AAT1151 Datasheet, PDF (12/16 Pages) Advanced Analogic Technologies – 850kHz 700mA Synchronous Buck DC/DC Converter

AAT1151
850kHz 700mA Synchronous Buck DC/DC Converter
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_DS(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:
P = ^{IO2 · (RDSON(HS) · VO + RDSON(LS) · (VIN · VO))}
V_IN
+ (t _SW · F · I _O · V_IN + I_Q) · V_IN
where I_q is the AAT1151 quiescent current.
Once the total losses have been determined, the
junction temperature can be derived from the θ_JA
for the MSOP-8 package.
T_J = P · θ_JA + T_AMB
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
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1151.2005.11.1.4