AAT1123
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
AAT1123 step-down converter: switching losses,
conduction losses, and quiescent current losses.
1
2
3
Enable
VIN
U1
AAT1123
1
2
3
4
8
7
6
5
EN
PGND
OUT PGND
118k
R1
L1
VIN
LX
PGND
AGND
VOUT
4.7μH
C1
22μF
C2
4.7μF
R2
59k
GND
GND2
LX
U1 AAT1123 SC70JW-8
L1 CDRH3D16-4R7
C1 22μF 6.3V 0805 X5R
C2 4.7μF 6.3V 0805 X5R
Figure 5: AAT1123 Adjustable Evaluation Board Schematic.
1123.2006.05.1.5
13
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
