PRODUCT DATASHEET
AAT2515
SystemPowerTM
Dual 600mA FastTransient High Frequency Buck Converter
Input Capacitor
Applications Information
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and
solve for C. The calculated value varies with input volt-
age and is a maximum when VIN is double the output
voltage.
Inductor Selection
The step-down converter uses peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. The output inductor value
must be selected so the inductor current down slope
meets the internal slope compensation requirements.
The internal slope compensation for the adjustable and
low-voltage fixed versions of the AAT2515 is 0.24A/μs.
This equates to a slope compensation that is 75% of the
inductor current down slope for a 1.5V output and 4.7μH
inductor.
VO
⎛
⎝
VO ⎞
VIN ⎠
⋅
1 -
VIN
CIN =
⎛ VPP
⎝ IO
⎞
- ESR
⋅
FS
⎠
This equation provides an estimate for the input capaci-
tor required for a single channel.
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.24
A
m =
=
L
4.7µH
µs
Configuration
Output Voltage
Inductor
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V to 3.3V
2.2μH
4.7μH
6.8μH
4.7μH
This is the internal slope compensation for the adjust-
able (0.6V) version or low-voltage fixed version. When
externally programming the 0.6V version to a 2.5V out-
put, the calculated inductance would be 7.5μH.
0.6V Adjustable With
External Feedback
Fixed Output
Table 1: Inductor Values.
0.75 ⋅ VO
m
0.75V
0.24A/µs
µs
L =
=
≈
3
⋅ VO
The equation below solves for input capacitor size for
both channels. It makes the worst-case assumptions
that both converters are operating at 50% duty cycle
and are synchronized.
A
µs
= 3
⋅ 2.5V = 7.5µH
A
In this case, a standard 6.8μH value is selected. For high-
voltage fixed versions (2.5V and above), m = 0.48A/μs.
Table 1 displays inductor values for the AAT2515 fixed
and adjustable options.
1
CIN =
⎛
VPP
⎞
⎠
- ESR · 4 · FS
⎝IO1 + IO2
Because the AAT2515 channels will generally operate at
different duty cycles and are not synchronized, the
actual ripple will vary and be less than the ripple (VPP)
used to solve for the input capacitor in the equation
above.
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 satura-
tion 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 consider the losses associated with the DCR and
its effect on the total converter efficiency when selecting
an inductor.
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10μF 6.3V X5R ceram-
ic capacitor with 5V DC applied is actually about 6μF.
The maximum input capacitor RMS current is:
The 4.7μH CDRH3D16 series inductor selected from
Sumida has a 105mΩ DCR and a 900mA DC current rat-
ing. At full load, the inductor DC loss is 37.8mW which
gives a 4.2% loss in efficiency for a 600mA 1.5V output.
VO1
⎛
V ⎞
V
⎛
V ⎞
⎛
⎞
⎛
⎞
O1
O2
IRMS = IO1
·
· 1 -
⎝
+ IO2
·
O2 · 1 -
VIN
VIN
V ⎠
IN
⎝
V ⎠
IN
w w w . a n a l o g i c t e c h . c o m
10
2515.2007.12.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
