AAT3601_08 Datasheet, PDF (28/37 Pages) Advanced Analogic Technologies – Total Power Solution for Portable Applications

PRODUCT DATASHEET
AAT3601178
Total Power Solution for Portable Applications
The AAT3601 is offered in a TQFN55-36 package which
can provide up to 4W of power dissipation when it is
properly bonded to a printed circuit board and has a
maximum thermal resistance of 25°C/W. Many consider-
ations should be taken into account when designing the
printed circuit board layout, as well as the placement of
the charger IC package in proximity to other heat gen-
erating devices in a given application design. The ambi-
ent temperature around the charger IC will also have an
effect on the thermal limits of a battery charging appli-
cation. The maximum limits that can be expected for a
given ambient condition can be estimated by the follow-
ing discussion. First, the maximum power dissipation for
a given situation should be calculated:
Where:
P_D(MAX) =
(T_J(MAX) - T_A)
θ_JA
P_D(MAX) = Maximum Power Dissipation (4W)
θ_JA = Package Thermal Resistance (25°C/W)
T_J(MAX) = Maximum Device Junction Temperature (°C)
(140°C)
T_A = Ambient Temperature (°C)
Next, the power dissipation for the charger can be cal-
culated by the following equation:
P_D = (V_CHGIN - V_BAT) · I_{CH_CC} + (V_CHGIN · I_OP) + (V_CHGIN - V_SYSOUT) · I_SYSOUT
+ (V_SYSOUT - V_OUT1) · I_OUT1 + (V_SYSOUT - V_OUT2) · I_OUT2
+ (V_SYSOUT - V_OUT3) · I_OUT3 + (V_SYSOUT - V_OUT4) · I_OUT4
+ (V_SYSOUT - V_OUT5) · I_OUT5
+
I²
OUTBUCK
·
⎛
⎝^RDS(ON)L
·
V_OUTBUCK
V_SYSOUT
+
R_DS(ON)H
·
[V_SYSOUT -
V_SYSOUT
V_OUTBUCK]⎞
⎠
Where:
P_D = Total Power Dissipation by the Device
V_CHGIN = CHGIN Input Voltage
V_BAT = Battery Voltage at the BAT Pin
I_{CH_CC} = Constant Charge Current Programmed for the
Application
I_OP = Quiescent Current Consumed by the IC for Normal
Operation (0.5mA)
V_SYSOUT and I_SYSOUT = Output Voltage and Load Current
from the SYSOUT Pin for the System LDOs and Step-
down Converter (3.9V out for SYSOUT)
R_DS(ON)H and R_DS(ON)L = On-Resistance of Step-down High
and Low Side MOSFETs (0.8Ω each)
V_OUTX and I_OUTX = Output Voltage and Load Currents for
the LDOs and Step-Down Converter (Default Output
Voltages)
By substitution, we can derive the maximum charge cur-
rent (I_{CH_CC(MAX)}) before reaching the thermal limit condi-
tion (T_REG = 100°C, Thermal Loop Regulation). The maxi-
mum charge current is the key factor when designing
battery charger applications.
I = _{CH_CC(MAX)}
(T_REG - T_A)
θ_JA
- (V_CHGIN · I_OP) - (V_CHGIN - V_SYSOUT) · I_SYSOUT)
- [(V_SYSOUT - V_OUT1) · I_OUT1] - (V_SYSOUT - V_OUT2) · I_OUT2
- [(V_SYSOUT - V_OUT3) · I_OUT3] - (V_SYSOUT - V_OUT4) · I_OUT4
- (V_SYSOUT - V_OUT5) · I_OUT5
-
I²
OUTBUCK
·
⎛⎝_RDS(ON)L
·
V_OUTBUCK
V_SYSOUT
+
R_DS(ON)H · (V_SYSOUT - V_OUTBUCK)⎞
V_SYSOUT
⎠
V_IN - V_BAT
In general, the worst condition is when there is the
greatest voltage drop across the charger, when battery
voltage is charged up to just past the preconditioning
voltage threshold and the LDOs and step-down con-
verter are sourcing full output current.
For example, if 700mA and 147mA are being sourced
from the 3.9V SYSOUT pin to the LDOs and Buck supply
channels respectively (300mA to LDO2, 100mA to LDO1
and 3-5, and 147mA to Buck; see buck efficiency graph
for 300mA output current) with a CHGIN supply of 5V,
and the battery is being charged at 3.0V, then the power
dissipated will be 3.49W. A reduction in the charge cur-
rent (through I²C) may be necessary in addition to
reduction provided by the internal thermal loop of the
charger itself.
For the above example at T_A = 30°C, the I_{CH_CC(MAX)} =
459mA.
Thermal Overload Protection
The AAT3601 integrates thermal overload protection
circuitry to prevent damage resulting from excessive
thermal stress that may be encountered under fault con-
ditions, for example. This circuitry disables all regulators
if the AAT3601 die temperature exceeds 140°C, and
prevents the regulators from being enable until the die
temperature drops by 15°C (typ).
28
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