AAT2556 Datasheet, PDF (13/27 Pages) Advanced Analogic Technologies – Battery Charger and Step-Down Converter for Portable Applications

DATA SHEET
AAT2556
Battery Charger and Step-Down Converter for Portable Applications
Current Limit, Over-Temperature Protection
For overload conditions, the peak input current is limited
at the step-down converter. As load impedance decreas-
es and the output voltage falls closer to zero, more
power is dissipated internally, which causes the internal
die temperature to rise. In this case, the thermal protec-
tion circuit completely disables switching, which protects
the device from damage.
The battery charger has a thermal protection circuit which
will shut down charging functions when the internal die
temperature exceeds the preset thermal limit threshold.
Once the internal die temperature falls below the thermal
limit, normal charging operation will resume.
Control Loop
The AAT2556 contains a compact, current mode step-
down DC/DC controller. The current through the
P-channel MOSFET (high side) is sensed for current loop
control, as well as short-circuit and overload protection.
A fixed slope compensation signal is added to the sensed
current to maintain stability for duty cycles greater than
50%. The peak current mode loop appears as a voltage-
programmed current source in parallel with the output
capacitor. The output of the voltage error amplifier pro-
grams the current mode loop for the necessary peak
switch current to force a constant output voltage for all
load and line conditions. Internal loop compensation
terminates the transconductance voltage error amplifier
output. The error amplifier reference is fixed at 0.6V.
Battery Charging Operation
Battery charging commences only after checking several
conditions in order to maintain a safe charging environ-
ment. The input supply (ADP) must be above the mini-
mum operating voltage (UVLO) and the enable pin must
be high (internally pulled down). When the battery is
connected to the BAT pin, the charger checks the condi-
tion of the battery and determines which charging mode
to apply. If the battery voltage is below V_MIN, the charger
begins battery pre-conditioning by charging at 10% of
the programmed constant current; e.g., if the pro-
grammed current is 150mA, then the pre-conditioning
current (trickle charge) is 15mA. Pre-conditioning is
purely a safety precaution for a deeply discharged cell
and will also reduce the power dissipation in the internal
series pass MOSFET when the input-output voltage dif-
ferential is at its highest.
Pre-conditioning continues until the battery voltage
reaches V_MIN. At this point, the charger begins constant-
current charging. The current level for this mode is pro-
grammed using a single resistor from the ISET pin to
ground. Programmed current can be set from a mini-
mum 15mA up to a maximum of 500mA. Constant cur-
rent charging will continue until the battery voltage
reaches the voltage regulation point, V_BAT. When the
battery voltage reaches V_BAT, the battery charger begins
constant voltage mode. The regulation voltage is factory
programmed to a nominal 4.2V (±0.5%) and will con-
tinue charging until the charging current has reduced to
10% of the programmed current.
After the charge cycle is complete, the pass device turns
off and the device automatically goes into a power-sav-
ing sleep mode. During this time, the series pass device
will block current in both directions, preventing the bat-
tery from discharging through the IC.
The battery charger will remain in sleep mode, even if
the charger source is disconnected, until one of the fol-
lowing events occurs: the battery terminal voltage drops
below the V_RCH threshold; the charger EN pin is recycled;
or the charging source is reconnected. In all cases, the
charger will monitor all parameters and resume charging
in the most appropriate mode.
Charge Complete Voltage
Regulated Current
Preconditioning
Trickle Charge
Phase
Constant Current
Charge Phase
I = Max CC
Constant Voltage
Charge Phase
Constant Current Mode
Voltage Threshold
Trickle Charge and I = CC / 10
Termination Threshold
Figure 1: Current vs. Voltage Profile During Charging Phases.
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