AAT2783_12 Datasheet, PDF (15/22 Pages) Skyworks Solutions Inc. – Triple-Output PMIC Dual Buck with Low-VIN LDO

DATA SHEET
AAT2783
Triple-Output PMIC: Dual Buck with Low-V_IN LDO
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input.
Under-voltage lockout (UVLO) guarantees sufficient V_IN
bias and proper operation of all internal circuitry prior to
activation.
Component Selection
Inductor Selection—Channel 1
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 inductor should be set equal to the output voltage
numeric value in μH. This guarantees that there is 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. For
Channel 1, the 1.5μH LQH32PN1R5NN0L series Murata
inductor has a 68.4m worst case DCR and a 1.75A DC
current rating. At full 1A load, the inductor DC loss is
68.4mW which gives less than 6% loss in efficiency for a
1A, 1.2V output.
Inductor Selection—Channel 2
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 AAT2783 is 0.6A/μs.
This equates to a slope compensation that is 75% of the
inductor current down slope for a 1.8V output and 2.2μH
inductor.
m
=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.8V
2.2µH
=
0.6
A
µs
L=
0.75 ·
m
V_O
=
0.75 · V_O
A
0.6 _µs
=
1.2
µs
A
1.7V
=
2.0µH
1.2
µs
A
·
V_O
In this case a standard 2.2μH value is selected.
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 induc-
tor’s saturation characteristics. The inductor should not
show any appreciable saturation under all 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 effi-
ciency when selecting an inductor.
For Channel 2, the 2.2μH GLF2518T2R2M series TDK
inductor has a 104mΩ worst case DCR and a 475mA DC
current rating. At full 300mA load, the inductor DC loss
is 10mW which gives less than 1% loss in efficiency for
a 300mA, 3.3V output.
Input Capacitor
Select a 10μF to 22μF X7R or X5R ceramic capacitor for
the VP1 and VP2 inputs. To estimate the required input
capacitor size, determine the acceptable input ripple
level (V_PP) and solve for C. The calculated value varies
with input voltage and is a maximum when V_IN is double
the output voltage.
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
C_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
1
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
202039A • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • June 8, 2012
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