AAT2789 Datasheet, PDF (14/22 Pages) Advanced Analogic Technologies – Low Noise, High Frequency Dual Step-Down Converter

SystemPower^TM
PRODUCT DATASHEET
AAT2789
Low Noise, High Frequency Dual Step-Down Converter
Applications Information
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.
Channel 1
The internal slope compensation for the adjustable and
low voltage fixed versions of Channel 1 is 0.75A/μs. This
equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 1.8μH
inductor.
m
=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.8V
1.8µH
=
0.75
A
µs
L=
0.75 ⋅ V_O
m
=
0.75
⋅
1.2V
A
=
1.2µH
0.75 _µs
The inductor should be set equal to the output voltage
numeric value in microhenries (μH). 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.
Channel 2
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:
m
=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.8V
1.8µH
=
0.75
A
µs
L=
0.75 ⋅ V_O
m
=
0.75
⋅
3.3V
A
=
3.3µH
0.75 _µs
Input Capacitor
Select a 10μF to 22μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (V_PP) and
solve for C_IN. The calculated value varies with input volt-
age and is a maximum when V_IN is double the output
voltage.
C_IN =
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_S
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
1
- ESR^⎞⎠ · 4 · F_S
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.
The maximum input capacitor RMS current is:
I_RMS = I_O ·
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
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.
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
=
D · (1 - D) =
0.5²
=
1
2
For V_IN = 2 · V_O
I = _RMS(MAX)
I_O
2
The term
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
appears in both the input voltage
ripple and input capacitor RMS current equations and is
a maximum when V_O is twice V_IN. This is why the input
voltage ripple and the input capacitor RMS current ripple
are a maximum at 50% duty cycle.
14
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2789.2008.03.1.0