AAT2550 Datasheet, PDF (26/35 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

AAT2550
Total Power Solution for Portable Applications
Step-Down Converter Losses
There are three types of losses are associated with
the AAT2550 step-down converter: switching losses
(t_SW · F_S), conduction losses (I² · R_DS(ON)), and qui-
escent current losses (I_Q · V_IN). At full load, assum-
ing continuous conduction mode, a simplified form
of the step-down converter losses is:
P_SD =
I_OA2 · (R_DS(ON)H · V_OA + R_DS(ON)L · (V_IN - V_OA)) + I_OB2 · (R_DS(ON)H · V_OB + R_DS(ON)L · (V_IN - V_OB))
V_IN
+ (t_SW · F_S · (I_OA + I_OB) + 2 · I_Q ) · V_IN
For the condition where one channel is in dropout
at 100% duty cycle (I_OA), the step-down converter
dissipation is:
P_SD = I_OA2 · R_DS(ON)H
+
I_OB2 · (R_DS(ON)H · V_OB + R_DS(ON)L · (V_IN - V_OB))
V_IN
+ (t_SW · F_S · I_OB + 2 · I_Q ) · V_IN
P_SD
V_IN
R_DS(ON)H
R_DS(ON)L
V_OA
V_OB
I_OA
I_OB
I_Q
t_SW
F_S
= Step-Down Converter Dissipation
= Converter Input Voltage
= High Side MOSFET On Resistance
= Low Side MOSFET On Resistance
= Converter A Output Voltage
= Converter B Output Voltage
= Converter A Load Current
= Converter B Load Current
= Converter Quiescent Current
= Switching Time Estimate
= Converter Switching Frequency
Always use the R_DS(ON) and quiescent current
value that corresponds to the applied input voltage.
Battery Charger Losses
The maximum battery charger loss is:
P_C = (V_ADP - V_MIN) · I_CH + V_ADP · I_QC
26
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 · 0.74mA
= 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 - (28°C/W) · 2W
= 54°C
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.
2550.2006.07.1.0