PRODUCT DATASHEET
AAT3183
TM
ChargePump
300mA Inductorless Step-Down Converter
CFLY
ESL
ESR
LTRACE
LTRACE
VOUT
ESL
ESL
AAT3183
VIN
RLOAD
COUT
CIN
IAC
ESR
ESR
GND
Figure 3: AC Parasitic Components of an AAT3183 Typical Application Circuit.
160°C/W. The maximum junction temperature (TJ(MAX)
)
Thermal Performance
of the device at 85°C ambient is estimated: This is below
the maximum recommended device junction tempera-
ture of 125°C.
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 VOUT = 1.5V and IOUT = 300mA, the estimated
worst-case operating efficiency (η) is 68% (VIN = 4.2V).
TJ(MAX) = TAMB(MAX) + (PD · RθJA
)
= 85ºC + (211.8mW · 160°C/W)
= 119°C
POUT
PIN
η =
(VOUT · IOUT
(VIN · IIN)
)
=
PCB Layout Guidelines
Proper circuit board layout will maximize efficiency while
minimizing switching noise and EMI. The following guide-
lines should be observed when designing the printed cir-
cuit board layout for the AAT3183 step-down converter:
Device power dissipation (PD) can be estimated:
PD = PIN - POUT
POUT
1. Place the three external capacitors as close to the
AAT3183 device as possible. Maintain the circuit
board traces as short and wide as possible. 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.
=
- POUT
η
(1 - η)
= VOUT · IOUT
·
η
(1 - 0.68)
0.68
= 1.5V · 0.3A ·
= 211.8mW
The typical junction-to-ambient thermal resistance (RθJA)
of a SC70JW-8 package mounted on an FR4 board is
w w w . a n a l o g i c t e c h . c o m
3183.2008.02.1.3
13
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
