AAT2552 Datasheet, PDF (24/33 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

AAT2552
Total Power Solution for Portable Applications
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 dis-
plays inductor 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
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 excessive loss-
es 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 rating. At full load, the inductor DC loss is
9.375mW which gives a 2.08% loss in efficiency for
a 250mA, 1.8V output.
Adjustable Output Voltage for the Step-
down Converter
Resistors R2 and R3 of Figure 5 program the out-
put of the step down converter and regulate at a
voltage higher than 0.6V. To limit the bias current
required for the external feedback resistor string
while maintaining good noise immunity, the sug-
gested 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 values 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.
24
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.
Adjustable Output Voltage for the LDO
The output voltage for the LDO can be pro-
grammed by an external resistor divider network.
As shown below, the selection of R4 and R5 is a
straightforward matter. R5 is chosen by considering
the tradeoff between the feedback network bias cur-
rent and resistor value. Higher resistor values allow
stray capacitance to become a larger factor in circuit
performance whereas lower resistor values increase
bias current and decrease efficiency. To select appro-
priate resistor values, first choose R5 such that the
feedback network bias current is reasonable. Then,
according to the desired V_OUT, calculate R4 according
to the equation below. An example calculation follows.
R₄ =
⎛
⎝
V_OUT
V_REF
-
1^⎞⎠
·
R₅
An R5 value of 59kΩ is chosen, resulting in a small
feedback network bias current of 1.24V/59kΩ ≈
21µA. The desired output voltage is 1.8V. From this
information, R4 is calculated from the equation below.
The result is R4 = 26.64kΩ. Since 26.64kΩ is not a
standard 1%-value, 26.7kΩ is selected. From this
example calculation, for V_OUT = 1.8V, use R5 = 59kΩ
and R4 = 26.7kΩ. Example output voltages and cor-
responding resistor values are provided in Table 4.
2552.2007.04.1.0