AAT1147 Datasheet, PDF (13/18 Pages) Advanced Analogic Technologies – High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter

DATA SHEET
AAT1147
High Efficiency, Low Noise, Fast Transient 400mA Step-Down Converter
VIN
VOUT
C1
10μF
GND
1
2
3
Enable
R1
118k
L1
4.7μH
R2
59k
U1
AAT1147
1 _{EN PGND} 8
2 _{OUT PGND} 7
3 _{VIN PGND} 6
4 _{LX AGND} 5
C2
4.7μF
GND2
LX
U1 AAT1147 SC70JW-8
L1 CDRH3D16-4R7
C1 10μF 10V 0805 X5R
C2 4.7μF 10V 0805 X5R
Figure 5: AAT1147 Evaluation Board Schematic.
Thermal Calculations
There are three types of losses associated with the
AAT1147 step-down converter: switching losses, conduc-
tion losses, and quiescent current losses. Conduction
losses are associated with the R_DS(ON) characteristics of the
power output switching devices. Switching losses are
dominated by the gate charge of the power output switch-
ing devices. At full load, assuming continuous conduction
mode (CCM), a simplified form of the losses is given by:
I_Q is the step-down converter quiescent current. The
term t_sw is used to estimate the full load step-down con-
verter switching losses.
P_TOTAL
=
I_O2
·
(R_DSON(H)
·
V_O
+ R_DSON(L)
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F_S · I_O + I_Q) · V_IN
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
P_TOTAL = I_O2 · R_DSON(H) + I_Q · V_IN
Since R_DS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the _JA for the SC70JW-8 pack-
age which is 160°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
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201986A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 23, 2012
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