FUNCTIONAL DEVICE OPERATION
BATTERY INTERFACE AND CONTROL
CHARGER OPERATION
USB CHARGING
The USB VBUS line in this case, is used to provide a supply within the USB voltage limits and with at least 500 mA of current
drive capability.
When trickle charging from the USB cable, it is important not to exceed the 100 mA, in case of a legacy USB bus. The
appropriate charge current level ICHRG[2:0] = (0001) is 80 mA typical which accounts for the additional current through the
charge LED indicator.
WALL CHARGING
No distinction can be made between a USB Host or a wall charger. Therefore, when attaching a wall charger, the CHRGSE1B
pin must be forced low as a charger attach indicator. The CHRGSE1B pin has a built-in weak pull-up to VCORE. In the
application, this pin is preferably pulled low, with for instance an NPN of which the base is pulled high through a resistor to
CHRGRAW. The state of the CHRGSE1B pin is reflected through the CHRGSE1BS bit. When CHRGSE1B changes state a
CHRGSE1BI is generated. No specific debounce is applied to the CHRGSE1B detector.
Table 76. Charger Detector Characteristics
Parameter
CHRGSE1B Pull Up
Logic Low
Condition
Min
-
Typ
Max
-
Units
kOhm
V
To VCORE
100
0
-
-
0.3
Logic High
1.0
VCORE
V
If an application is to support wall chargers and USB on separate connectors, it is advised to separate the VBUS and the
CHRGRAW on the PCB. For these applications, charging from USB is no longer possible. For proper operation, a 120 kOhm
pull-down resistor should be placed at VBUS.
STANDALONE CHARGING
A standalone charge mode of operation is provided to minimize software interaction. It also allows for a completely discharged
battery to be revived without processor control. This is especially important when charging from a USB host or when in single
path configuration (M3 replaced by short, BATTFET floating). Since the default voltage and current setting of the charge path
regulator may not be the optimum choice for a given application, these values can be reprogrammed through the SPI if the
CHGAUTOVIB bit is set. Note that the power limiter can be programmed independent of this bit being set.
Upon connecting a USB host to the application with a dead battery, the trickle cycle is started and the current set to the lowest
charge current level (80 mA). When the battery voltage rises above the BATTON = 3.4 V threshold, a power up sequence is
automatically initiated. The lowest charge current level remains selected until a higher charge current level is set through the SPI
after negotiation with the USB host. In case of a power up failure, a second power up will not be initiated to avoid an ambulance
mode, the charger circuitry will though continue to charge. The USB dead battery operation following the low power boot scheme
is described further in this chapter.
Upon connecting a charger to an application with a dead battery the behavior will be different for serial path and single path
configurations.
In serial path (M3 present), the application will be powered up with the current through M1M2 set to 500 mA minimum. The
internal trickle charge current source will be enabled, set to its lowest level (40 mA) up to BATTMIN, followed by the highest
setting (80 mA). The internal trickle charge current is not programmable, but can be turned off by the SPI. In this mode, the
voltage and current regulation to BP through the external pass devices M1M2 can be reprogrammed through the SPI. Once the
battery is greater than BATTON, it will be connected to BP and further charged through M1/M2 at the same time as the
application.
In single path (M3 replaced by a short, BATTFET floating), the battery (and therefore BP) is below the BPON threshold. This
will be detected and the external charge path will be used to precharge the battery, up to BATTMIN at the lowest level (80 mA),
and above at the 500 mA minimum level. Once exceeding BPON, a turn on event is generated and the voltage and current levels
can be reprogrammed.
When in the serial path and upon initialization of the charger circuitry, and it appears BP stays below BPON, the application
will not be powered up, and the same charging scheme is followed as for single path.
13892
Analog Integrated Circuit Device Data
Freescale Semiconductor
95
FREESCALE [ Freescale ]
