Output Capacitor (Required)

Output Capacitor Information

TurboTrans Output Capacitance

Non-TurboTrans Output Capacitance

PTH08T255W

The PTH08T255W requires a minimum output capacitance of 1000µF of polymer-aluminum, tantulum, or

polymer-tantalum type. Ceramic output capacitance can be used in addition to the required non-ceramic 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.

When selecting output capacitors, the main considerations are capacitor type, temperature stability, and ESR.

When using the TurboTrans feature, the capacitance × 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 10µF 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 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 (µF) × ESR (m

Ω) 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) (use Type A capacitors in addition to Type B or C)

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 330µF and an ESR of 5m

Ω, has a C×ESR product of 1650µFxmΩ (330µF ×

5m

Ω). This is a Type B capacitor. A capacitor with a capacitance of 1000µF and an ESR of 8mΩ, has a C×ESR

product of 8000µFxm

Ω (1000µF × 8mΩ). 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.

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 PTH08T255W, observe the minimum ESR of the entire output capacitor bank. The minimum

ESR limit of the output capacitor bank is 7m

Ω. A list of preferred low-ESR type capacitors, are identified in

Table 3.

When using the PTH08T255W without the TurboTrans feature, the maximum amount of capacitance is 1000 µF

of ceramic type. Large amounts of capacitance may reduce system stability.

Utilizing the TurboTrans feature improves system stability, improves transient response, and reduces the

amount of output capacitance required to meet system transient design requirements.

10

Copyright © 2008, Texas Instruments Incorporated

Product Folder Link(s): PTH08T255W