BQ24075T Datasheet, PDF (14/29 Pages) Texas Instruments – 1.5A USB-Friendly Li-Ion Battery Charger and Power-Path Management IC

bq24075T
bq24079T
SLUS937 – DECEMBER 2009
www.ti.com
OVERVOLTAGE PROTECTION (OVP)
The bq2407xT accepts inputs up to 28V without damage. Additionally, an overvoltage protection (OVP) circuit is
implemented that shuts off the internal LDO and discontinues charging when V_IN > V_OVP for a period longer than
t_DGL(OVP). When in OVP, the system output (OUT) is connected to the battery and PGOOD is high impedance.
Once the OVP condition is removed, a new power on sequence starts (See the POWER ON section). The safety
timers are reset and a new charge cycle will be indicated by the CHG output.
DYNAMIC POWER-PATH MANAGEMENT
The bq2407xT features an OUT output that powers the external load connected to the battery. This output is
active whenever a source is connected to IN or BAT. The following sections discuss the behavior of OUT with a
source connected to IN to charge the battery and a battery source only.
INPUT SOURCE CONNECTED (ADAPTER or USB)
With a source connected, the dynamic power-path management (DPPM) circuitry of the bq2407xT monitors the
input current continuously. The OUT output for the bq24075T/ 79T is regulated to a fixed voltage (V_O(REG)). The
current into IN is shared between charging the battery and powering the system load at OUT. The bq2407xT has
internal selectable current limits of 100mA (USB100) and 500mA (USB500) for charging from USB ports, as well
as a resistor-programmable input current limit.
The bq2407xT is USB IF compliant for the inrush current testing. The USB spec allows up to 10μF to be hard
started, which establishes 50μC as the maximum inrush charge value when exceeding 100mA. The input current
limit for the bq2407xT prevents the input current from exceeding this limit, even with system capacitances greater
than 10μF. Note that the input capacitance to the device must be selected small enough to prevent a violation
(<10μF), as this current is not limited. Figure 24 demonstrates the startup of the bq2407xT and compares it to
the USB-IF specification.
Figure 24. USB-IF Inrush Current Test
The input current limit selection is controlled by the state of the EN1 and EN2 pins as shown in Table 1. When
using the resistor-programmable current limit, the input current limit is set by the value of the resistor connected
from the ILIM pin to VSS, and is given by the equation:
II_N-MAX = K_ILIM / R_ILIM
The input current limit is adjustable up to 1.5A. The valid resistor range is 1.07 kΩ to 7.5kΩ.
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