AAT2550
Total Power Solution for Portable Applications
Conditions:
Total Power Loss Examples
The most likely high power scenario is when the
charger and step-down converter are both opera-
tional and powered from the adapter. To examine
the step-down converter maximum current capabil-
ity for this condition, it is necessary to determine
the step-down converter MOSFET RDS(ON), quies-
cent current, and switching losses at the adapter
voltage level (5V). This example shows that with a
600mA battery charge current, the buck converter
output current capability is limited 400mA. This lim-
its the junction temperature to 110°C and avoids
the thermal loop charge reduction at a 70°C ambi-
ent temperature.
VOA
VOB
IQ
2.5V @
400mA
1.8V @
400mA
70µA
Step-Down Converter A
Step-Down Converter B
Converter Quiescent
Current
VIN
=
5.0V
3.0V
Charger and Step-Down
Converter Input Voltage
Battery Preconditioning
Threshold Voltage
VADP
VMIN
ICH
IOP
0.6A
Battery Charge Current
Charger Operating Current
0.75mA
2
2
IOA
·
(RDS(ON)H · VOA
+
RDS(ON)L · (VIN - VOA)) + IOB
VIN
· (RDS(ON)H · VOB + RDS(ON)L · (VIN - VOB))
PTOTAL
=
=
+ ( SW
t
· 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
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
TJ(MAX) = TAMB + θJA · PLOSS
= 70°C + (28°C/W) · 1.38W
= 108°C
The step-down converter load current capability is
greatest when the battery charger is disabled. The
following example demonstrates the junction tem-
perature rise for conditions where the battery charg-
er is disabled and full load is applied to both con-
verter outputs at the nominal battery input voltage.
Conditions:
VO1
2.5V @
600mA
1.8V @
600mA
70µA
Step-Down Converter A
Step-Down Converter B
VO2
IQ
Converter Quiescent
Current
VIN
3.6V
0A
Charger and Step-Down
Converter Input Voltage
Charger Disabled
ICH
=
IOP
2550.2006.07.1.0
27
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