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Designing for Load Transients

PTH04070W

SLTS227 – SEPTEMBER 2004

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/

EQUIVALENT

VENDOR

MAXIMUM

PHYSICAL

COMPONENT

WORKING

VALUE

SERIES

INPUT

OUTPUT

NUMBER

RIPPLE

SIZE

SERIES

VOLTAGE

µF

RESISTANCE

BUS

CURRENT

(mm)

(ESR)

Panasonic WA (SMT)

10 V

120

0.035

Ω

2800 mA

8

×6.9

1

FC (SMT)

25 V

47

0.400

Ω

230 mA

8

×6.2

1

Panasonic SL SP-cap(SMT)

6.3 V

47

0.018

Ω

2500 mA

7.3

×4.3

1

≤ 3

EEFCD0J470R

6.3 V

56

0.009

Ω

3000 mA

7.3

× 4.3

1

≤ 2

EEFSL0J560R

United Chemi-con PXA (SMT)

10 V

47

0.031

Ω

2250 mA

6.3

×5.7

1

1

PXA10VC470MF60TP

FS

10 V

100

0.040

Ω

2100 mA

6.3

×9.8

1

≤ 5

10FS100M

LXZ

16 V

100

0.250

Ω

290 mA

6.3

×11.5

1

1

LXZ16VB101M6X11LL

MVZ (SMT)

16 V

100

0.440

Ω

230mA

6.3

×5.7

1

1

MVZ16VC101MF60TP

Nichicon UWG (SMT)

16 V

100

0.400

Ω

230mA

8

×6.2

1

1

UWG1C101MCR1GS

F559(Tantalum)

10 V

100

0.055

Ω

2000mA

7.7

× 4.3

1

≤ 5

F551A107MN

PM

10 V

100

0.550

Ω

210 mA

6

×11

1

1

UPM1A101MEH

Sanyo Os-con\ POS-Cap SVP

10V

68

0.025

Ω

2400 mA

7.3

×4.3

1

≤ 5

10TPE68M

(SMT)

6.3 V

47

0.074

Ω

1110 mA

5

×6

1

≤ 5

6SVP47M

SP

10 V

56

0.045

Ω

1710 mA

6.3

×5.0

1

≤ 5

10SP56M

AVX Tantalum TPS (SMD)

10 V

47

0.100

Ω

1100 mA

7.3L

× 4.3W

1

≤ 5

TPSD476M010R0100

10 V

47

0.060

Ω

> 412 mA

× 4.1H

1

≤ 5

TPSB476M010R0500

Kemet T520 (SMD)

10 V

68

0.060

Ω

>1200 mA

7.3L

× 5.7W

1

≤ 5

T520V686M010ASE060

AO-CAP

6.3 V

47

0.028

Ω

>1100 mA

× 4.0H

1

≤ 3

A700V476M006AT

Vishay/Sprague 594D/595D

10 V

68

0.100

Ω

>1000 mA

7.3L

× 6.0W

1

≤ 5

594D686X0010C2T

(SMD)

10 V

68

0.240

Ω

680 mA

× 4.1H

1

≤ 5

595D686X0010C2T

94SL

16 V

47

0.070

Ω

1550 mA

8

× 5

1

≤ 5

94SL476X0016EBP

TDK Ceramic X5R

6.3 V

22

0.002

Ω

>1400 mA

1210 case

≤ 3

C3225X5R0J226KT/MT

Murata Ceramic X5R

6.3 V

22

0.002

Ω

>1000 mA

3225 mm

≤ 3

GRM32ER61J223M

Kemet

6.3 V

22

0.002

Ω

>1000 mA

≤ 3

C1210C226K9PAC

TDK Ceramic X5R

6.3 V

47

0.002

Ω

>1400 mA

1210 case

≥ 1

≤ 3

C3225X5R0J476KT/MT

Murata Ceramic X5R

6.3 V

47

0.002

Ω

>1000 mA

3225 mm

≥ 1

≤ 3

GRM32ER60J476M/6.3

Kemet

6.3 V

47

0.002

Ω

>1000 mA

≥ 1

≤ 3

C1210C476K9PAC

(1)

A ceramic capacitor is required on both the input and the output. An electrolytic capacitor can be added to the output for improved

transient response.

(2)

An optional through-hole capacitor available.

(3)

A total capacitance of 44 µF is an acceptable replacement for a single 47-µF capacitor.

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