AAT2552_08 Datasheet, PDF (22/31 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

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PRODUCT DATASHEET
AAT2552178
Total Power Solution for Portable Applications
age droop during the three switching cycles to the output
capacitance can be estimated by:
C_OUT
=
3 · ΔI_LOAD
V_DROOP · F_S
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion establishes a limit on the minimum value for the
output capacitor with respect to load transients.
The internal voltage loop compensation also limits the
minimum output capacitor value to 4.7μF. This is due to
its effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capac-
itance will reduce the crossover frequency with greater
phase margin.
The maximum output capacitor RMS ripple current is
given by:
I = _RMS(MAX)
1
2·
·
3
V_OUT · (V_IN(MAX) - V_OUT)
L · F_S · V_IN(MAX)
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
Inductor Selection
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 AAT2552 is
0.45A/μsec. This equates to a slope compensation that
is 75% of the inductor current down slope for a 1.8V
output and 3.0μH inductor.
m=
0.75 ⋅
L
V_O
=
0.75 ⋅ 1.8V
3.0µH
=
0.45
A
µsec
L=
0.75 ⋅ V_O
m
=
0.75 ⋅ V_O
A
≈
1.67
µsec
A
⋅
V_O
0.45A _µsec
For most designs, the step-down converter operates with
inductor values from 1μH to 4.7μH. Table 6 displays induc-
tor values for the AAT2552 for various output voltages.
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.
The 3.0μH CDRH2D09 series inductor selected from
Sumida has a 150mΩ DCR and a 470mA DC current rat-
ing. At full load, the inductor DC loss is 9.375mW which
gives a 2.08% loss in efficiency for a 250mA, 1.8V out-
put.
Adjustable Output Voltage
for the Step-down Converter
Resistors R2 and R3 of Figure 5 program the output of
the step down converter and regulate at a voltage high-
er than 0.6V. To limit the bias current required for the
external feedback resistor string while maintaining good
noise immunity, the suggested value for R3 is 59kΩ.
Decreased resistor values are necessary to maintain
noise immunity on the FBB pin, resulting in increased
quiescent current. Table 3 summarizes the resistor val-
ues for various output voltages.
R2
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R3
=
⎛ ^3.3V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
267kΩ
With enhanced transient response for extreme pulsed
load application, an external feed-forward capacitor (C8
in Figure 5) can be added.
V_OUT (V)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
R3 = 59kΩ
R2 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R3 = 221kΩ
R2 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 3: Adjustable Resistor Values For
Step-Down Converter.
22
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