AAT4900 Datasheet, PDF (8/13 Pages) Advanced Analogic Technologies – 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.
Eq. 1:
D
=
V_O
V_IN
D is the duty cycle.
Eq. 2:
ΔI =
V_O
L · F_S
⎛⎝_1-
V_O ⎞
V_IN⎠
ΔI is the peak-to-peak inductor ripple current.
High Side Switch RMS Current
Eq. 3:
I_RMS(HS) ⁼
⎛
⎝
I_O2
+
ΔI ²
12
⎞
⎠
·
D
Low Side Switch RMS Current
The low side RMS current is estimated by the fol-
lowing equation.
Eq. 4:
I_RMS(LS) ⁼
⎛
⎝
I_O2
+
ΔI ²
12
⎞
⎠
·
(1
-
D)
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:
AAT4900
Buffered Power Half-Bridge
Eq.
5:
P_LOSS
=
I_O2 · (R_DS(ON)H · V_O + R_DS(ON)L · (V_IN -V_O))
V_IN
+ (t_sw · F_S · I_O + I_Q) · 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
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
V_IN = 4.2V, V_O = 2.5V
1.5
V_IN = 5.0V, V_O = 3.3V
T_JMAX = 150°C
1.25
T_JMAX = 120°C
1
V_IN = 4.2V, V_O = 2.5V
0.75
V_IN = 5.0V, V_O = 3.3V
0.5
25
35
45
55
65
75
85
Ambient Temperature (°C)
8
4900.2006.05.1.3