AAT2550
Total Power Solution for Portable Applications
(tSW · FS), conduction losses (I2 · RDS(ON)), and qui-
escent current losses (IQ · VIN). At full load, assum-
ing continuous conduction mode, a simplified form
of the step-down converter losses is:
Step-Down Converter Losses
There are three types of losses are associated with
the AAT2550 step-down converter: switching losses
2
2
IOA
·
(RDS(ON)H · VOA
+
RDS(ON)L · (VIN - VOA)) + IOB
VIN
· (RDS(ON)H · VOB + RDS(ON)L · (VIN - VOB))
PSD
=
+ (tSW · FS · (IOA + IOB) + 2 · IQ ) · VIN
For the condition where one channel is in dropout
at 100% duty cycle (IOA), the step-down converter
dissipation is:
PC = Total Charger Dissipation
VADP = Adapter Voltage
VMIN = Preconditioning Voltage Threshold
ICH = Programmed Charge Current
2
PSD
=
+
IOA
·
RDS(ON)H
IQC = Charger Quiescent Current Consumed by
the Charger
2
IOB
·
(RDS(ON)H · VOB
+ RDS(ON)L · (VIN - VOB))
VIN
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
+ (tSW · FS · IOB + 2 · IQ ) · VIN
threshold VMIN
.
PSD
VIN
= Step-Down Converter Dissipation
= Converter Input Voltage
RDS(ON)H = High Side MOSFET On Resistance
RDS(ON)L = Low Side MOSFET On Resistance
PC = (VADP - VMIN) · ICH + VADP · IQC
= (5.0V - 3.0V) · 1A + 5.0V · 0.74mA
= 2W
VOA
VOB
IOA
IOB
IQ
= Converter A Output Voltage
= Converter B Output Voltage
= Converter A Load Current
= Converter B Load Current
= Converter Quiescent Current
= Switching Time Estimate
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:
tSW
FS
= Converter Switching Frequency
Always use the RDS(ON) and quiescent current
value that corresponds to the applied input voltage.
TA = TLOOP_IN - θJA · PC
Battery Charger Losses
= 110°C - (28°C/W) · 2W
= 54°C
The maximum battery charger loss is:
PC = (VADP - VMIN) · ICH + VADP · IQC
Therefore, under the given conditions, the
AAT2550 battery charger will enter the thermal
loop charge current reduction at an ambient tem-
perature greater than 54°C.
26
2550.2006.07.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
