PRODUCT DATASHEET
AAT1140
SwitchRegTM
FastTransient 600mA Step-Down Converter
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Thermal Calculations
There are three types of losses associated with the
AAT1140 step-down converter: switching losses, con-
duction losses, and quiescent current losses. Conduction
losses are associated with the RDS(ON) characteristics of
the power output switching devices. Switching losses are
dominated by the gate charge of the power output
switching devices. At full load, assuming continuous con-
duction mode (CCM), a simplified form of the LDO losses
is given by:
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the SOT23-5 pack-
age which is 150°C/W.
TJ(MAX) = PTOTAL · ΘJA + TAMB
Output Dropout
At dropout, the duty cycle of AAT1140 switching is
100%. The minimum dropout voltage is determined by
RDS(ON)H and the inductor copper loss resistor. AAT1140
has 0.53Ω RDS(ON)H. The inductor copper loss resistor var-
ies with different inductor values and manufacturer. The
safe dropout voltage is 0.5V for a 600mA load.
IO2 · (RDSON(HS) · VO + RDSON(LS) · [VIN - VO])
PTOTAL
=
VIN
+ (tsw · F · IO + IQ) · VIN
IQ is the step-down converter quiescent current. The
term tsw is used to estimate the full load step-down con-
verter switching losses.
For example, when load current is 600mA, the voltage
dropped across RDS(ON)H is 0.32V; if the inductor copper
loss resistor is 135mΩ, the voltage drop across the
inductor is 0.08V. So the total voltage drop is 0.4V.
Considering manufacturer’s tolerances, the inductor cop-
per loss resistor and RDS(ON)H will vary from part to part,
a 0.5V dropout window is safe.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
PTOTAL = IO2 · RDSON(HS) + IQ · VIN
SW
U1
AAT1140
L1
4.7μH
4
3
IN
LX
VIN
VOUT = 1.8V
C1
4.7μF
C3
100pF
C2
10μF
R1
2
1
GND
EN
442k
1
2
3
Enable
5
R2
221k
OUT
Figure 5: AAT1140 Adjustable Evaluation Board Schematic.
w w w . a n a l o g i c t e c h . c o m
14
1140.2007.12.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
