AAT4900_06 ANALOGICTECH [Advanced Analogic Technologies], AAT4900_06 Datasheet - Page 8

• Manufacturer Part Number: AAT4900_06
• Description: Buffered Power Half-Bridge
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

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
switching device. At the full load condition, assum-
ing continuous conduction mode (CCM), the on
losses can be derived from the following equations.
Eq. 1:
D is the duty cycle.
Eq. 2:
ΔI is the peak-to-peak inductor ripple current.
High Side Switch RMS Current
Eq. 3:
Low Side Switch RMS Current
The low side RMS current is estimated by the fol-
lowing equation.
Eq. 4:
Total Losses
A simplified form of the above results (where the
above descriptions of I
with I
8
o
) is given by:
I
I
RMS(LS)
RMS(HS)
DS(ON)
ΔI =
=
=
L · F
D =
RMS
characteristics of the output
⎛
⎝
⎛
⎝
V
I
I
O
O
O
2
2
S
has been approximated
V
V
+
+
IN
O
⎛
⎝
Δ
Δ
1
12
12
I
I
-
2
2
V
V
⎞
⎠
⎞
⎠
IN
O
· (1 - D)
· D
⎞
⎠
Eq. 5:
Substitution of the I
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
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:
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
estimated at 3.75mΩ for a 10°C increase in junc-
tion temperature.
0.75
1.25
1.75
0.5
1.5
1
25
P
T
LOSS
Buffered Power Half-Bridge
JMAX
SW
V
=
+ (t
= 120°C
IN
35
Step-Down Converter Limits
is approximated at 18 nsec and I
I
T
= 4.2V, V
O
sw
2
J(MAX)
· (R
· F
Ambient Temperature (°C)
RMS
S
V
45
DS(ON)H
IN
· I
= P
O
V
= 5.0V, V
O
(F
= 2.5V
IN
equations with I
+ I
LOSS
S
= 4.2V, V
= 1MHz)
· V
DS(ON)
Q
55
) · V
O
· Θ
O
V
+ R
V
= 3.3V
IN
IN
JC
IN
O
vs. temperature is
DS(ON)L
= 2.5V
= 5.0V, V
AAT4900
65
= T
T
AMB
4900.2006.05.1.3
JMAX
· (V
O
O
75
IN
= 150°C
results in
= 3.3V
-V
O
))
Q
85
is