UCC28951
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ZHCSIQ7A –AUGUST 2018 –REVISED DECEMBER 2021
8.2.2.7 Output Capacitance (COUT
)
The output capacitor is selected based on holdup and transient (VTRAN) load requirements.
The time it takes LOUT to change 90% of its full load current is calculated in 方程式67:
L
OUT ´POUT ´0.9
VOUT
tHU
=
= 7.5ms
VOUT
(67)
During load transients most of the current will immediately go through the capacitors equivalent series resistance
(ESRCOUT). 方程式 68 and 方程式 69 are used to select ESRCOUT and COUT based on a 90% load step in
current. The ESR is selected for 90% of the allowable transient voltage (VTRAN), while the output capacitance
(COUT) is selected for 10% of VTRAN
.
V
TRAN ´0.9
ESRCOUT
£
= 12mW
P
OUT ´0.9
VOUT
(68)
(69)
P
OUT ´0.9´ tHU
VOUT
TRAN ´0.1
COUT
³
» 5.6mF
V
Before selecting the output capacitor, the output capacitor RMS current (ICOUT_RMS) must be calculated using 方
程式70.
DILOUT
ICOUT _RMS
=
» 5.8A
3
(70)
To meet the design requirements five 1500-µF, aluminum electrolytic capacitors are chosen for the design from
United Chemi-Con™, part number EKY-160ELL152MJ30S. These capacitors have an ESR of 31 mΩ.
The number of output capacitors (n) is 5.
The total output capacitance is calculated using 方程式71:
COUT = 1500mF´n » 7500mF
(71)
The effective output capacitance ESR is calculated using 方程式72:
31mW
ESRCOUT
=
= 6.2mW
n
(72)
(73)
Calculate output capacitor loss (PCOUT) using 方程式73:
2
PCOUT = ICOUT _RMS ´ESRCOUT » 0.21W
Recalculate the remaining Power Budget using 方程式74:
Copyright © 2023 Texas Instruments Incorporated
English Data Sheet: SLUSDB2
52
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