PRODUCT DATASHEET
AAT2789
SystemPowerTM
Low Noise, High Frequency Dual Step-Down Converter
Input Capacitor
Applications Information
Select a 10ꢀF to 22ꢀF 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 CIN. The calculated value varies with input volt-
age and is a maximum when VIN is double the output
voltage.
Inductor Selection
Both step-down converters use peak current mode con-
trol with slope compensation to maintain stability for
duty cycles greater than 50%. When the duty cycle
exceeds 50%, the inductor value must be selected to
maintain the prescribed down-slope in accordance with
the internal slope compensation requirements.
VO
⎛
VO ⎞
· 1 -
⎝
VIN
VIN
⎠
CIN =
Channel 1
⎛ VPP
⎝ IO
⎞
- ESR ·FS
⎠
The internal slope compensation for the adjustable and
low voltage fixed versions of Channel 1 is 0.75A/ꢀs. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 1.8ꢀH
inductor.
VO
⎛
VO ⎞
1
· 1 -
⎝
=
for VIN = 2 · VO
VIN
VIN ⎠
4
1
CIN(MIN)
=
⎛ VPP
⎝ IO
⎞
0.75 ⋅ VO 0.75 ⋅ 1.8V
= 0.75
A
- ESR · 4 · FS
m =
L =
=
⎠
L
1.8µH
µs
Always examine the ceramic capacitor DC voltage coef-
ficient characteristics when selecting the proper value.
For example, the capacitance of a 10ꢀF, 6.3V, X5R ceram-
ic capacitor with 5.0V DC applied is actually about 6ꢀF.
0.75 ⋅ VO 0.75
m
⋅
1.2V
A
=
= 1.2µH
0.75
µs
The inductor should be set equal to the output voltage
numeric value in microhenries (ꢀH). This guarantees suf-
ficient internal slope compensation. Manufacturer’s spec-
ifications 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 exces-
sive 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.
The maximum input capacitor RMS current is:
VO
⎛
VO ⎞
IRMS = IO ·
· 1 -
⎝
VIN
VIN
⎠
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
⎛
VO ⎞
VIN ⎠
1
· 1 -
⎝
=
D · (1 - D) = 0.52 =
VIN
2
Channel 2
For VIN = 2 · VO
The slope compensation for Channel 2 output is set at
0.75A/us. This equates to a slope compensation that is
75% of the inductor current down slope for a 1.8V out-
put and 1.8μH inductor:
IO
IRMS(MAX)
=
2
0.75 ⋅ VO 0.75 ⋅ 1.8V
= 0.75
A
VO
⎛
⎝
VO
⎞
⎠
·
1 -
m =
L =
=
VIN
VIN
L
1.8µH
µs
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.
0.75 ⋅ VO 0.75
m
⋅
3.3V
A
=
= 3.3µH
0.75
µs
w w w . a n a l o g i c t e c h . c o m
14
2789.2008.03.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
