W |
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