Datasheet

2 2
O OUT
IC _ rms = Idiode _ rms - I
OUT
O
SW CO
I × D
C =
× VDf
fd OUT
Pdiode = V × I
OUT
I
Idiode _ peak = 2 ×
1 - D
( )
1
2
OUT
1
Idiode _ rms = 2 × I ×
3 × 1 - D
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÷
ç
÷
ç
÷
ç
÷
÷
ç
è ø
( )
SEC
IN PRI
OUT
PRI
N
Vdiode _ max = V - V × + V
N
TPS55010
SLVSAV0A –APRIL 2011–REVISED JUNE 2011 www.ti.com
SECONDARY SIDE DIODE
The diode should be selected to handle the voltage stress and rms current calculated in Equation 26 and
Equation 27. Typically, a low duty cycle or high turns ratio design will have a larger voltage stress on the diode.
At the maximum input voltage of 5.5V, the Vdiode_max voltage is calculated at 13.3 V. The rms current is
calculated as 0.31 A. The diode peak current is 0.71 A using Equation 28 and the power dissipated in the diode
is 0.1 W. The B120 diode is used which is rated for 20 V and 1 A.
(26)
Spacer
(27)
Spacer
(28)
Spacer
(29)
SECONDARY SIDE CAPACITOR
The ΔV
CO
voltage should be 0.25% to 1% of V
CO
voltage. The converter transfers energy each switching period
to the secondary, since the converter has primary side feedback, at light or no load conditions the output voltage
may rise above the desired output. If the application will experience a no load condition, attention to the capacitor
voltage ratings should be considered. Adding a ballast load, zener diode or linear regulator can help prevent the
overvoltage at light or no load.
The output capacitance is calculated to be 10.1 µF using Equation 30 and the rms current is 0.24 A.
Two 10 µF/10V X5R ceramic capactors are used. The effective capacitance is lower than the 20 µF, because of
dc voltage bias.
(30)
Spacer
(31)
INPUT CAPACITOR
The ΔV
CIN
voltage should be 0.25% to 1% of V
IN
. The TPS55010 requires a high quality ceramic, type X5R or
X7R, input decoupling capacitor of at least 2.2 µF of effective capacitance or larger coupled to VIN and GND
pins and in some applications additional 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 TPS55010.
The input ripple current can be calculated using Equation 33. The value of a ceramic capacitor varies significantly
overtemperature 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 overtemperature. 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.
In applications with significant unload transients, the bulk input capacitance must be sized to include energy
transfer from the primary side capacitor to the input capacitor.
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