Datasheet

Iout max 0.25
ΔVin =
Cin sw
´
´ ¦
( )
Vin min Vout
Vout
Icirms = Iout
Vin min Vin min
-
´ ´
TPS54240
www.ti.com SLVSAA6A –APRIL 2010–REVISED DECEMBER 2012
Input Capacitor
The TPS54240 requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 3 μF of
effective capacitance and in some applications a bulk capacitance. 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 TPS54240.
The input ripple current can be calculated using Equation 38.
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 60V voltage rating is required to support the
maximum input voltage. Common standard ceramic capacitor voltage ratings include 4V, 6.3V, 10V, 16V, 25V,
50V or 100V so a 100V capacitor should be selected. For this example, two 2.2μF, 100V capacitors in parallel
have been selected. Table 2 shows a selection of high voltage capacitors. The input capacitance value
determines the input ripple voltage of the regulator. The input voltage ripple can be calculated using Equation 39.
Using the design example values, Ioutmax = 2.5 A, Cin = 4.4μF, ƒsw = 300 kHz, yields an input voltage ripple of
206 mV and a rms input ripple current of 1.15 A.
(38)
(39)
Table 2. Capacitor Types
VENDOR VALUE (μF) EIA Size VOLTAGE DIALECTRIC COMMENTS
1.0 to 2.2 100 V
1210 GRM32 series
1.0 to 4.7 50 V
Murata
1.0 100 V
1206 GRM31 series
1.0 to 2.2 50 V
1.0 10 1.8 50 V
2220
1.0 to 1.2 100 V
Vishay VJ X7R series
1.0 to 3.9 50 V
2225
1.0 to 1.8 100 V
X7R
1.0 to 2.2 100 V
1812 C series C4532
1.5 to 6.8 50 V
TDK
1.0. to 2.2 100 V
1210 C series C3225
1.0 to 3.3 50 V
1.0 to 4.7 50 V
1210
1.0 100 V
AVX X7R dielectric series
1.0 to 4.7 50 V
1812
1.0 to 2.2 100 V
Slow Start Capacitor
The slow start capacitor determines the minimum amount of time it will take for the output voltage to reach its
nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This
is also used if the output capacitance is large and would require large amounts of current to quickly charge the
capacitor to the output voltage level. The large currents necessary to charge the capacitor may make the
TPS54240 reach the current limit or excessive current draw from the input power supply may cause the input
voltage rail to sag. Limiting the output voltage slew rate solves both of these problems.
Copyright © 2010–2012, Texas Instruments Incorporated Submit Documentation Feedback 33
Product Folder Links: TPS54240