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Designing for Load Transients
PTN78020A
SLTS244A – 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/s. 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.
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
(F)
RESISTANCE
BUS
(V)
CURRENT
(mm)
(ESR) (
)
(mArms)
FC( Radial)
35
180
0.090
755
10 × 16
1
EEUFC1V331 (VI < 30 V)
FK (SMD)
50
330
0.12
900
12,5 × 13,5
1 (1)
1
EEVFK1H181Q
United Chemi-Con PXA (SMD)
16
150
0.026
3430
10 × 12,2
≤1
PXA16VC151MJ80TP (VI, |VO|
1 (1)
< 14 V)
PS
25
100
0.020
4320
10 × 12,5
1 (1)
≤ 1
25PS100M J12 (VI < 20 V)
LXZ
35
220
0.090
760
10 × 12,5
1 (1)
1
LXZ35VB221M10X12LL
MVZ (SMD)
25
470
0.090
670
10 × 10
1
MVZ25VC471MJ10TP (VI < 20
V)
Nichicon UWG (SMD)
35
330
0.15
670
10 × 10
1
UWG1V331MNR1GS
F550 (Tantalum)
10
100
0.055
2000
7,7 × 4,3
N/R (2)
≤ 3 (3)
F551A107MN (VO ≤ 5 V)
HD
50
150
0.061
979
10 × 12,5
1
UHD1H151MHR
Sanyo Os-Con SVP (SMD)
20
150
0.020
4300
10 × 12,7
1 (1)
≤ 1
20SVP150M (VI ≤ 16 V)
SP
20
120
0.024
3110
8 × 10,5
1 (1)
≤ 1
20SP180M (VI ≤ 16 V)
20
100
0.085
1543
7,3 L × 4,3
N/R (2)
≤ 3
TPSV107M020R0085
W × 4,1 H
(|VO| ≤ 10 V)
AVX Tantalum TPS (SMD)
20
100
0.200
> 817
3225
N/R (2)
≤ 3
TPSE107M020R0200
(|VO| ≤ 10 V)
Murata X5R Ceramic
6.3
100
0.002
>1000
3225
N/R (2)
≤ 2
GRM32ER60J107M
(|VO| ≤ 5.5 V)
TDK X5R Ceramic
6.3
100
0.002
>1000
3225
N/R (2)
≤ 2
C3225X5R0J107MT
(|VO| ≤ 5.5 V)
Murata X5R Ceramic
16
47
0.002
>1000
3225
4 (1)
≤ 4
GRM32ER61C476M
(|Vo| ~ VI ≤ 13.5 V)
Kemet X5R Ceramic
6.3
47
0.002
>1000
3225
N/R (2)
≤ 4
C1210C476K9PAC
(|VO| ≤ 5.5 V)
TDK X5R Ceramic
6.3
47
0.002
>1000
3225
N/R (2)
≤ 4
C3225X5R0J476MT
(|VO| ≤ 5.5 V)
Murata X5R Ceramic
6.3
47
0.002
>1000
3225
N/R (2)
≤ 4
GRM42-2X5R476M6.3
(|VO| ≤ 5.5 V)
TDK X7R Ceramic
25
2.2
0.002
>1000
3225
≥ 8 (4)
1
C3225X7R1E225KT/MT
(|VO| ≤ 20 V)
Murata X7R Ceramic
25
2.2
0.002
>1000
3225
≥ 8 (4)
1
GRM32RR71E225K
(|VO| ≤ 20 V)
Kemet X7R Ceramic
25
2.2
0.002
>1000
3225
≥ 8 (4)
1
C1210C225K3RAC
(VI, |VO| ≤ 20 V)
AVX X7R Ceramic
25
2.2
0.002
>1000
3225
≥ 8 (4)
1
C12103C225KAT2A
(VI, |VO| ≤ 20 V)
(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)
Not recommended (N/R). The voltage rating does not meet the minimum operating limits in most applications.
(3)
The maximum voltage rating of the capacitor must be selected for the desired set-point voltage (VO ). To operate at a higher output
voltage, select a capacitor with a higher voltage rating.
(4)
The maximum rating of the ceramic capacitor limits the regulator operating input voltage to 20 V. Select a alternative ceramic
component to operate at a higher input voltage.
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