AAT1156 Datasheet, PDF (10/14 Pages) Advanced Analogic Technologies – 1MHz 700mA Step-Down DC-DC Converter

AAT1156
1MHz 700mA Step-Down DC-DC Converter
Thermal Calculations
There are three types of losses associated with the AAT1156: MOSFET switching losses, conduction losses,
and quiescent current losses. The conduction losses are due to the R_DS(ON) characteristics of the internal P-
and N-channel MOSFET power devices. At full load, assuming continuous conduction mode (CCM), a simpli-
fied form of the total losses is given by:
P=
I_O2 ⋅ (R_DSON(HS) ⋅ V_O + R_DSON(LS) ⋅
V_IN
(V_IN - V_O)) _{+ (tsw ⋅ F ⋅ IO ⋅ VIN + IQ) ⋅ VIN}
where I_Q is the AAT1156 quiescent current.
Once the total losses have been determined, the junction temperature can be derived from the θ_JA for the
QFN33-16 package.
Adjustable Output
T_J = P · Θ_JA + T_AMB
Resistors R3 and R4 of Figure 1 force the output to regulate higher than 0.6V. The optimum value for R4 is
59kΩ. Values higher than this may cause problems with stability, while lower values can degrade light load
efficiency. For a 2.5V output with R4 set to 59kΩ, R3 is 187kΩ.
R3
=
⎛ ^VO
⎝V_REF
-1^⎞⎠
·
R4
=
⎛ ^2.5V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
187kΩ
500
450 ^R4=59kΩ
400
350
300
250
200
150
100
50
0
1 1.5
2 2.5
3
3.5
4 4.5
5
5.5
Output Voltage (V)
Figure 4: R3 vs. V_OUT for Adjustable Output Using the AAT1156.
10
1156.2005.11.1.2