AAT1151 AAT, AAT1151 Datasheet - Page 12

• Manufacturer Part Number: AAT1151
• Description: DC/DC Converter
• Manufacturer: AAT

Thermal Calculations
There are two types of losses associated with the
AAT1151 output switching MOSFET: switching loss-
es and conduction losses. Conduction losses are
associated with the R
output switching device. At the full load condition,
assuming continuous conduction mode (CCM), a
simplified form of the total losses is given by:
where I
12
P =
+ (t
SW
I
O
q
·
2
F I
is the AAT1151 quiescent current.
· (R
·
O
DSON(HS)
·
V
IN
+ I
· V
Q
) · V
DS(ON)
O
V
+ R
IN
IN
DSON(LS)
characteristics of the
· (V
IN
850kHz 700mA Synchronous Buck DC/DC Converter
· V
O
))
Once the total losses have been determined, the
junction temperature can be derived from the θ
for the MSOP-8 package.
Adjustable Output
For applications requiring an output other than the
fixed available, the 1V version can be programmed
externally. Resistors R3 and R4 of Figure 7 force
the output to regulate higher than 1 volt. R4 should
be 100 times less than the 1MΩ internal resistance
of the FB pin (recommended 10kΩ). Once R4 is
selected, R3 can be calculated. For a 1.25 volt out-
put with R4 set to 10.0kΩ, R3 is 2.55kΩ.
R3 = (V
O
- 1) · R4 = 0.25 · 10kΩ = 2.55kΩ
T
J
= P · θ
JA
+ T
AMB
AAT1151
1151.2005.11.1.4
JA