AAT2503
Adjustable 3-Channel Regulator
the acceptable input ripple level (VPP) and solve for
CIN. The calculated value varies with input voltage
and is a maximum when VIN is double the output
voltage.
Applications Information
Step-Down Converter Inductor Selection
The step-down converter uses peak current mode
control with slope compensation to maintain stability
for duty cycles greater than 50ꢀ. The output induc-
tor value must be selected so the inductor current
down slope meets the internal slope compensation
requirements. The internal slope compensation for
the AAT2503 step-down converter is 0.51A/µsec.
This equates to a slope compensation that is 75ꢀ of
the inductor current down slope for a 1.5V output
and 2.2µH inductor.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
-
CIN =
⎛
⎝
VPP
IO
⎞
- ESR
·
FS
⎠
VO
VIN
⎛
VO
VIN
⎞
⎠
1
4
· 1
-
=
for VIN = 2 · VO
⎝
1
CIN(MIN)
=
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.51
A
⎛
⎝
VPP
IO
⎞
⎠
m =
=
- ESR
·
4
·
FS
L
2.2μH
μsec
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 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.
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF,
6.3V, X5R ceramic capacitor with 5.0V DC applied
is actually about 6µF.
The maximum input capacitor RMS current is:
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
IRMS = IO
·
-
The 2.2µH CDRH2D14 series Sumida inductor has
a 94mΩ DCR and a 1.5A DC current rating. At full
800mA load, the inductor DC loss is 17mW which
gives a 2.8ꢀ loss in efficiency for a 800mA, 1.8V
output.
The input capacitor RMS ripple current varies with
the input and output voltage and will always be less
than or equal to half of the total DC load current.
VO
VIN
⎛
· 1
⎝
VO
VIN
⎞
⎠
1
2
Input Capacitor
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input of the step-down converter. To esti-
mate the required input capacitor size, determine
-
=
D
· (1 - D) = 0.52 =
Configuration
Output Voltage
Inductor
1.5µH
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.9V Adjustable With
External Feedback
2.2µH
3.3µH
Table 1: Inductor Values.
14
2503.2007.04.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
