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SLTS265K – NOVEMBER 2005 – REVISED JUNE 2010
Output Capacitor (Required)
The PTH08T231W requires a minimum output capacitance of 200 mF of ceramic type.
The PTH08T230W requires a minimum output capacitance of 200 mF ceramic type. An optional 100 mF of
non-ceramic,
low-ESR
capacitance
is
recommended
for
improved
performance.
See
the
Electrical
Characteristics table for maximum capacitor limits.
The required capacitance above the minimum will be 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.
When using the TurboTrans feature, the capacitance x ESR product should also be considered (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 mF 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. When
using TurboTrans, the capacitor's capacitance (in mF) × ESR (in m) product determines its capacitor type; Type
A, B, or C. These three types are defined as follows:
Type A = (100
≤ capacitance × ESR ≤ 1000) (e.g. ceramic)
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 data sheet.
Working Examples:
A capacitor with a capacitance of 330 mF and an ESR of 5 m, has a C × ESR product of 1650 mF x m (330 ×
5). This is a Type B capacitor. A capacitor with a capacitance of 1000 mF and an ESR of 8 m, has a C × ESR
product of 8000 mF x m (1000 × 8). 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.
Non-TurboTrans Output Capacitance
If the TurboTrans feature is not used, minimum ESR and maximum capacitor limits must be followed. System
stability may be effected and increased output capacitance may be required without TurboTrans.
When using the PTH08T230W without the TurboTrans feature, observe the minimum ESR of the entire output
capacitor bank. The minimum ESR limit of the output capacitor bank is 7 m
. A list of preferred low-ESR type
capacitors, are identified in
Table 3. Large amounts of capacitance may reduce system stability when not using
the TurboTrans feature.
When using the PTH08T231W without the TurboTrans feature, the maximum amount of capacitance is 5000 mF
of ceramic type. Large amounts of capacitance may reduce system stability.
Using the TurboTrans feature improves system stability, improves transient response, and reduces the
amount of output capacitance required to meet system transient design requirements.
Copyright 2005–2010, Texas Instruments Incorporated
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