AAT2782 Datasheet, PDF (15/22 Pages) Advanced Analogic Technologies – Triple Output PMIC: Dual Buck with Low-VIN LDO

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PRODUCT DATASHEET
AAT2782
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 57mΩ worst case DCR and a 1.75A DC
current rating. At full 1.2A load, the inductor DC loss is
128mW which gives 9% loss in efficiency for a 1.2A,
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 AAT2782 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
µs
1.2 _A · V_O
0.6 _µs
µs
= 1.2 _A 1.7V = 2.0µH
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 NLCV32T-2R2M series TDK
inductor has a 130mΩ worst case DCR and a 770mA DC
current rating. At full 600mA load, the inductor DC loss
is 47mW which gives less than 3% loss in efficiency for
a 600mA, 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.
C_IN =
V_{O · 1 -} V_O
V_IN
V_IN
V_PP
I_O
- ESR
· F_OSC
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
C_IN(MIN) =
1
V_PP
I_O
- ESR
· 4 · F_OSC
2782.2008.05.1.0
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