Output Capacitor (Required)

Output Capacitor Information

TurboTrans Output Capacitance

Designing for Fast Load Transients

PTH04T240F

The PTH04T240F requires a minimum output capacitance of 1000

µF of aluminum, polymer-aluminum, tantulum,

or polymer-tantalum type.

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.

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

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 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 (mΩ) product determines its capacitor type; Type B,

or Type C. These two 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 5mΩ, has a C×ESR product of 1650µFxmΩ (330µF ×

5m

Ω). This is a Type B capacitor. A capacitor with a capacitance of 1000

µF and an ESR of 8mΩ, has a C×ESR

product of 8000

µFxmΩ (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.

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 10

µF 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.

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