Buffered Power Half-Bridge
Synchronous Buck DC/DC Converter
Application
The losses associated with the AAT4900 high side
switching MOSFET are due to switching losses and
conduction losses. The conduction losses are asso-
ciated with the R
DS(ON)
characteristics of the output
switching device. At the full load condition, assum-
ing continuous conduction mode (CCM), the on
losses can be derived from the following equations.
I
O2
· (R
DS(ON)H
· V
O
+ R
DS(ON)L
· (V
IN
-V
O
))
V
IN
AAT4900
Eq. 5:
P
LOSS
=
+ (t
sw
· F
S
· I
O
+ I
Q
) · V
IN
Eq. 1:
D is the duty cycle.
D=
V
O
V
IN
Eq. 2:
ΔI
=
V
⎞
V
O
⎛
1-
O
L · F
S
⎝
V
IN
⎠
Substitution of the I
RMS
equations with I
O
results in
very little error when the inductor ripple current is
20% to 40% of the full load current. The equation
also includes switching and quiescent current loss-
es where t
SW
is approximated at 18 nsec and I
Q
is
the no load quiescent current of the AAT4900.
Quiescent current losses are associated with the
gate drive of the output stage and biasing. Since
the gate drive current varies with frequency and
voltage, the bias current must be checked at the
frequency, voltage, and temperature of operation
with no load attached to the LX node. Once the
above losses have been determined, the maximum
junction temperature can be calculated.
Eq. 6:
T
J(MAX)
=
P
LOSS
·
Θ
JC
= T
AMB
ΔI
is the
peak-to-peak
inductor ripple current.
High Side Switch RMS Current
Eq. 3:
2
⎛
2
Δ
I
⎞
=
·D
I
O
+
I
RMS(HS)
⎝
12
⎠
Low Side Switch RMS Current
The low side RMS current is estimated by the fol-
lowing equation.
Using the above equations, the graph below shows
the current capability for some typical applications
with maximum junction temperatures of 150°C and
120°C. The increase in R
DS(ON)
vs. temperature is
estimated at 3.75mΩ for a 10°C increase in junc-
tion temperature.
Step-Down Converter Limits
(F
S
= 1MHz)
1.75
Output Current (A)
Eq. 4:
I
RMS(LS)
=
⎛
2
Δ
I
⎞
· (1 - D)
I
+
⎝
O
12
⎠
2
1.5
1.25
V
IN
= 4.2V, V
O
= 2.5V
V
IN
= 5.0V, V
O
= 3.3V
T
JMAX
= 150°C
T
JMAX
= 120°C
Total Losses
A simplified form of the above results (where the
above descriptions of I
RMS
has been approximated
with I
o
) is given by:
1
0.75
0.5
25
V
IN
= 4.2V, V
O
= 2.5V
V
IN
= 5.0V, V
O
= 3.3V
35
45
55
65
75
85
Ambient Temperature (°C)
8
4900.2006.05.1.3
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
