Datasheet
PTH08T250W
www.ti.com
SLTS278G –NOVEMBER 2010–REVISED MARCH 2009
Designing for Fast Load Transients
The transient response of the dc/dc converter has been characterized using a load transient with a di/dt of
2.5 A/µs. 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 100 A/ms, adding
multiple 10 mF ceramic capacitors plus 10 × 1 mF, and numerous high frequency ceramics (≤ 0.1 mF) 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.
Capacitor Table
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.
Ripple
Capacitor Vendor,
Working ESR at
Value Current Physical Input
Type Series (Style)
No
Voltage 100 Vendor Part No.
TurboTrans
(µF) at 85°C Size (mm) Bus
Turbo
(V) kHz
(Cap Type)
(2)
(Irms)
Trans
(Ω)
(mA)
Panasonic 25 1000 0.043 1690 16 × 15 1 ≥ 2
(3)
N/R
(4)
EEUFC1E102S
FC (Radial) 25 1800 0.029 2205 16 × 20 1 ≥ 1
(3)
N/R
(4)
EEUFC1E182
FC (SMD) 25 2200 0.028 2490 18 × 21,5 1 ≥ 1
(3)
N/R
(4)
EEVFC1E222N
FK (SMD) 25 1000 0.060 1100 12,5×13,5 1 ≥ 2
(5)
N/R
(4)
EEVFK1V102Q
United Chemi-Con
PTB Poly-Tant (SMD) 6.3 330 0.025 2600 7,3x4,3x 2,8 N/R
(6)
2 - 4
(3)
(C) ≥ 2
(2)
4PTB337MD6TER
LXZ, Aluminum (Radial) 25 680 0.068 1050 10 × 16 1 1 - 3
(3)
N/R
(4)
LXZ25VB681M10X20LL
PS, Poly-Alum (Radial) 16 330 0.014 5060 10 × 12,5 2 2 - 3 (B) ≥ 2
(2)
16PS330MJ12
PXA, Poly-Alum (SMD) 16 330 0.014 5050 10 × 12,2 2 2 - 3 (B) ≥ 2
(2)
PXA16VC331MJ12TP
PS, Poly-Alum (Radial) 6.3 680 0.010 5500 10 × 12,5 N/R
(6)
1 - 2 (C) ≥ 1
(2)
6PS680MJ12
PXA, Poly-Alum (Radial) 6.3 680 0.010 5500 10 × 12,2 N/R
(6)
1 - 2 (C) ≥ 1
(2)
PXA6.3VC681MJ12TP
(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. In some instances, the capacitor product life cycle may be in decline and have short-term
consideration for obsolescence.
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 TransTrans Application information for Capacitor Selection
Capacitor Type Groups by ESR (Equivalent Series Resistance) :
(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 of ≥ 15mΩ to ≤ 30mΩ requires an additional ≥ 200 mF of ceramic
capacitor.
(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) Output bulk capacitor's maximum ESR is ≥ 30 mΩ. Additional ceramic capacitance of ≥ 200 mF is required.
(6) N/R – Not recommended. The voltage rating does not meet the minimum operating limits.
Copyright © 2010–2009, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): PTH08T250W