Designing for Fast Load Transients

PTH04T240W, PTH04T241W

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.

Capacitor Characteristics

Quantity

Max

Max

Ripple

Capacitor Vendor,

Working

ESR

Turbo-

Value

Current

Physical

Input

Type Series (Style)

No

Voltage

at 100

Vendor Part No.

Trans

(

µF)

at 85°C

Size (mm)

Bus

Turbo-

(V)

kHz

Capacitor

(Irms)

Trans

(m

Ω)

(mA)

Panasonic

SP series (UE)

6.3

220

15

3000

7,3×4,3

2

1

≤ 2

B

EEFUE0J221R

FC (Radial)

6.3

390

117

555

8 X 11,5

1

≥ 1

EEUFC0J391

FK (SMD)

6.3

470

160

600

10 X 10,2

1

≥ 1

EEVFK0J471P

United Chemi-Con

PTB, Poly-Tantalum(SMD)

6.3

330

25

2600

7,3×4,3×2,8

1

1

≤ 3

C

6PTB337MD6TER

LXZ, Aluminum (Radial)

6.3

680

120

555

8 X 12

1

1

LXZ6.3VB681M8X12LL

PS, Poly-Alum (Radial)

6.3

390

12

4770

8 X 11,5

1

≤ 1

B

6PS390MH11

PT Poly-Tantalum (SMD)

6.3

330

40

3000

7,3×4,3

1

1

6PT337MD8TER

MVY, Aluminum (SMD)

10

680

150

670

10 × 10

1

1

B

MVY10VC681MJ10TP

PXA, Poly-Alum (Radial)

10 V

330

14

4420

8 × 12,2

1

1

≤ 2

B

PXA10VC331MH12

Nichicon, Aluminum

WG (SMD)

10

470

150

670

10 × 10

1

1

UWG1A471MNR1GS

HD (Radial)

10

470

72

760

8 X 11,5

1

1

UHD1A471MPR

Panasonic, Poly-Aluminum

SE Series (SMD)

2.0

560

5

4000

7,3×4,3×4,2

B

(7)

(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.

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)

Additional output capacitance must include the required 100

µF of ceramic type.

(3)

Required capacitors with TurboTrans. See the TurboTrans Application information for Capacitor Selection

Capacitor Types:

a. Type A = (100 < capacitance × ESR

≤ 1000)

b. Type B = (1,000 < capacitance × ESR

≤ 5,000)

c. Type C = (5,000 < capacitance × ESR

≤ 10,000)

(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)

N/R – Not recommended. The voltage rating does not meet the minimum operating limits.

(6)

N/R – Not recommended. The ESR value of this capacitor is below the required minimum when not using TurboTrans.

(7)

The voltage rating of this capacitor only allows it to be used for output voltage that is equal to or less than 80% of the working voltage.

16

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