External Component Selection
2-20
of power losses and additional voltage drops through non-ideal components.
Equation (4) should be sufficiently accurate for the first frequency estimate at
the beginning of a design.
2.2.5 Power MOSFET Selection
The TPS56xx is designed to drive N-channel power MOSFETs in a
synchronous rectifier configuration. The MOSFET chosen for this design is the
Siliconix Si4410DY. This device is chosen for its low rDS(on) of 13.5 mΩ and
drain-to-source breakdown voltage rating of 30 V.
Power dissipation for the switching MOSFETs, which includes both conduction
and switching losses, is given by:
P
D
(
Q
1) +ǒ
I
2
O
r
DS
(
on
)
D
Ǔ)ǒ0.5
V
i
I
O
t
r
)
f
f
sw
Ǔ
P
D
(
Q
2) +ǒ
I
2
O
r
DS
(
on
) (1
–D
)Ǔ)ǒ0.5
V
i
I
O
t
r
)
f
f
sw
Ǔ
An example MOSFET power dissipation calculation for Q1 and Q3 is shown
below with the following assumptions:
The total switching time,
t
r+f
= 100 ns,
An
r
DS (on)
high temperature adjustment factor = 1.4,
A 60°C maximum ambient temperature,
VI = 5.0 V, VO = 3.3 V, and IO = 6 A then :
P
D
(
Q
1) +(6)2 (0.0135 1.4) 0.7 )0.5 5 6 100 10
–
9 135 103
+0.48 )0.20 +0.68
W
P
D
(
Q
2) +(6)2 (0.0135 1.4) 0.3 )0.5 5 6 100 10
–
9 135 103
+0.20 )0.20 +0.40
W
The thermal impedance of these devices, RθJA = 90°C/W for FR-4 with 2-oz.
copper and a one-inch-square pattern, thus:
T
J
(
Q
1) +
T
A
)ǒ
R
JA
P
D
Ǔ+60 )(50 0.90)+94°
C
T
J
(
Q
2) +
T
A
)ǒ
R
JA
P
D
Ǔ+60 )(50 1.40)+80°
C
It is good design practice to check power dissipation at the extreme limits of
input voltage to find the worst case.