PRODUCT DATASHEET
AAT2503178
SystemPowerTM
Adjustable 3-Channel Regulator
Applications Information
VO
VIN
⎛
VO ⎞
VIN ⎠
· 1 -
⎝
CIN =
Step-Down Converter Inductor Selection
⎛ VPP
⎝ IO
⎞
⎠
- ESR ·FS
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 AAT2503 step-
down converter is 0.51A/μsec. This equates to a slope
compensation that is 75% of the inductor current down
slope for a 1.5V output and 2.2μH inductor.
VO
VIN
⎛
VO ⎞
VIN ⎠
1
· 1 -
⎝
=
for VIN = 2 · VO
4
1
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
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 5.0V DC applied is actually about 6μF.
0.75 ⋅ VO 0.75 ⋅ 1.5V
= 0.51
A
m =
=
L
2.2μH
μsec
The maximum input capacitor RMS current is:
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.
VO
VIN
⎛
VO ⎞
VIN ⎠
IRMS = IO ·
· 1 -
⎝
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
⎛
VO ⎞
VIN ⎠
1
2
· 1 -
⎝
=
D · (1 - D) = 0.52 =
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 2.8%
loss in efficiency for a 800mA, 1.8V output.
for VIN = 2 · VO:
IO
Configuration
Output Voltage
Inductor
IRMS(MAX)
=
2
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
1.5μH
2.2μH
3.3μH
0.9V Adjustable
With External
Feedback
VO
VIN
⎛
⎝
VO
VIN
⎞
⎠
·
1 -
The term
appears in both the input voltage
ripple and input capacitor RMS current equations and is
a maximum when VO is twice VIN. This is why the input
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle.
Table 1: Inductor Values.
Input Capacitor
The input capacitor provides a low impedance loop for
the edges of pulsed current drawn by the AAT2503. Low
ESR/ESL X7R and X5R ceramic capacitors are ideal for
this function. To minimize stray inductance, the capacitor
should be placed as closely as possible to the IC. This
keeps the high frequency content of the input current
localized, minimizing EMI and input voltage ripple.
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input of the step-down converter. To estimate the
required input capacitor size, determine 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.
w w w . a n a l o g i c t e c h . c o m
14
2503.2008.02.1.4
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
