AAT2505 Datasheet, PDF (18/26 Pages) Advanced Analogic Technologies – Dual Channel, Step-Down Converter/Linear Regulator

AAT2505
Dual Channel, Step-Down
Converter/Linear Regulator
VOUTBUCK
3
2
1
LDO Input
V_IN1
GND
LX1
C2
10µF
C3
10µF
C7
0.01µF
C1
4.7µF¹
L1
Table 3
R1
C9 Table 3
U1
n/a
AAT2505
1
12
PGND SGND
R2
59k
2
11
LX
FB
3
VP
10
EN
4
9
VCC ENLDO
5
IN
8
GND
6
OUT
7
POK
C4
4.7µF
C8¹
3
2
1
Buck Enable
3
2
1
LDO Enable
GND
V_OUTLDO
R3
100k
1
POK
Figure 5: AAT2505 Evaluation Board Schematic.
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_DS(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:
P_TOTAL
=
I²
OBUCK
·
(R_DSON(H)
·
V_OBUCK + R_DSON(L)
V_IN
·
[V_IN
-
V_OBUCK])
+ (t_sw · F_S · I_OBUCK + I_QBUCK + I_QLDO) · V_IN + I_OLDO · (V_IN - V_OLDO)
I_QBUCK is the step-down converter quiescent cur-
rent and I_QLDO is the LDO quiescent current. The
term t_sw is used to estimate the full load step-down
converter switching losses.
For the condition where the buck converter is in
dropout at 100% duty cycle, the total device dissi-
pation reduces to:
P_TOTAL
=
I²
OBUCK
·
R_DSON(H)
+
I_OLDO
·
(V_IN
-
V_OLDO)
+ (I_QBUCK + I_QLDO) · V_IN
Since R_DS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
1. For step-down converter, enhanced transient configuration C8 = 100pF and C1 = 10µF.
18
2505.2006.06.1.1