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Designing for Load Transients

PTN78060A

SLTS245A – APRIL 2005 – REVISED MAY 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

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 of

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

WORKING

MAXIMUM

PHYSICAL

COMPONENT

VALUE

SERIES

INPUT

OUTPUT

NUMBER

VOLTAGE

RIPPLE

SIZE

SERIES

(µF)

RESISTANCE

BUS

BUS

(V)

CURRENT

(mm)

(ESR) (

Ω)

FC( Radial)

35

100

0.117

555

8 x 11,5

≥1

1

EEUFC1V01

FC (SMD)

35

100

0.015

670

10 x10,2

≥1

1

EEVFC1V101P

United Chemi-Con PXA (SMD)

16

180

0.016

4360

8 x 12

≤1

14 V)

PS

25

100

0.020

4300

10 x 12,5

≤ 1

LXZ

50

100

0.22

485

8 x 12,5

1

LXZ50VB101M8X12LL

MVY(SMD)

50

100

0.300

500

10 x 10

≥1

1

MVY50VC101M10X10TP

Nichicon UWG (SMD)

50

100

0.300

500

10 x 10

≥1

1

UWG1H101MNR1GS

F550 (Tantalum)

10

100

0.055

2000

7,7 x 4,3

HD

50

120

0.072

979

10 x12,5

≥1

1

UHD1H151MHR

Sanyo Os-Con SVP (SMD)

20

100

0.024

2500

8 x 12

≤ 1

SP

16

100

0.032

2890

10 x 5

≤ 1

20

100

0.085

1543

7,3 L x 4,3

≤ 3

TPSV107M020R0085

W x 4,1 H

AVX Tantalum TPS (SMD)

20

100

0.200

> 817

≤ 3

TPSE107M020R0200

Murata X5R Ceramic

16

47

0.002

>1000

3225

≤ 3

GRM32ER61C476M

Murata X5R Ceramic

6.3

47

0.002

>1000

3225

≤ 3

GRM42-2X5R476M6.3

TDK X7R Ceramic

25

2.2

0.002

>1000

3225

1

C3225X7R1E225KT/MT

Murata X7R Ceramic

25

2.2

0.002

>1000

3225

1

GRM32RR71E225K

Kemet X7R Ceramic

25

2.2

0.002

>1000

3225

1

C1210C225K3RAC

AVX X7R Ceramic

25

2.2

0.002

>1000

3225

1

C12103C225KAT2A

Murata X7R Ceramic

50

4.7

0.002

>1000

3225

≥ 3

1

GRM32ER71H475KA88L

TDK X7R Ceramic

50

2.2

0.002

>1000

3225

≥ 6

1

C3225X7R1H225KT

Murata Radial Through-hole

50

2.2

0.004

>1000

10 H x 10 W

≥ 6

1

RPER71H2R2KK6F03

x 4 D

(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 the next higher voltage rating.

(2)

voltage, select a capacitor with a higher voltage rating.

(3)

Not recommended (N/R). The voltage rating does not meet the minimum operating limits in most applications.

(4)

The maximum rating of the ceramic capacitor limits the regulator's operating input voltage to 20 V. Select an alternative ceramic

component to operate at a higher input voltage.

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