AAT2550_08 Datasheet, PDF (25/34 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

SystemPower^TM
PRODUCT DATASHEET
AAT2550178
Total Power Solution for Portable Applications
Battery Charger Losses
The maximum battery charger loss is:
P_C = (V_ADP - V_MIN) · I_CH + V_ADP · I_QC
P_C = Total Charger Dissipation
V_ADP = Adapter Voltage
V_MIN = Preconditioning Voltage Threshold
I_CH = Programmed Charge Current
I_QC = Charger Quiescent Current Consumed by the
Charger
For an application where no load is applied to the step-
down converters and the charger current is set to 1A
with V_ADP = 5.0V, the maximum charger dissipation
occurs at the preconditioning voltage threshold V_MIN.
P_C = (V_ADP - V_MIN) · I_CH + V_ADP · I_QC
= (5.0V - 3.0V) · 1A + 5.0V · 0.75mA
= 2W
The charger thermal loop begins reducing the charge
current at a 110°C junction temperature (T_{LOOP_IN}). The
ambient temperature at which the charger thermal loop
begins reducing the charge current is:
T_A = T_{LOOP_IN} - θ_JA · P_C
= 110°C - (50°C/W · 2W)
= 10°C
Therefore, under the given conditions, the AAT2550 bat-
tery charger will enter the thermal loop charge current
reduction at an ambient temperature greater than 10°C.
Total Power Loss Examples
The most likely high power scenario is when the charger
and step-down converter are both operational and pow-
ered from the adapter. To examine the step-down con-
verter maximum current capability for this condition, it is
necessary to determine the step-down converter MOSFET
R_DS(ON), quiescent current, and switching losses at the
adapter voltage level (5V). This example shows that with
a 600mA battery charge current, the buck converter out-
put current capability is limited 400mA. This limits the
junction temperature to 110°C and avoids the thermal
loop charge reduction at a 70°C ambient temperature.
Conditions:
V_OA
V_OB
I_Q
V_IN = V_ADP
V_MIN
I_CH
I_OP
2.5V @ 400mA
1.8V @ 400mA
70μA
5.0V
3.0V
0.6A
0.75mA
Step-Down Converter A
Step-Down Converter B
Converter Quiescent Current
Charger and Step-Down
Battery Preconditioning
Threshold Voltage
Battery Charge Current
Charger Operating Current
The step-down converter load current capability is great-
est when the battery charger is disabled. The following
example demonstrates the junction temperature rise for
conditions where the battery charger is disabled and full
load is applied to both converter outputs at the nominal
battery input voltage.
P_TOTAL
=
I_OA2
·
(R_DS(ON)H
·
V_OA
+
R_DS(ON)L
·
(V_IN
-
V_OA)) + I_OB2
V_IN
·
(R_DS(ON)H
·
V_OB
+
R_DS(ON)L
·
(V_IN
-
V_OB))
+ (t_SW · F_S · (I_OA + I_OB) + 2 · I_Q) · V_IN + (V_ADP - V_MIN) · I_CH + V_ADP · I_OP
=
0.4A²
·
(0.475Ω
·
2.5V
+
0.45Ω
·
(5.0V
-
2.5V)) + 0.4A²
5.0V
·
(0.475Ω
·
1.8V
+
0.45Ω
·
(5.0V
-
1.8V))
+ 2 · (5ns · 1.4MHz · 0.4A + 70µA) · 5.0V + (5.0V - 3.0V) · 0.6A + 5.0V · 0.75mA = 1.38W
T_J(MAX) = T_AMB + (θ_JA · P_LOSS)
= 70°C + (50°C/W · 1.38W)
= 139°C
Conditions:
V_OA
V_OB
I_Q
V_IN
I_CH = I_OP
2.5V @ 600mA
1.8V @ 600mA
70μA
3.6V
0A
Step-Down Converter A
Step-Down Converter B
Converter Quiescent Current
Charger and Step-Down Con-
verter Input Voltage
Charger Disabled
P_TOTAL
=
I_OA2
·
(R_DS(ON)H
·
V_OA
+
R_DS(ON)L
·
(V_IN
-
V_OA)) + I_OB2
V_IN
·
(R_DS(ON)H
·
V_OB
+
R_DS(ON)L
·
(V_IN
-
V_OB))
+ (t_SW · F_S · (I_OA + I_OB) + 2 · I_Q) · V_IN + (V_ADP - V_MIN) · I_CH + V_ADP · I_OP
=
0.6A² · (0.58Ω · 2.5V + 0.56Ω · (3.6V - 2.5V)) + 0.2A² · (0.58Ω · 1.8V + 0.56Ω · (3.6V - 1.8V))
3.6V
+ 2 · (5ns · 1.4MHz · 0.4A + 70µA) · 3.6V = 0.443W
T_J(MAX) = T_AMB + (θ_JA · P_LOSS)
= 85°C + (50°C/W · 0.443W)
= 107.15°C
2550.2008.02.1.3
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