AAT2550_08 ANALOGICTECH [Advanced Analogic Technologies], AAT2550_08 Datasheet - Page 24

• Manufacturer Part Number: AAT2550_08
• Description: Total Power Solution for Portable Applications
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

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 (100pF) typi-
cally requires a larger output capacitor for stability.
Thermal Considerations
The AAT2550 is available in a 4x4mm QFN package,
which has a typical thermal resistance of 50°C/W when
the exposed paddle is soldered to a printed circuit board
(PCB) in the manner discussed in the Printed Circuit
Board Layout section of this datasheet. Thermal resis-
tance 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
temperature is limited by either the design derating cri-
teria, the over-temperature shutdown temperature, or
the thermal loop charge current reduction control. To
calculate the junction temperature, sum the step-down
converter losses with the battery charger losses. Multiply
the total losses by the package thermal resistance and
add to the ambient temperature to determine the junc-
tion temperature rise.
24
SystemPower
Table 6: Adjustable Resistor Values for Use With
V
OUT
1.85
R8 =
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
2.0
2.5
3.3
(V)
⎛
⎝
V
V
OUT
REF
0.6V Step-Down Converter.
T
R7, R9 = 59kΩ
R8, R10 (kΩ)
J(MAX)
-1 · R7 =
⎞
⎠
TM
= (P
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
SD
⎛
⎝
+ P
0.6V
1.5V
C
) · θ
- 1 · 59kΩ = 88.5kΩ
⎞
⎠
JA
R7, R9 = 221kΩ
+ T
R8, R10 (kΩ)
AMB
1000
113
150
187
221
261
301
332
442
464
523
715
75
w w w . a n a l o g i c t e c h . c o m
Total Power Solution for Portable Applications
P
verters and P
The total losses will vary considerably depending on
input voltage, load, and charging current. While charg-
ing a battery, the current capability of the step-down
converters is limited.
Step-Down Converter Losses
There are three types of losses are associated with the
AAT2550 step-down converter: switching losses (t
F
rent losses (I
conduction mode, a simplified form of the step-down
converter losses is:
P
For the condition where one channel is in dropout at
100% duty cycle (I
tion is:
Always use the R
corresponds to the applied input voltage.
P
V
R
R
V
V
I
I
I
t
F
SW
SD
OA
OB
Q
S
SD
SD
S
IN
DS(ON)H
DS(ON)L
OA
OB
), conduction losses (I2 · R
+ ( t
=
is the total loss associated with both step-down con-
I
SW
OA
P
2
· F
SD
· (R
S
= Step-Down Converter Dissipation
= Converter Input Voltage
= Low Side MOSFET On Resistance
= Converter A Output Voltage
= Converter B Output Voltage
= Converter A Load Current
= Converter B Load Current
= Converter Quiescent Current
= Switching Time Estimate
= Converter Switching Frequency
= High Side MOSFET On Resistance
· ( I
= I
+
+ ( t
DS(ON)H
OA
OA
I
+ I
OB
SW
· V
2
OB
Q
C
2
) + 2 · I
· R
OA
· F
· (R
· V
is the loss associated with the charger.
+ R
S
DS(ON)H
DS(ON)
IN
DS(ON)L
· I
DS(ON)H
Q
OA
). At full load, assuming continuous
) · V
OB
), the step-down converter dissipa-
PRODUCT DATASHEET
· (V
IN
+ 2 · I
and quiescent current value that
IN
· V
- V
OA
OB
)) + I
Q
V
DS(ON)
V
AAT2550178
+ R
) · V
IN
IN
OB
2
· (R
DS(ON)L
IN
), and quiescent cur-
DS(ON)H
· (V
· V
2550.2008.02.1.3
OB
+ R
IN
- V
DS(ON)L
OB
· (V
))
IN
SW
- V
OB
))
·