PTH08T240F_15 Datasheet, PDF (13/39 Pages) Texas Instruments – 10-A, 4.5-V to 14-V INPUT, NON-ISOLATED POWER MODULE FOR 3-GHz DSP SYSTEMS

PTH08T240F
www.ti.com ............................................................................................................................................... SLTS277D – DECEMBER 2006 – REVISED MARCH 2009
Output Capacitor (Required)
The PTH08T240F requires a minimum output capacitance of 1000 µF of aluminum, polymer-aluminum, tantulum,
or polymer-tantalum type.
The required capacitance above the minimum is determined by actual transient deviation requirements. See the
TurboTrans Technology application section within this document for specific capacitance selection.
Output Capacitor Information
When selecting output capacitors, the main considerations are capacitor type, temperature stability, and ESR.
Consider the capacitance X ESR product (see the following section).
Ceramic output capacitors added for high-frequency bypassing should be located as close as possible to the
load to be effective. Ceramic capacitor values below 10 µF should not be included when calculating the total
output capacitance value.
When the operating temperature is below 0°C, the ESR of aluminum electrolytic capacitors increases. For these
applications, OS-CON, poly-aluminum, and polymer-tantalum types should be considered.
TurboTrans Output Capacitance
TurboTrans allows the designer to optimize the output capacitance according to the system transient design
requirement. High quality, ultra-low ESR capacitors are required to maximize TurboTrans effectiveness. The
capacitor's capacitance (µF) × ESR (mΩ) product determines its capacitor type; Type B, or C. Capacitor types
are defined as follows:
Type B = (1000 < capacitance × ESR ≤ 5000) (e.g. polymer-tantalum)
Type C = (5000 < capacitance × ESR ≤ 10,000) (e.g. OS-CON)
When using more than one type of output capacitor, select the capacitor type that makes up the majority of your
total output capacitance. When calculating the C×ESR product, use the maximum ESR value from the capacitor
manufacturer's datasheet.
Working Examples:
A capacitor with a capacitance of 330 µF and an ESR of 5 mΩ, has a C × ESR product of 1650 µF x mΩ (330 µF
× 5 mΩ). This is a Type B capacitor. A capacitor with a capacitance of 1000 µF and an ESR of 8 mΩ, has a
C × ESR product of 8000 µF x mΩ (1000 µF × 8 mΩ). This is a Type C capacitor.
See the TurboTrans Technology application section within this document for specific capacitance selection.
Table 3 includes a preferred list of capacitors by type and vendor. See the Output Bus / TurboTrans column.
Designing for Fast Load Transients
The transient response of the dc/dc converter has been characterized using a load transient with a di/dt of
2.5 A/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 100 A/µs, adding
multiple 10 µF ceramic capacitors plus 10 × 1 µF, and numerous high frequency ceramics (≤ 0.1 µF) 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.
Copyright © 2006–2009, Texas Instruments Incorporated
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