Designing for Fast Load Transients

Capacitor Table

PTH08T255W

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.

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

(Irms)

Trans

(

Ω)

(mA)

Panasonic

25

1000

0.043

1690

16 × 15

1

EEUFC1E102S

FC (Radial)

25

1800

0.029

2205

16 × 20

1

EEUFC1E182

FC (SMD)

25

2200

0.028

2490

18 × 21,5

1

EEVFC1E222N

FK (SMD)

25

1000

0.060

1100

12,5×13,5

1

EEVFK1V102Q

United Chemi-Con

PTB Poly-Tant (SMD)

6.3

330

0.025

2600

7,3x4,3x 2,8

(C)

4PTB337MD6TER

LXZ, Aluminum (Radial)

25

680

0.068

1050

10 × 16

1

LXZ25VB681M10X20LL

PS, Poly-Alum (Radial)

16

330

0.014

5060

10 × 12,5

2

3

(B)

16PS330MJ12

PXA, Poly-Alum (SMD)

16

330

0.014

5050

10 × 12,2

2

3

(B)

PXA16VC331MJ12TP

PS, Poly-Alum (Radial)

6.3

680

0.010

5500

10 × 12,5

2

(C)

6PS680MJ12

PXA, Poly-Alum (Radial)

6.3

680

0.010

5500

10 × 12,2

2

(C)

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

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