AAT2550_08 Datasheet, PDF (24/34 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2550178
Total Power Solution for Portable Applications
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
R7, R9 = 59kΩ
R8, R10 (kΩ)
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
R7, R9 = 221kΩ
R8, R10 (kΩ)
75
113
150
187
221
261
301
332
442
464
523
715
1000
Table 6: Adjustable Resistor Values for Use With
0.6V Step-Down Converter.
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.
R8
=
⎛ ^VOUT
⎝ V_REF
-1^⎞⎠
·
R7
=
⎛ ^1.5V
⎝ 0.6V
-
1 ^⎞⎠
·
59kΩ
=
88.5kΩ
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.
T_J(MAX) = (P_SD + P_C) · θ_JA + T_AMB
P_SD is the total loss associated with both step-down con-
verters and P_C is the loss associated with the charger.
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_SW ·
F_S), conduction losses (I2 · R_DS(ON)), and quiescent cur-
rent losses (I_Q · V_IN). At full load, assuming continuous
conduction mode, a simplified form of the step-down
converter losses is:
P_SD =
I_OA2 · (R_DS(ON)H · V_OA + R_DS(ON)L · (V_IN - V_OA)) + I_OB2 · (R_DS(ON)H · V_OB + R_DS(ON)L · (V_IN - V_OB))
V_IN
+ (t_SW · F_S · (I_OA + I_OB) + 2 · I_Q ) · V_IN
For the condition where one channel is in dropout at
100% duty cycle (I_OA), the step-down converter dissipa-
tion is:
P_SD = I_OA2 · R_DS(ON)H
+
I_OB2 · (R_DS(ON)H · V_OB + R_DS(ON)L · (V_IN - V_OB))
V_IN
+ (t_SW · F_S · I_OB + 2 · I_Q ) · V_IN
P_SD = Step-Down Converter Dissipation
V_IN
= Converter Input Voltage
R_DS(ON)H = High Side MOSFET On Resistance
R_DS(ON)L = Low Side MOSFET On Resistance
V_OA = Converter A Output Voltage
V_OB = Converter B Output Voltage
I_OA
= Converter A Load Current
I_OB
= Converter B Load Current
I_Q
= Converter Quiescent Current
t_SW
= Switching Time Estimate
F_S
= Converter Switching Frequency
Always use the R_DS(ON) and quiescent current value that
corresponds to the applied input voltage.
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