Texas Instruments SLVU013 - Manual (Page 40)

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 r
DS(on)
of 13.5 m
Ω
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 (Q1)
+
ǒ
I
2
O
r
DS(on)
D
Ǔ
)
ǒ
0.5 V
i
I
O
t
r ) f
f
sw
Ǔ
P
D (Q2)
+
ǒ
I
2
O
r
DS(on)
(1–D)
Ǔ
)
ǒ
0.5 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,
V
I
= 5.0 V, V
O
= 3.3 V, and I
O
= 6 A then :
P
D (Q1)
+ (6)
2
(0.0135 1.4) 0.7 ) 0.5 5 6 100 10–9 135 10
3
+
0.48
)
0.20
+
0.68 W
P
D (Q2)
+ (6)
2
(0.0135 1.4) 0.3 ) 0.5 5 6 100 10–9 135 10
3
+
0.20
)
0.20
+
0.40 W
The thermal impedance of these devices, R θ
JA
= 90
°
C/W for FR-4 with 2-oz.
copper and a one-inch-square pattern, thus:
T
J (Q1)
+ T
A
)
ǒ
R
q JA
P
D
Ǔ
+ 60 ) (50 0.90) + 94
°
C
T
J (Q2)
+
T
A
)
ǒ
R
q 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.