AAT1151
850kHz 700mA Synchronous Buck DC/DC Converter
Once the total losses have been determined, the
junction temperature can be derived from the θJA
for the MSOP-8 package.
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 RDS(ON) characteristics of the
output switching device. At the full load condition,
assuming continuous conduction mode (CCM), a
simplified form of the total losses is given by:
TJ = P · θJA + TAMB
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.
IO2 · (RDSON(HS) · VO + RDSON(LS) · (VIN ·VO))
P =
V
IN
F I V + I ) · V
· ·
O IN
+ (tSW
·
Q
IN
where Iq is the AAT1151 quiescent current.
R3 = (VO - 1) · R4 = 0.25 · 10kΩ = 2.55kΩ
12
1151.2005.11.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
