AAT2505 Analogic Corporation, AAT2505 Datasheet - Page 18

• Manufacturer Part Number: AAT2505
• Manufacturer: Analogic Corporation

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 R
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:
1. For step-down converter, enhanced transient configuration C8 = 100pF and C1 = 10µF.
18
P
TOTAL
+ (t
=
V
OUTBUCK
I
OBUCK
sw
LDO Input
· F
2
S
· (R
· I
3
2
1
OBUCK
DSON(H)
+ I
· V
V
QBUCK
V
OUTLDO
OBUCK
IN1
GND
+ I
V
+ R
IN
QLDO
Figure 5: AAT2505 Evaluation Board Schematic.
DSON(L)
) · V
C2
10µF
IN
· [V
+ I
LX1
OLDO
IN
- V
C3
10µF
· (V
OBUCK
IN
])
- V
DS(ON)
OLDO
C1
4.7µF
Table 3
)
C7
0.01µF
L1
1
C4
4.7µF
I
rent and I
term t
converter switching losses.
For the condition where the buck converter is in
dropout at 100% duty cycle, the total device dissi-
pation reduces to:
Since R
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
P
QBUCK
1
2
3
4
5
6
TOTAL
PGND
LX
VP
VCC
IN
OUT
U1
AAT2505
R3
100k
sw
+ (I
= I
is the step-down converter quiescent cur-
ENLDO
SGND
is used to estimate the full load step-down
DS(ON)
GND
POK
OBUCK
Converter/Linear Regulator
QBUCK
EN
QLDO
FB
Dual Channel, Step-Down
12
11
10
9
8
7
2
, quiescent current, and switching
+ I
is the LDO quiescent current. The
· R
C9
n/a
QLDO
DSON(H)
Table 3
R1
) · V
R2
59k
+ I
IN
OLDO
C8
1
· (V
IN
Buck Enable
LDO Enable
- V
2505.2006.06.1.1
3
2
1
3
2
1
1
POK
OLDO
GND
)