TPS65273V_15 Datasheet, PDF (28/46 Pages) Texas Instruments – 4.5-V TO 18-V INPUT VOLTAGE, 3.5-A/3.5-A DUAL SYNCHRONOUS STEP-DOWN CONVERTER WITH I2C CONTROLLED VID AND CURRENT SHARING

TPS65273V
SLVSBV9A – FEBRUARY 2013 – REVISED JANUARY 2014
www.ti.com
Equation 11 calculates the minimum output capacitance needed to meet the output voltage ripple specification.
Where f_SW is the switching frequency, V_oripple is the maximum allowable output voltage ripple, and I_oripple is the
inductor ripple current.
C_o
>
1
8 × f_sw
×
1
V_oripple
I_oripple
(11)
Equation 12 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple
specification.
R_esr
<
V_oripple
I_oripple
(12)
Additional capacitance de-ratings for aging, temperature and DC bias should be factored in which increases this
minimum value.
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 13 can be used
to calculate the RMS ripple current the output capacitor needs to support.
I_corms
=
V_out × (V_inmax - V_out )
12 × V_inmax × L × f_sw
(13)
Input Capacitor Selection
The TPS65273V requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 10-µF
of effective capacitance on the PVIN input voltage pins. In some applications additional bulk capacitance may
also be required for the PVIN input. The effective capacitance includes any DC bias effects. The voltage rating of
the input capacitor must be greater than the maximum input voltage. The capacitor must also have a ripple
current rating greater than the maximum input current ripple of the TPS65273V. The input ripple current can be
calculated using Equation 14.
( ) _{Iinrms = Iout ×}
V_{out ×} ^V_inmin ^{- V}_out
V_{in min}
V_{in min}
(14)
The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the
capacitor. The capacitance variations due to temperature can be minimized by selecting a dielectric material that
is stable over temperature. X5R and X7R ceramic dielectrics are usually selected for power regulator capacitors
because they have a high capacitance to volume ratio and are fairly stable over temperature. The output
capacitor must also be selected with the DC bias taken into account. The capacitance value of a capacitor
decreases as the DC bias across a capacitor increases. For this example design, a ceramic capacitor with at
least a 25-V voltage rating is required to support the maximum input voltage. TPS65273V may operate from a
single supply. The input capacitance value determines the input ripple voltage of the regulator. The input voltage
ripple can be calculated using Equation 15.
DV_in
=
I_{out max} × 0.25
C_in × f_sw
(15)
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