TPS54308 Datasheet, PDF (17/28 Pages) Texas Instruments – 4.5-V to 28-V Input, 3-A Output, Synchronous 350-kHz FCCM Step-Down Converter in SOT23 Package

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TPS54308
SLUSCV2 – JUNE 2017
C_O
!
2 u 'I_OUT
f_sw u 'V_OUT
(11)
where:
∆I_OUT is the change in output current
ƒ_SW is the switching frequency of the regulator
∆V_{(OUT )}b is the allowable change in the output voltage
For this example, the transient load response is specified as a 5% change in the output voltage, V_OUT, for a load
step of 1.5 A. For this example, ΔI_OUT = 1.5 A and ΔV_OUT = 0.05 × 3.3 = 0.165 V. Using these values results in a
minimum capacitance of 52 μF. This value does not consider the ESR of the output capacitor in the output
voltage change. For ceramic capacitors, the ESR is usually small enough to ignore in this calculation.
Equation 12 calculates the minimum output capacitance required to meet the output voltage ripple specification.
In this case, the maximum output voltage ripple is 30 mV. Under this requirement, Equation 12 yields 9.9 μF.
C_O
>
1
8 ´ f_SW
´
1
V_OUTripple
I_ripple
(12)
where:
ƒ_SW is the switching frequency
V_(OUTripple) is the maximum allowable output voltage ripple
I_(ripple) is the inductor ripple current
Use Equation 13 to calculate the maximum ESR an output capacitor can have to meet the output-voltage ripple
specification. Equation 13 indicates the ESR should be less than 36 mΩ. In this case, the ESR of the ceramic
capacitor is much smaller than 36 mΩ.
R_ESR
<
V_OUTripple
I_ripple
(13)
The output capacitor can affect the crossover frequency ƒ_o. Considering to the loop stability and effect of the
internal parasitic parameters, choose the crossover frequency less than 40 kHz without considering the feed
forward capacitor. A simple estimation for the crossover frequency without feed forward capacitor C6 is shown in
Equation 14, assuming C_OUT has small ESR.
f_o
=
5.1
V_OUT u C_O
(14)
Additional capacitance deratings for aging, temperature, and DC bias should be considered which increases this
minimum value. For this example, two 22-uF 25-V, X7R ceramic capacitors are 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
RMS value of the maximum ripple current. Use Equation 15 to calculate the RMS ripple current that the output
capacitor must support. For this application, Equation 15 yields 120 mA for each capacitor.
I_{COUT RMS}
1
12
§
u
¨
¨
¨©
V_OUT u
V_{IN MAX}
V_{IN MAX} V_OUT
u L_O u f_SW u N_C
·
¸
¸
¸¹
(15)
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