AAT1155 Datasheet, PDF (13/17 Pages) Advanced Analogic Technologies – 1MHz 2.5A Step-Down DC/DC Converter

AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Thermal
Losses associated with the AAT1155 output switch-
ing MOSFET are due to switching losses and con-
duction losses. The conduction losses are associ-
ated with the R_DS(ON) characteristics of the output
switching device. At the full load condition, assum-
ing continuous conduction mode (CCM), an accu-
rate calculation of the R_DS(ON) losses can be
derived from the following equations:
P_ON =
I2
RMS
· R_DS(ON)
R_DS(ON) losses
I_RMS =
⎛
⎝
I
2
O
+
∆I ²
12
⎞
⎠
·D
Internal switch RMS current
D is the duty cycle and V_F is the forward voltage
drop of the Schottky diode.
D = ^{VO + VF}
V_IN+ V_F
∆I is the peak-to-peak inductor ripple current.
A simplified form of calculating the R_DS(ON) and
switching losses is given by:
P=
I
2
O
·
R
DS(ON)
V_IN
V_o
+ t_SW ·F · I _O + I _Q· V_IN
where I_Q is the AAT1155 quiescent current.
Once the total losses have been determined, the
junction temperature can be derived. The thermal
resistance (Θ_JA) for the MSOP-8 package mounted
on an FR4 printed circuit board in still air is 150°C/W.
T_J = P Q_JA + T_AMB
T_AMB is the maximum ambient temperature and T_J
is the resultant maximum junction temperature.
1155.2005.11.1.6
Design Example
(see Figures 2 and 5 for reference)
I_OUT
I_RIPPLE
V_OUT
V_IN
F_S
T_MAX
2.5A
40% of Full Load at Max V_IN
2.5V
5V ±5%
1MHz
70°C
Inductor Selection
L = ^VOUT · ^⎛1- ^VOUT⎞
I_O · k · F ⎝ V_IN ⎠
=
2.5A
3.3V
· 0.4 ·
1MHz
·
⎛
⎝
1-
3.3V ⎞
5.25V^⎠
=
1.23µH
Use standard value of 1.5µH
Sumida inductor Series CDRH6D38.
∆I = ^VO
L·F
⎛_1- ^V_O ⎞ ₌
⎝ V_IN ⎠
3.3V
1.5µH ·1MHz
⎛⎝_1-
3.3V ⎞
5.25V^⎠
=
0.82A
I _PK
=
I_OUT +
∆I
2
=
2.5A + 0.41 = 2.91A
Efficiency vs. Load Current
(V_IN = 5.0V; V_OUT = 3.3V)
100
95
90
85
80
75
70
65
60
0.01
0.1
1
10
Output Current (A)
Figure 5: 5V Input, 3.3V Output.
13