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I
CAP(RMS)
+
ƪǒ
I
LOAD(max)
* I
IN(avg)
2
Ǔ
)
DI
2
12
ƫ
D ) I
IN(avg)
2
(
1 * D)
Ǹ
(23)
I
D
+
V
O
V
IN(min)
ǒ
I
LOAD(max)
2
)
DI
2
12
Ǔ
Ǹ
(24)
TPS40075
SLUS676A – MAY 2006 – REVISED SEPTEMBER 2007
3.1.3. Input Capacitor, C
IN
, ELCO and MLCC
The input capacitor is selected to handle the ripple current of the buck stage. Also a relative large capacitance is
used to keep the ripple voltage on the supply line low. This is especially important where the supply line is high
impedance. It is recommended that the supply line be kept low impedance. The input capacitor ripple current can
be calculated using Equation 23 .
where
• I
IN(avg)
is the average input current
This is calculated simply by multiplying the output DC current by the duty cycle. The ripple current in the input
capacitor is 5.05 A. A 1206 MLCC using X7R material has a typical dissipation factor of 5%. For a 2.2 μ F
capacitor at 400 kHz the ESR is approximately 7.2 m Ω . If two capacitors are used in parallel the power
dissipation in each capacitor is less than 46 mW.
A 470 μ F/16 V electrolytic capacitor is added to maintain the voltage on the input rail.
3.1.4 Switching MOSFET, QSW
The following key parameters must be met by the selected MOSFET.
• Drain source voltage, V
DS
, must be able to withstand the input voltage plus spikes that may be on the
switching node. For this design a V
DS
rating of 25 V to 30 V is recommended.
• Drain current, I
D
, at 25 ° C, must be greater than that calculated using Equation 24 . For this design, I
D
should
be greater than 5 A.
• Gate source voltage, V
GS
must be able to withstand the gate voltage from the control device . For the
TPS40075 this is 9 V.
Once the above boundary parameters are defined the next step in selecting the switching MOSFET is to select
the key performance parameters. Efficiency is the performance characteristic which drives the other selection
criteria. Target efficiency for this design is 90%. Based on 1.5-V output and 15 A this equates to a power loss in
the converter of 2.5 W. Using this figure a target of 0.5 W dissipated in the switching MOSFET was chosen.
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