PRODUCT DATASHEET
AAT2550178
SystemPowerTM
Total Power Solution for Portable Applications
PSD is the total loss associated with both step-down con-
verters and PC is the loss associated with the charger.
The total losses will vary considerably depending on
input voltage, load, and charging current. While charg-
ing a battery, the current capability of the step-down
converters is limited.
R7, R9 = 59kΩ
R7, R9 = 221kΩ
R8, R10 (kΩ)
VOUT (V)
R8, R10 (kΩ)
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.85
2.0
2.5
3.3
19.6
29.4
39.2
49.9
59.0
68.1
78.7
88.7
118
124
137
187
267
75
113
150
187
221
261
301
332
442
464
523
715
1000
Step-Down Converter Losses
There are three types of losses are associated with the
AAT2550 step-down converter: switching losses (tSW
·
FS), conduction losses (I2 · RDS(ON)), and quiescent cur-
rent losses (IQ · VIN). At full load, assuming continuous
conduction mode, a simplified form of the step-down
converter losses is:
Table 6: Adjustable Resistor Values for Use With
0.6V Step-Down Converter.
IOA2 · (RDS(ON)H · VOA + RDS(ON)L · (VIN - VOA)) + IOB2 · (RDS(ON)H · VOB + RDS(ON)L · (VIN - VOB))
VIN
PSD
=
+ (tSW · FS · (IOA + IOB) + 2 · IQ ) · VIN
The AAT2550, combined with an external feedforward
capacitor (C10 and C11 in Figure 4), delivers enhanced
transient response for extreme pulsed load applications.
The addition of the feedforward capacitor (100pF) typi-
cally requires a larger output capacitor for stability.
For the condition where one channel is in dropout at
100% duty cycle (IOA), the step-down converter dissipa-
tion is:
V
V
1.5V
0.6V
⎛
⎝
⎞
⎛
⎝
⎞
⎠
PSD = IOA2 · RDS(ON)H
R8 =
OUT -1 · R7 =
- 1 · 59kΩ = 88.5kΩ
⎠
REF
IOB2 · (RDS(ON)H · VOB + RDS(ON)L · (VIN - VOB))
VIN
+
Thermal Considerations
The AAT2550 is available in a 4x4mm QFN package,
which has a typical thermal resistance of 50°C/W when
the exposed paddle is soldered to a printed circuit board
(PCB) in the manner discussed in the Printed Circuit
Board Layout section of this datasheet. Thermal resis-
tance will vary with the PCB area, ground plane area,
size and number of other adjacent components, and the
heat they generate. The maximum ambient operating
temperature is limited by either the design derating cri-
teria, the over-temperature shutdown temperature, or
the thermal loop charge current reduction control. To
calculate the junction temperature, sum the step-down
converter losses with the battery charger losses. Multiply
the total losses by the package thermal resistance and
add to the ambient temperature to determine the junc-
tion temperature rise.
+ (tSW · FS · IOB + 2 · IQ ) · VIN
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
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
= Converter Switching Frequency
tSW
FS
Always use the RDS(ON) and quiescent current value that
corresponds to the applied input voltage.
TJ(MAX) = (PSD + PC) · θJA + TAMB
w w w . a n a l o g i c t e c h . c o m
24
2550.2008.02.1.3
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
