AAT2514
Dual Channel 600mA Step-Down Converter
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
Inductor Selection
For most designs, the AAT2514 operates with
inductor values of 1µH to 4.7µH. Low inductance
values are physically smaller, but require faster
switching, which results in some efficiency loss.
The inductor value can be derived from the follow-
ing equation:
the peak current rating, which is determined by the
saturation characteristics. The inductor should not
show any appreciable saturation under normal load
conditions. Some inductors may meet the peak and
average current ratings yet result in excessive loss-
es due to a high DCR. Always consider the losses
associated with the DCR and its effect on the total
converter efficiency when selecting an inductor. For
example, the 2.2ꢀH CR43 series inductor selected
from Sumida has a 71.2mΩ DCR and a 1.75ADC
current rating. At full load, the inductor DC loss is
25mW which gives a 2.8% loss in efficiency for a
600mA, 1.5V output.
VOUT · (VIN - VOUT
VIN · ∆IL · fOSC
)
L =
Where ∆IL is inductor ripple current. Large value
inductors lower ripple current and small value
inductors result in high ripple currents. Choose
inductor ripple current approximately 35% of the
maximum load current 600mA, or ∆IL = 210mA.
Slope Compensation
The AAT2514 step-down converter uses peak cur-
rent mode control with a unique adaptive slope
compensation scheme to maintain stability with
lower value inductors for duty cycles greater than
50%. Using lower value inductors provides better
overall efficiency and also makes it easier to stan-
dardize on one inductor for different required output
voltage levels. In order to do this and keep the
step-down converter stable when the duty cycle is
greater than 50%, the AAT2514 separates the
slope compensation into 2 phases. The required
slope compensation is automatically detected by
an internal circuit using the feedback voltage VFB
For output voltages above 2.0V, when light-load
efficiency is important, the minimum recommended
inductor size is 2.2µH. For optimum voltage-posi-
tioning load transients, choose an inductor with DC
series resistance in the 50mΩ to 150mΩ range. For
higher efficiency at heavy loads (above 200mA), or
minimal load regulation (with some transient over-
shoot), the resistance should be kept below
100mΩ. The DC current rating of the inductor
should be at least equal to the maximum load cur-
rent plus half the ripple current to prevent core sat-
uration (600mA + 105mA). Table 2 lists some typi-
cal surface mount inductors that meet target appli-
cations for the AAT2514.
before the error amp comparison to VREF
.
Part
L (µH)
Max DCR (mΩ)
Rated DC Current (A)
Size WxLxH (mm)
1.5
2.2
3.3
4.7
1.0
2.2
3.3
4.7
1.5
2.2
3.3
4.7
80
1.35
1.10
0.9
120
173
238
45
CDRH2D11/HP
3.2x3.2x1.2
0.75
1.72
1.32
1.04
0.84
1.29
1.14
0.98
0.79
75
Sumida CDRH4D18
Toko D312C
4.7x4.7x2.0
3.6x3.6x1.2
110
162
120
140
180
240
Table 2: Typical Surface Mount Inductors.
10
2514.2007.06.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
