AAT2550 Datasheet, PDF (27/35 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

AAT2550
Total Power Solution for Portable Applications
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 R_DS(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.
Conditions:
V_OA
2.5V @
400mA
V_OB
1.8V @
400mA
I_Q
70µA
V_IN =
V_ADP
V_MIN
5.0V
3.0V
I_CH
0.6A
I_OP
0.75mA
Step-Down Converter A
Step-Down Converter B
Converter Quiescent
Current
Charger and Step-Down
Converter Input Voltage
Battery Preconditioning
Threshold Voltage
Battery Charge Current
Charger Operating Current
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 + (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:
V_O1
2.5V @
600mA
V_O2
1.8V @
600mA
I_Q
70µA
V_IN
3.6V
I_CH =
0A
I_OP
Step-Down Converter A
Step-Down Converter B
Converter Quiescent
Current
Charger and Step-Down
Converter Input Voltage
Charger Disabled
2550.2006.07.1.0
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