Datasheet

Designing for Fast Load Transients
PTH08T240W , PTH08T241W
www.ti.com ................................................................................................................................................... SLTS264J – 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. Input/Output Capacitors
(1)
Capacitor Characteristics Quantity
Max Output Bus
Max. Ripple
Capacitor Vendor,
Working ESR Current
Turbo-
Value Physical Input
Type Series (Style)
No
Voltage at 100 at Vendor Part No.
Trans
( µ F) Size (mm) Bus
Turbo-
(V) kHz 85 ° C
(Cap
Trans
(m Ω ) (Irms)
Type)
(2)
(mA)
Panasonic
FC (Radial) 25 270 90 755 10 × 12,5 ≥ 1
(3)
≥ 1
(4)
N/R
(5)
EEUFC1E271
FC (Radial) 25 560 65 1205 12,5 × 15 ≥ 1
(3)
≥ 1
(4)
N/R
(5)
EEUFC1E561S
FC(SMD) 25 470 65 1200 12,5 × 16,5 ≥ 1
(3)
≥ 1
(4)
N/R
(5)
EEVFC1E471LQ
FK(SMD) 25 470 80 850 10 × 10,2 ≥ 1
(3)
≥ 1
(4)
N/R
(5)
EEVFK1E471P
United Chemi-Con
PTB(SMD) Poly-Tantalum 6.3 330 25 2600 7,3x4,3x2.8 N/R
(6)
1 ~ 4
(4)
C ≥ 2
(2)
4PTB337MD6TER (V
O
≤ 5.1V)
(7)
LXZ, Aluminum (Radial) 25 330 90 760 10 × 12,5 ≥ 1
(3)
≥ 1
(4)
N/R
(5)
LXZ25VB331M10X12LL
PS, Poly-Alum(Radial) 16 330 14 5060 10 × 12,5 ≥ 1
(3)
1 ~ 3 B ≥ 2
(2)
16PS330MJ12
PXA, Poly-Alum(SMD) 16 330 14 5050 10 × 12,2 ≥ 1
(3)
1 ~ 3 B ≥ 2
(2)
PXA16VC331MJ12TP
PS, Poly-Alum(Radial) 10 270 14 4420 8 × 11,5 N/R
(6)
1 ~ 2 B ≥ 2
(2)
10PS270MH11
PXA, Poly-Alum(Radial) 10 330 14 4420 8 × 12 N/R
(6)
1 ~ 2 B ≥ 2
(2)
PXA10VC331MH12
Nichicon, Aluminum
HD (Radial) 25 220 72 760 8 × 11,5 ≥ 1
(3)
≥ 1
(4)
N/R
(5)
UHD1E221MPR
PM (Radial) 25 330 95 750 10 × 15 ≥ 1
(3)
≥ 1
(4)
N/R
(5)
UPM1E331MPH6
PM (Radial) 35 560 48 1360 16 × 15 ≥ 1
(3)
≥ 2
(4)
N/R
(5)
UPM1V561MHH6
Panasonic, Poly-Aluminum 2.0 390 5 4000 7,3 × 4,3 × 4,2 N/R
(6)
N/R
(6)
B ≥ 2
(2)
EEFSE0J391R (V
O
≤ 1.6V)
(8)
(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) In addition to the required input electrolytic capacitance, ≥ 20 µ F of ceramic capacitance is required to reduce the high-frequency
reflected ripple current.
(4) Total bulk nonceramic capacitors on the output bus with ESR of ≥ 15m Ω to ≤ 30m Ω requires an additional ≥ 200 µ F of ceramic
capacitance.
(5) Aluminum Electrolytic capacitor not recommended for the TurboTrans due to higher capacitance × ESR products. Aluminum and higher
ESR capacitors can be used in conjunction with lower ESR capacitance.
(6) N/R – Not recommended. The voltage rating does not meet the minimum operating limits.
(7) The voltage rating and derating of this capacitor only allows it to be used for voltages that are equal or less than 5.1 V.
(8) 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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