AAT2550_08 ANALOGICTECH [Advanced Analogic Technologies], AAT2550_08 Datasheet - Page 25

• Manufacturer Part Number: AAT2550_08
• Description: Total Power Solution for Portable Applications
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

Battery Charger Losses
The maximum battery charger loss is:
For an application where no load is applied to the step-
down converters and the charger current is set to 1A
with V
occurs at the preconditioning voltage threshold V
The charger thermal loop begins reducing the charge
current at a 110°C junction temperature (T
ambient temperature at which the charger thermal loop
begins reducing the charge current is:
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.
Total Power Loss Examples
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
R
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
P
V
V
I
I
SystemPower
2550.2008.02.1.3
CH
QC
C
ADP
MIN
DS(ON)
= Total Charger Dissipation
= Adapter Voltage
= Preconditioning Voltage Threshold
= Programmed Charge Current
= Charger Quiescent Current Consumed by the
, quiescent current, and switching losses at the
Charger
ADP
P
C
= 5.0V, the maximum charger dissipation
= (V
= (5.0V - 3.0V) · 1A + 5.0V · 0.75mA
= 2W
P
C
T
= (V
A
ADP
= T
= 110°C - (50°C/W · 2W)
= 10°C
TM
ADP
- V
LOOP_IN
MIN
- V
) · I
MIN
- θ
CH
) · I
JA
+ V
CH
· P
+ V
ADP
C
ADP
· I
QC
· I
QC
LOOP_IN
w w w . a n a l o g i c t e c h . c o m
MIN
). The
.
Total Power Solution for Portable Applications
junction temperature to 110°C and avoids the thermal
loop charge reduction at a 70°C ambient temperature.
Conditions:
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.
Conditions:
P
P
TOTAL
TOTAL
V
I
IN
CH
+ ( t
=
+ 2 · ( 5ns · 1.4MHz · 0.4A + 70µA) · 5.0V + (5.0V - 3.0V) · 0.6A + 5.0V · 0.75mA = 1.38W
=
=
+ ( t
=
+ 2 · ( 5ns · 1.4MHz · 0.4A + 70µA) · 3.6V = 0.443W
V
V
V
V
V
V
= V
I
I
I
= I
I
I
0.4A
MIN
I
0.6A
CH
OP
OA
OB
OA
OB
OA
Q
IN
OA
Q
SW
SW
2
2
· F
· (R
· F
· (R
2
OP
ADP
2
· (0.475Ω · 2.5V + 0.45Ω · (5.0V - 2.5V)) + 0.4A
S
· (0.58Ω · 2.5V + 0.56Ω · (3.6V - 2.5V)) + 0.2A
S
DS(ON)H
· ( I
DS(ON)H
· ( I
OA
OA
+ I
+ I
· V
2.5V @ 400mA
1.8V @ 400mA
2.5V @ 600mA
1.8V @ 600mA
· V
OB
OB
OA
) + 2 · I
OA
) + 2 · I
+ R
+ R
0.75mA
70μA
70μA
DS(ON)L
5.0V
3.0V
0.6A
3.6V
DS(ON)L
Q
0A
T
Q
) · V
T
) · V
J(MAX)
J(MAX)
· (V
IN
PRODUCT DATASHEET
· (V
IN
+ (V
= T
= 85°C + (50°C/W · 0.443W)
= 107.15°C
+ (V
= T
= 70°C + (50°C/W · 1.38W)
= 139°C
IN
IN
- V
AMB
ADP
AMB
- V
ADP
OA
+ (θ
OA
- V
+ (θ
)) + I
- V
)) + I
V
MIN
V
IN
JA
MIN
JA
IN
OB
) · I
Charger and Step-Down Con-
· P
5.0V
3.6V
Converter Quiescent Current
· P
OB
Converter Quiescent Current
) · I
2
AAT2550178
Charger Operating Current
2
LOSS
· (R
CH
LOSS
· (R
CH
Charger and Step-Down
Step-Down Converter A
Step-Down Converter B
Battery Preconditioning
Battery Charge Current
Step-Down Converter A
Step-Down Converter B
+ V
)
DS(ON)H
+ V
)
DS(ON)H
verter Input Voltage
2
ADP
2
Threshold Voltage
· (0.475Ω · 1.8V + 0.45Ω · (5.0V - 1.8V))
ADP
· (0.58Ω · 1.8V + 0.56Ω · (3.6V - 1.8V))
Charger Disabled
· I
· V
· I
OP
· V
OP
OB
OB
+ R
+ R
DS(ON)L
DS(ON)L
· (V
· (V
IN
IN
- V
- V
OB
))
OB
))
25