AAT2514_08 ANALOGICTECH [Advanced Analogic Technologies], AAT2514_08 Datasheet - Page 10

• Manufacturer Part Number: AAT2514_08
• Description: Dual Channel 600mA Step-Down Converter
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

For output voltages above 2.0V, when light-load efficiency
is important, the minimum recommended inductor size is
2.2μH. For optimum voltage-positioning 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 inductor 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 induc-
tors that meet target applications for the AAT2514.
Manufacturer's specifications list both the inductor DC cur-
rent rating, which is a thermal limitation, and the peak
current rating, which is determined by the saturation char-
acteristics. 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 consider the
losses associated with the DCR and its effect on the total
converter efficiency when selecting an inductor. For exam-
ple, the 2.2μH 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.
Slope Compensation
The AAT2514 step-down converter uses peak current
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 standardize on one inductor for different required out-
10
SwitchReg
Sumida CDRH4D18
CDRH2D11/HP
Toko D312C
Part
TM
L (μH)
1.5
2.2
3.3
4.7
1.0
2.2
3.3
4.7
1.5
2.2
3.3
4.7
Table 2: Typical Surface Mount Inductors.
Max DCR (mΩ)
w w w . a n a l o g i c t e c h . c o m
120
173
238
110
162
120
140
180
240
80
45
75
Dual Channel 600mA Step-Down Converter
put 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 automati-
cally detected by an internal circuit using the feedback
voltage V
When below 50% duty cycle, the slope compensation 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.
Below 50% duty cycle, the slope compensation require-
ment is:
Therefore:
Above 50% duty cycle,
Rated DC Current (A)
FB
1.35
1.10
0.75
1.72
1.32
1.04
0.84
1.29
1.14
0.98
0.79
0.9
before the error amp comparison to V
V
V
FB
REF
m =
m =
L =
PRODUCT DATASHEET
1.25
2 · L
2 · L
0.625
5
m
= 0.284A/µs
= 1.136A/µs
Error Amp
= 2.2µH
Size WxLxH (mm)
AAT2514
3.2x3.2x1.2
4.7x4.7x2.0
3.6x3.6x1.2
2514.2008.02.1.1
REF
.