PRODUCT DATASHEET
AAT2550178
SystemPowerTM
Total Power Solution for Portable Applications
junction temperature to 110°C and avoids the thermal
loop charge reduction at a 70°C ambient temperature.
Battery Charger Losses
The maximum battery charger loss is:
Conditions:
PC = (VADP - VMIN) · ICH + VADP · IQC
VOA
VOB
IQ
2.5V @ 400mA
1.8V @ 400mA
70μA
Step-Down Converter A
Step-Down Converter B
Converter Quiescent Current
Charger and Step-Down
PC = Total Charger Dissipation
VADP = Adapter Voltage
VMIN = Preconditioning Voltage Threshold
ICH = Programmed Charge Current
IQC = Charger Quiescent Current Consumed by the
Charger
VIN = VADP
5.0V
Battery Preconditioning
Threshold Voltage
Battery Charge Current
VMIN
3.0V
ICH
IOP
0.6A
0.75mA
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.
For an application where no load is applied to the step-
down converters and the charger current is set to 1A
with VADP = 5.0V, the maximum charger dissipation
occurs at the preconditioning voltage threshold VMIN
.
PC = (VADP - VMIN) · ICH + VADP · IQC
= (5.0V - 3.0V) · 1A + 5.0V · 0.75mA
= 2W
IOA2 · (RDS(ON)H · VOA + RDS(ON)L · (VIN - VOA)) + IOB2 · (RDS(ON)H · VOB + RDS(ON)L · (VIN - VOB))
VIN
PTOTAL
=
+ (tSW · FS · (IOA + IOB) + 2 · IQ) · VIN + (VADP - VMIN) · ICH + VADP · IOP
0.4A2 · (0.475Ω · 2.5V + 0.45Ω · (5.0V - 2.5V)) + 0.4A2 · (0.475Ω · 1.8V + 0.45Ω · (5.0V - 1.8V))
5.0V
=
The charger thermal loop begins reducing the charge
current at a 110°C junction temperature (TLOOP_IN). The
ambient temperature at which the charger thermal loop
begins reducing the charge current is:
+ 2 · (5ns · 1.4MHz · 0.4A + 70µA) · 5.0V + (5.0V - 3.0V) · 0.6A + 5.0V · 0.75mA = 1.38W
TJ(MAX) = TAMB + (θJA · PLOSS
)
= 70°C + (50°C/W · 1.38W)
= 139°C
TA = TLOOP_IN - θJA · PC
= 110°C - (50°C/W · 2W)
= 10°C
Conditions:
VOA
VOB
IQ
2.5V @ 600mA
1.8V @ 600mA
Step-Down Converter A
Step-Down Converter B
Converter Quiescent Current
Charger and Step-Down Con-
verter Input Voltage
70μA
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.
VIN
3.6V
0A
ICH = IOP
Charger Disabled
Total Power Loss Examples
IOA2 · (RDS(ON)H · VOA + RDS(ON)L · (VIN - VOA)) + IOB2 · (RDS(ON)H · VOB + RDS(ON)L · (VIN - VOB))
VIN
PTOTAL
=
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
+ (tSW · FS · (IOA + IOB) + 2 · IQ) · VIN + (VADP - VMIN) · ICH + VADP · IOP
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
RDS(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
TJ(MAX) = TAMB + (θJA · PLOSS
)
= 85°C + (50°C/W · 0.443W)
= 107.15°C
w w w . a n a l o g i c t e c h . c o m
2550.2008.02.1.3
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