AAT1126
600mA, 1MHz Step-Down Converter
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 switching devices.
At full load, assuming continuous conduction mode
(CCM), a simplified form of the losses is given by:
Ω
Ω
R2 = 221k
R2 = 59k
Ω
Ω
VOUT (V)
R1 (k )
R1 (k )
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
75
113
150
187
221
261
301
332
442
464
523
715
1000
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
PTOTAL
=
VIN
+ (tsw · F · IO + IQ) · VIN
124
137
187
267
IQ is the step-down converter quiescent current.
The term tsw is used to estimate the full load step-
down converter switching losses.
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
Table 2: Adjustable Resistor Values For Use
With 0.6V Step-Down Converter.
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
Thermal Calculations
There are three types of losses associated with the
AAT1126 step-down converter: switching losses,
conduction losses, and quiescent current losses.
JP1
1
2
3
VIN
1
2
3
GND
EN
5
VIN
L1
4.7µH
Enable
4
OUT
VOUT
C2
LX
4.7µF
R1
118K
AAT1126
C1
10µF
U1 AAT1126 SOT23-5
L1 CDRH3D16-4R7
C2 4.7µF 10V 0805 X5R
C1 10µF 6.3V 0805 X5R
R2
59K
GND
LX
Figure 5: AAT1126 Adjustable Evaluation Board Schematic.
1126.2006.05.1.1
13
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