AAT1121IES-0.6-T1 Advanced Analog Technology, Inc., AAT1121IES-0.6-T1 Datasheet - Page 13

• Manufacturer Part Number: AAT1121IES-0.6-T1
• Description: 1.5MHz, 250mA Step-Down Converter
• Manufacturer: Advanced Analog Technology, Inc.

Thermal Calculations
There are three types of losses associated with
the AAT1121 step-down converter: switching loss-
es, conduction losses, and quiescent current loss-
es. Conduction losses are associated with the
R
ing devices. Switching losses are dominated by
the gate charge of the power output switching
devices. At full load, assuming continuous conduc-
tion mode (CCM), a simplified form of the losses is
given by:
I
The term t
down converter switching losses.
1121.2007.03.1.2
Q
P
GND
DS(ON)
TOTAL
VIN
is the step-down converter quiescent current.
+ (t
=
characteristics of the power output switch-
I
O
sw
sw
2
· (R
· F · I
is used to estimate the full load step-
C1
4.7μF
DSON(H)
O
+ I
Q
· V
) · V
O
+ R
IN
V
DSON(L)
IN
1
2
3
4
VP
VIN
GND
FB
· [V
Figure 1: AAT1121 Schematic.
IN
AAT1121
- V
U1
O
])
PGND
N/C
EN
LX
1.5MHz, 250mA Step-Down Converter
8
7
6
5
LX
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
Since R
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
Given the total losses, the maximum junction tem-
perature can be derived from the θ
TDFN22-8 package which is 50°C/W.
L1
DS(ON)
P
TOTAL
T
J(MAX)
, quiescent current, and switching
= I
C2
4.7μF
= P
O
2
· R
TOTAL
DSON(H)
R2
59kΩ
R1
Adj.
· Θ
JA
+ I
+ T
Q
AAT1121
· V
AMB
C3
(optional)
100pF
IN
JA
+VOUT
GND
for the
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