AAT3183 Datasheet, PDF (14/16 Pages) Advanced Analogic Technologies – 300mA Inductorless Step-Down Converter

AAT3183
300mA Inductorless Step-Down Converter
Thermal Performance
Power de-rating of the AAT3183 is not necessary in
most cases due to the low thermal resistance of the
SC70JW-8 package, and the limited device losses.
Under operating conditions V_OUT = 1.5V and I_OUT =
300mA, the estimated worst-case operating effi-
ciency (η) is 68% (V_IN = 4.2V).
η=
P_OUT
P_IN
=
(V_OUT
(V_IN
·
·
I_OUT)
I_IN)
Device power dissipation (P_D) can be estimated:
P_D = P_IN - P_OUT
=
P_OUT
η
- P_OUT
(1 - η)
= V_OUT · I_OUT · _η
=
1.5V
·
0.3A
·
(1
- 0.68)
0.68
= 211.8mW
The typical junction-to-ambient thermal resistance
(R_θJA) of a SC70JW-8 package mounted on an FR4
board is 160ºC/W. The maximum junction temper-
ature (T_J(MAX)) of the device at 85ºC ambient is esti-
mated: This is below the maximum recommended
device junction temperature of 125ºC.
T_J(MAX) = T_AMB(MAX) + (P_D · R_θJA)
= 85ºC + (211.8mW · 160°C/W)
= 119°C
PCB Layout Guidelines
Proper circuit board layout will maximize efficiency
while minimizing switching noise and EMI. The fol-
lowing guidelines should be observed when
designing the printed circuit board layout for the
AAT3183 step-down converter:
1. Place the three external capacitors as close to
the AAT3183 device as possible. Maintain the
circuit board traces as short and wide as pos-
sible. This will minimize noise resulting from
parasitic ESR and ESL in the AC current path.
2. Maintain short and wide traces from ground
plane to circuit nodes. This will minimize stray
parasitics.
3. A good example of an optimal layout for the
AAT3183 is the AAT3183 evaluation board
shown in Figures 4 and 5. The evaluation board
schematic is shown in Figure 6.
14
3183.2007.07.1.2