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

DATA SHEET
AAT1151
850kHz 700mA Synchronous Buck DC/DC Converter
Thermal Calculations
There are two types of losses associated with the
AAT1151 output switching MOSFET: switching losses 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 conduc-
tion mode (CCM), a simplified form of the total losses is
given by:
P = ^{IO2 · (RDS(ON)H · VO + RDS(ON)L · (VIN · VO))}
V _IN
+ (t _SW · F_S · 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 _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 regu-
late higher than 1V. R4 should be 100 times less than the
1M internal resistance of the FB pin (recommended
10k). Once R4 is selected, R3 can be calculated. For a
1.25 volt output with R4 set to 10.0k, R3 is 2.55k.
R3 = (V_O - 1) · R4 = 0.25 · 10kΩ = 2.55kΩ
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