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Designing for Load Transients

PTH08080W

SLTS235A – FEBRUARY 2005 – REVISED MARCH 2005

The transient response of the dc/dc converter has been characterized using a load transient with a di/dt of

1 A/µs. The typical voltage deviation for this load transient is given in the data-sheet specification table using the

optional 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 output

capacitor decoupling. In these cases, special attention must be paid to the type, value, and ESR of the

capacitors selected.

If the transient performance requirements exceed those specified in the data sheet, the selection of output

capacitors becomes more important. Review the minimum ESR in the characteristic data sheet for details on the

capacitance maximum.

Table 3. Recommended Input/Output Capacitors

CAPACITOR CHARACTERISTICS

QUANTITY

85

°C

CAPACITOR VENDOR/

WORK-

EQUIVALENT

VENDOR

MAXIMUM

PHYSICAL

OUTPUT

COMPONENT

ING

VALUE

SERIES

INPUT

NUMBER

RIPPLE

SIZE

BUS

SERIES

VOLTAGE

(µF)

RESISTANCE

CURRENT

(mm)

(Optional)

(V)

(ESR) (

Ω)

Panasonic WA (SMT)

20

150

0.026

3700

10

× 10,2

1

≤ 2

EEFWA1D151P

FC (SMT)

25

220

0.150

670

10

× 10,2

1

1

EEVFC1E221P

Panasonic SL

6.3

47

0.018

2500

7,3

× 4,3

≤ 3

EEFCD0J470R

SP-cap(SMT)

6.3

120

0.007

3500

7,3

× 4,3

≤ 1

EEFSD0J121R

United Chemi-con PXA

16

150

0.026

3400

10

× 7,7

1

≤ 2

PXA16VC151MJ80TP

(SMT)

PS

25

100

0.020

4300

10

× 12,5

1

≤ 2

25PS100MJ12

LXZ

35

220

0.180

760

10

× 12,5

1

1

LXZ35VB221M10X12LL

MVZ (SMT)

25

470

0.090

670

10

× 10

1

1

MVZ25VC471MJ10TP

Nichicon UWG (SMT)

35

100

0.150

670

10

× 10

1

1

UWG1V101MNR1GS

F559(Tantalum)

10

100

0.055

2000

7,7

× 4,3

≤ 3

F551A107MN

HD

25

220

0.072

760

8

× 11,5

1

1

UHD1E221MPR

Sanyo Os-con\ POS-Cap

10

68

0.025

2400

7,3

× 4,3

≤ 2

10TPE68M

SVP (SMT)

20

150

0.020

4320

10

× 12,7

1

≤ 1

20SVP150M

SP

20

120

0.024

3110

8

× 10,5

1

≤ 2

20SP120M

AVX Tantalum TPS (SMD)

35

47

0.100

1430

7,3 L

× 4,3

2

≤ 4

TPSV476M035R0100

25

47

0.100

1150

W

× 4,1 H

2

≤ 4

TPSE476M025R0100

Kemet T520 (SMD)

10

100

0.025

> 2000

7,3 L

× 5,7

≤ 1

T520V107M010ASE025

AO-CAP

6.3

100

0.018

> 2900

W

× 4 H

≤ 1

A700V107M006AT

Vishay/Sprague

35

47

0.280

> 1000

7,3 L

× 6 W

2

≤ 5

595D476X0035R2T

594D/SVP(SMD)

20

100

0.025

3200

× 4,1 H 8 ×

1

≤ 2

94SVP107X0020E12

12

94SS

20

150

0.030

3200

10

× 10,5

1

≤ 2

94SS157X0020FBP

Murata Ceramic X5R

16

47

0.002

> 1400

3225

≤ 3

GRM32ER61C476M

TDK ceramic X5R

6.3

47

0.002

> 1400

3225

≤ 3

C3225X5R0J476MT

Kemet Ceramic X5R

6.3

47

0.002

> 1400

3225

≤ 3

C1210C476K9PAC

TDK Ceramic X7R

25

10

0.002

> 1400

3225

≤ 4

C3225X7R1E106K

Murata Ceramic X5R

25

10

0.002

> 1400

≤ 4

GRM32DR61E106KA12

Kemet

16

10

0.002

> 1400

≤ 4

C1210C106M4PAC

TDK Ceramic X7R

25

2.2

0.002

> 1400

3225

1

1

C3225X7R1E225KT/MT

Murata Ceramic X7R

25

2.2

0.002

> 1400

1

1

GRM32RR71J225KC01L

Kemet

25

2.2

0.002

> 1400

1

1

C1210C225K3RAC

(1)

The voltage rating of the input capacitor must be selected for the desired operating input voltage range of the regulator. To operate the

regulator at a higher input voltage, select a capacitor with a higher voltage rating.

(2)

The voltage rating of the input capacitor must be selected for the desired operating input voltage range of the regulator. To operate the

regulator at a higher input voltage, select a capacitor with a higher voltage rating.

(3)

Ceramic capacitors can be used to complement electrolytic types at the input bus by reducing high-frequency ripple current.

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