Datasheet

( ) ( )
( )
( ) ( )
( )
-
> ´
-
2 2
OH OL
OUT O
2 2
f i
I I
C L
V V
OUT
OUT
SW OUT
2 I
C
Vf
´ D
>
´ D
TPS54160 , TPS54160A
www.ti.com SLVSB56C –MAY 2012–REVISED FEBRUARY 2014
9.2.2.3 Output Capacitor
There are three primary considerations for selecting the value of the output capacitor. The output capacitor will
determine the modulator pole, the output voltage ripple, and how the regulators responds to a large change in
load current. The output capacitance needs to be selected based on the more stringent of these three criteria.
The desired response to a large change in the load current is the first criteria. The output capacitor needs to
supply the load with current when the regulator can not. This situation would occur if there are desired hold-up
times for the regulator where the output capacitor must hold the output voltage above a certain level for a
specified amount of time after the input power is removed. The regulator also will temporarily not be able to
supply sufficient output current if there is a large, fast increase in the current needs of the load such as
transitioning from no load to a full load. The regulator usually needs two or more clock cycles for the control loop
to see the change in load current and output voltage and adjust the duty cycle to react to the change. The output
capacitor must be sized to supply the extra current to the load until the control loop responds to the load change.
The output capacitance must be large enough to supply the difference in current for twoclock cycles while only
allowing a tolerable amount of droop in the output voltage. Equation 32 shows the minimum output capacitance
necessary to accomplish this.
Where ΔI
OUT
is the change in output current, ƒsw is the regulators switching frequency and ΔVout is the
allowable change in the output voltage. For this example, the transient load response is specified as a 4%
change in V
OUT
for a load step from 0 A (no load) to 1.5 A (full load). For this example, ΔI
OUT
= 1.5-0 = 1.5 A and
ΔV
OUT
= 0.04 × 3.3 = 0.132 V. Using these numbers gives a minimum capacitance of 18.9 μF. This value does
not take the ESR of the output capacitor into account in the output voltage change. For ceramic capacitors, the
ESR is usually small enough to ignore in this calculation. Aluminum electrolytic and tantalum capacitors have
higher ESR that should be taken into account.
The catch diode of the regulator cannot sink current so any stored energy in the inductor produces an output
voltage overshoot when the load current rapidly decreases, see Figure 53. The output capacitor must also be
sized to absorb energy stored in the inductor when transitioning from a high load current to a lower load current.
The excess energy that gets stored in the output capacitor increases the voltage on the capacitor. The capacitor
must be sized to maintain the desired output voltage during these transient periods. Equation 33 is used to
calculate the minimum capacitance to keep the output voltage overshoot to a desired value. Where L is the value
of the inductor, I
OH
is the output current under heavy load, I
OL
is the output under light load, Vf is the final peak
output voltage, and Vi is the initial capacitor voltage. For this example, the worst case load step will be from 1.5
A to 0 A. The output voltage increases during this load transition and the stated maximum in our specification is
4% of the output voltage. This will make Vf = 1.04 × 3.3 = 3.432. Vi is the initial capacitor voltage which is the
nominal output voltage of 3.3 V. Using these numbers in Equation 33 yields a minimum capacitance of 25.3 μF.
Equation 34 calculates the minimum output capacitance needed to meet the output voltage ripple specification.
Where f
SW
is the switching frequency, V
OUT(ripple)
is the maximum allowable output voltage ripple, and I
ripple
is the
inductor ripple current. Equation 34 yields 0.7 μF.
Equation 35 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple
specification. Equation 35 indicates the ESR should be less than 147 mΩ.
The most stringent criteria for the output capacitor is 25.3 μF of capacitance to keep the output voltage in
regulation during an unload transient.
Additional capacitance de-ratings for aging, temperature and dc bias should be factored in which increases this
minimum value. For this example, a 47 μF 6.3V X7R ceramic capacitor with 5 mΩ of ESR is used.
Capacitors generally have limits to the amount of ripple current they can handle without failing or producing
excess heat. An output capacitor that can support the inductor ripple current must be specified. Some capacitor
data sheets specify the Root Mean Square (RMS) value of the maximum ripple current. Equation 36 can be used
to calculate the RMS ripple current the output capacitor needs to support. For this application, Equation 36 yields
64.8 mA.
(32)
(33)
Copyright © 2012–2014, Texas Instruments Incorporated Submit Documentation Feedback 33
Product Folder Links: TPS54160 TPS54160A