PRODUCT DATASHEET
AAT2146W
TM
SwitchReg
Low Noise, FastTransient 600mA Step-Down Converter
switching devices. A simplified form of the losses is given
R2 = 59kW
R1 (kW)
R2 = 221kW
by:
IO2 · (RDS(ON)HS · VO + RDS(ON)LS · [VIN - VO])
VOUT (V)
R1 (kW)
PTOTAL
=
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
75
113
150
187
221
261
301
332
442
464
523
715
1000
VIN
+ (tsw · fsw · IO + IQ) · VIN
IQ is the step-down converter quiescent current. The
term tsw is used to estimate the full load step-down con-
verter switching losses.
Forꢀ theꢀ conditionꢀ whereꢀ theꢀ step-downꢀ converterꢀ isꢀ inꢀ
dropoutꢀatꢀ100%ꢀdutyꢀcycle,ꢀtheꢀtotalꢀdeviceꢀdissipationꢀ
reduces to:
PTOTAL = IO2 · RDS(ON)HS + IQ · VIN
Table 2: Adjustable Resistor Values For Use With
0.6V Step-Down Converter.
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Thermal Calculations
Given the total losses, the maximum junction tempera-
ture can be derived from the qJA for the SC70JW-8 pack-
ageꢀwhichꢀisꢀ160°C/W.ꢀ
There are three types of losses associated with the
AAT2146W step-down converter: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the RDS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
TJ(MAX) = PTOTAL · QJA + TA
1
2
3
Enable
VIN
C4
100pF
U1
AAT2146W
1
2
3
4
8
7
6
5
EN
PGND
VOUT =1.8V
R1
118k
4.7µH
OUT PGND
L1
VIN
LX
PGND
AGND
C1
10µF
R2
59k
C2
4.7µF
C3
n/a
GND
LX
U1 AAT2146W SC70JW-8
L1 CDRH3D16-4R7
C2 4.7µF 10V 0805 X5R
C1 10µF 6.3V 0805 X5R
Figure 5: AAT2146W Adjustable Evaluation Board Schematic.
w w w . a n a l o g i c t e c h . c o m
2146W.2011.02.1.0
13
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