AAT1123IJS-1.2-T1 Advanced Analog Technology, Inc., AAT1123IJS-1.2-T1 Datasheet - Page 13

• Manufacturer Part Number: AAT1123IJS-1.2-T1
• Description: 1MHz Step-Down Converter
• Manufacturer: Advanced Analog Technology, Inc.

Thermal Calculations
There are three types of losses associated with the
AAT1123 step-down converter: switching losses,
conduction losses, and quiescent current losses.
1123.2007.02.1.6
Table 2: Adjustable Resistor Values For Use
V
OUT
1.85
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.3
(V)
With 0.6V Step-Down Converter.
R2 = 59kΩ Ω
R1 (kΩ Ω )
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
Figure 5: AAT1123 Adjustable Evaluation Board Schematic.
V
22μF
GND
GND2
OUT
C1
R2 = 221kΩ Ω
R1 (kΩ Ω )
V
IN
1000
150
187
221
261
301
332
442
464
523
715
113
75
R1
L1
4.7μH
Enable
118k
1
2
3
59k
R2
LX
C1 22μF 6.3V 0805 X5R
C2 4.7μF 6.3V 0805 X5R
U1 AAT1123 SC70JW-8
L1 CDRH3D16-4R7
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 losses is given by:
I
The term t
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:
Q
P
4.7μF
C2
TOTAL
1
2
3
4
is the step-down converter quiescent current.
EN
OUT
VIN
LX
U1
AAT1123
+ (t
=
1MHz Step-Down Converter
I
PGND
PGND
PGND
AGND
P
O
sw
sw
2
TOTAL
· (R
· F · I
is used to estimate the full load step-
8
7
6
5
DSON(HS)
= I
O
O
+ I
2
· R
Q
) · V
· V
DSON(HS)
O
IN
+ R
V
IN
DSON(LS)
+ I
AAT1123
Q
· V
· [V
IN
IN
- V
DS(ON)
O
])
13