Datasheet

Designing for Fast Load Transients
Capacitor Table
PTH08T220W , PTH08T221W
www.ti.com ................................................................................................................................................... SLTS252K – NOVEMBER 2005 – REVISED JUNE 2009
The transient response of the dc/dc converter has been characterized using a load transient with a di/dt of
2.5A/ µ s. 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/ µ s, adding
multiple 10 µ F ceramic capacitors plus 10 × 1 µ F, and numerous high frequency ceramics ( ≤ 0.1 µ F) 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.
Table 3 identifies the characteristics of acceptable capacitors from a number of vendors. The recommended
number of capacitors required at both the input and output buses is identified for each capacitor.
This is not an extensive capacitor list. Capacitors from other vendors are available with comparable
specifications. Those listed are for guidance. The RMS ripple current rating and ESR (at 100 kHz) are critical
parameters necessary to ensure both optimum regulator performance and long capacitor life.
Table 3. Input/Output Capacitors
(1)
Capacitor Characteristics Quantity
Max Output Bus
Max.
Capacitor Vendor,
Ripple
Working Value ESR Physical Input
Type Series (Style)
No Turbo-
Current Vendor Part No.
Voltage ( µ F) at 100 Size (mm) Bus
Turbo- Trans
at 85 ° C
kHz
Trans Cap Type
(2)
(Irms)
Panasonic
FC (Radial) 25 V 1000 43m Ω 1690mA 16 × 15 1 ≥ 2
(3)
N/R
(4)
EEUFC1E102S
FC (Radial) 25 V 820 38m Ω 1655mA 12 × 20 1 ≥ 1
(3)
N/R
(4)
EEUFC1E821S
FC (SMD) 35 V 470 43m Ω 1690mA 16 × 16,5 1 ≥ 1
(3)
N/R
(4)
EEVFC1V471N
FK (SMD) 35 V 1000 35m Ω 1800mA 16 × 16,5 1 ≥ 2
(3)
N/R
(4)
EEVFK1V102M
United Chemi-Con
PTB, Poly-Tantalum(SMD) 6.3 V 330 25m Ω 2600mA 7,3 × 4,3 × 2.8 N/R
(5)
1 ~ 4
(3)
C ≥ 2
(2)
6PTB337MD6TER (V
O
≤ 5.1V)
(6)
LXZ, Aluminum (Radial) 35 V 680 38m Ω 1660mA 12,5 × 20 1 1 ~ 3
(3)
N/R
(4)
LXZ35VB681M12X20LL
PS, Poly-Alum (Radial) 16 V 330 14m Ω 5060mA 10 × 12,5 1 1 ~ 3 B ≥ 2
(2)
16PS330MJ12
PS, Poly-Alum (Radial) 6.3 V 390 12m Ω 5500mA 8 × 12,5 N/R
(5)
1 ~ 2 B ≥ 1
(2)
6PS390MH11 (V
O
≤ 5.1V)
(6)
PXA, Poly-Alum (SMD) 16 V 330 14m Ω 5050mA 10 × 12,2 1 1 ~ 3 B ≥ 2
(2)
PXA16VC331MJ12TP
PXA, Poly-Alum (Radial) 10 V 330 14m Ω 4420mA 8 × 12,2 N/R
(5)
1 ~ 2 B ≥ 1
(2)
PXA10VC331MH12
(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) 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)
(3) Total bulk nonceramic capacitors on the output bus with ESR ≥ 15m Ω to ≤ 30m Ω requires an additional 200 µ F of ceramic capacitance.
(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) 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.
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