AAT1155(2004) Ver la hoja de datos (PDF) - Analog Technology Inc
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AAT1155 Datasheet PDF : 16 Pages
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Thermal
The losses associated with the AAT1155 output
switching MOSFET are due to switching losses
and conduction losses. The conduction losses are
associated with the RDS(ON) characteristics of the
output switching device. At the full load condition,
assuming continuous conduction mode (CCM), an
accurate calculation of the RDS(ON) losses can be
derived from the following equations.
PON =
I2
RMS
· RDS(ON)
RDS(ON) losses
IRMS =
I
2
O
+
∆I 2
12
·D
Internal switch RMS current
D is the duty cycle and VF is the forward drop of the
Schottky diode.
D = VO + VF
VIN+ VF
IQ is the peak to peak inductor ripple current.
A simplified form of calculating the RDS(ON) and
switching losses is given by
P=
I O2 · R DS(ON)
VIN
Vo
+ tSW ·F · I O + I Q· VIN
where IQ is the AAT1155 quiescent current.
Once the total losses have been determined the
junction temperature can be derived. The thermal
resistance (ΘJA) for the MSOP-8 package mounted
on an FR4 printed circuit board in still air is 150°C/W.
TJ = P QJA + TAMB
TAMB is the maximum ambient temperature and TJ
is the resultant maximum junction temperature.
AAT1155
1MHz 2.5A Buck DC/DC Converter
Design Example
IOUT 2.5A
IRIPPLE 40% of full load at max Vin
VOUT 2.5V
VIN 5V ± 5%
FS 1MHz
TMAX = 70°C
Inductor Selection
L = VOUT · 1- VOUT
IO · k · F VIN
=
2.5A
3.3V
· 0.4 ·
1MHz
·
1-
3.3V
5.25V
=
1.23µH
Use standard value of 1.5 µH
Sumida inductor Series CDRH6D38.
∆I = VO
L·F
1- VO =
VIN
3.3V
1.5µH ·1MHz
1-
3.3V
5.25V
= 0.82A
I PK
=
IOUT +
∆I
2
=
2.5A + 0.41 = 2.91A
AAT1155 Junction Temperature
PON = IO2
·
RDS(ON)
VIN
·
VO
+ tSW
·
F·
2
IO
+
IQ
·
VIN
=
2.52
·
70mΩ
5V
·
3.3V
+
20ns
·
1MHz
2
·
2.5A
+
690µA
0.42 Watts
TJ(MAX)= TAMB + ΘJA · P =
70°C + 150°C/W · 0.42W = 133°C
12
1155.2004.08.1.2
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