AAT2505
Dual Channel, Step-Down
Converter/Linear Regulator
LX1
VOUTBUCK
R1
Table 3
C7
0.01µF
C9
n/a
U1
C81
AAT2505
PGND SGND
1
2
3
4
5
6
12
11
10
9
C1
4.7µF1
R2
59k
C2
10µF
LX
VP
FB
EN
L1
Table 3
3
2
1
VIN1
VCC ENLDO
3
2
1
Buck Enable
8
IN
GND
POK
7
3
2
1
OUT
LDO Input
C3
10µF
C4
4.7µF
LDO Enable
GND
GND
R3
100k
VOUTLDO
1
POK
Figure 5: AAT2505 Evaluation Board Schematic.
IQBUCK is the step-down converter quiescent cur-
rent and IQLDO is the LDO quiescent current. The
term tsw is used to estimate the full load step-down
converter switching losses.
Thermal Calculations
There are three types of losses associated with the
AAT2505 step-down converter: switching losses,
conduction 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 switching devices.
At full load, assuming continuous conduction mode
(CCM), a simplified form of the step-down convert-
er and LDO losses is given by:
For the condition where the buck converter is in
dropout at 100% duty cycle, the total device dissi-
pation reduces to:
PTOTAL = IOBUCK2 · RDSON(H) + IOLDO · (VIN - VOLDO
+ (IQBUCK + IQLDO) · VIN
)
IOBUCK2 · (RDSON(H) · VOBUCK + RDSON(L) · [VIN - VOBUCK])
PTOTAL
=
Since RDS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
VIN
+ (tsw · FS · IOBUCK + IQBUCK + IQLDO) · VIN + IOLDO · (VIN - VOLDO
)
1. For step-down converter, enhanced transient configuration C8 = 100pF and C1 = 10µF.
18
2505.2006.06.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
