AAT2550
Total Power Solution for Portable Applications
The maximum output capacitor RMS ripple current
is given by:
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
- 1 ·
R8 =
OUT -1
·
R7 =
59kΩ = 88.5kΩ
⎠
⎠
REF
1
V
OUT · (VIN(MAX) - VOUT
FS VIN(MAX)
)
The adjustable version of the AAT2550, combined
with an external feedforward capacitor (C10 and
C11 in Figure 4), delivers enhanced transient
response for extreme pulsed load applications. The
addition of the feedforward capacitor typically
requires a larger output capacitor for stability.
IRMS(MAX)
=
·
L
·
·
2 · 3
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.
Thermal Considerations
Adjustable Output Resistor Selection
The AAT2550 is available in a 4x4mm QFN pack-
age, which has a typical thermal resistance of
28°C/W when the exposed paddle is soldered to a
printed circuit board (PCB) in the manner dis-
cussed in the Printed Circuit Board Layout section
of this datasheet. Thermal resistance will vary with
the PCB area, ground plane area, size and number
of other adjacent components, and the heat they
generate. The maximum ambient operating tem-
perature is limited by either the design derating cri-
teria, the over-temperature shutdown temperature,
or the thermal loop charge current reduction con-
trol. To calculate the junction temperature, sum the
step-down converter losses with the battery charg-
er losses. Multiply the total losses by the package
thermal resistance and add to the ambient temper-
ature to determine the junction temperature rise.
For applications requiring an adjustable output volt-
age, the 0.6V version can be externally pro-
grammed. Resistors R7 through R10 of Figure 4 pro-
gram the output to regulate at a voltage higher than
0.6V. To limit the bias current required for the exter-
nal feedback resistor string while maintaining good
noise immunity, the minimum suggested value for
R7 and R9 is 59kΩ. Although a larger value will fur-
ther reduce quiescent current, it will also increase
the impedance of the feedback node, making it more
sensitive to external noise and interference. Table 6
summarizes the resistor values for various output
voltages with R7 and R9 set to either 59kΩ for good
noise immunity or 221kΩ for reduced no load input
current.
R7, R9 = 59kΩ R7, R9 = 221kΩ
VOUT (V)
R8, R10 (kΩ)
R8, R10 (kΩ)
TJ(MAX) = (PSD + PC) · θJA + TAMB
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
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
75
113
150
187
221
261
301
332
442
464
523
715
1000
PSD is the total loss associated with both step-down
converters and PC is the loss associated with the
charger. The total losses will vary considerably
depending on input voltage, load, and charging
current. While charging a battery, the current capa-
bility of the step-down converters is limited.
124
137
187
267
Table 6: Adjustable Resistor Values for Use
With 0.6V Step-Down Converter.
2550.2006.07.1.0
25
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