AAT1106
600mA Step-Down Converter
Part
L (µH)
Max DCR (mΩ)
Rated DC Current (A)
Size WxLxH (mm)
1.4
2.2
3.3
4.7
1.0
2.2
3.3
4.7
1.5
2.2
3.3
4.7
56.2
71.2
86.2
108.7
4.5
2.52
1.75
1.44
1.15
1,72
1.32
1.04
0.84
1.29
1.14
0.98
0.79
Sumida CR43
4.5x4.0x3.5
75
110
Sumida CDRH4D18
Toko D312C
4.7x4.7x2.0
3.6x3.6x1.2
162
120
140
180
240
Table 2: Typical Surface Mount Inductors.
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 overshoot), the resistance should be kept
below 100mΩ. The DC current rating of the induc-
tor should be at least equal to the maximum load
current plus half the ripple current to prevent core
saturation (600mA + 105mA). Table 2 lists some
typical surface mount inductors that meet target
applications for the AAT1106.
Slope Compensation
The AAT1106 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 out-
put voltage levels. In order to do this and keep the
step-down converter stable when the duty cycle is
greater than 50%, the AAT1106 separates the
slope compensation into 2 phases. The required
slope compensation is automatically detected by
an internal circuit using the feedback voltage VFB
before the error amp comparison to VREF
.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
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 losses due to a high DCR. Always con-
sider the losses associated with the DCR and its
effect on the total converter efficiency when select-
ing 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.
VREF
VFB
Error Amp
When below 50% duty cycle, the slope compensa-
tion is 0.284A/µs; but when above 50% duty cycle,
the slope compensation is set to 1.136A/µs. The
output inductor value must be selected so the
inductor current down slope meets the internal
slope compensation requirements.
12
1106.2007.07.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
