BQ24300 Datasheet, PDF (12/22 Pages) Texas Instruments – OVERVOLTAGE AND OVERCURRENT PROTECTION IC AND Li+ CHARGER FRONT-END PROTECTION IC

bq24300
bq24304
SLUS764A – AUGUST 2007 – REVISED OCTOBER 2007
www.ti.com
DETAILED FUNCTIONAL DESCRIPTION
The bq24300 and bq24304 are highly integrated circuits designed to provide protection to Li-ion batteries from
failures of the charging circuit. The IC continuously monitors the input voltage, the input current and the battery
voltage, and protects down-stream circuitry from damage if any of these parameters exceeds safe values. The IC
also monitors its own die temperature and switches off if it becomes too hot.
The IC also offers optional protection against reverse voltage at the input with an external P-channel MOSFET.
POWER DOWN
The device remains in power down mode when the input voltage at the IN pin is below the under-voltage
threshold V_UVLO. The FET Q1 (see Figure 1) connected between IN and OUT pins is off.
POWER-ON RESET
The device resets all internal timers when the input voltage at the IN pin exceeds the UVLO threshold. The gate
driver for the external P-FET is enabled. The IC then waits for duration t_DGL(PGOOD) for the input voltage to
stabilize. If, after t_DGL(PGOOD), the input voltage and battery voltage are safe, FET Q1 is turned ON. The IC has a
soft-start feature to control the inrush current. This soft-start minimizes voltage ringing at the input (the ringing
occurs because the parasitic inductance of the adapter cable and the input bypass capacitor form a resonant
circuit). Figure 2 shows the power-up behavior of the device. Because of the deglitch time at power-on, if the
input voltage rises rapidly to beyond the OVP threshold, the device will not switch on at all, as shown in Figure 3.
OPERATION
The device continuously monitors the input voltage, the input current and the battery voltage as described in
detail in the following sections:
Input Overvoltage Protection
As long as the input voltage is less than V_O(REG), the output voltage tracks the input voltage (less the drop caused
by R_DSON of Q1). If the input voltage is greater than V_O(REG) (plus the R_DSON drop) and less than V_OVP, the
device acts like a series linear regulator, with the output voltage regulated to V_O(REG). If the input voltage rises
above V_OVP, the output voltage is clamped to V_O(REG) for a blanking duration t_BLANK(OVP). If the input voltage
returns below V_OVP within t_BLANK(OVP), the device continues normal operation (see Figure 4). This provides
protection against turning power off due to transient overvoltage spikes while still protecting the system.
However, if the input voltage remains above V_OVP for more than t_BLANK(OVP), the internal FET is turned off,
removing power from the circuit (see Figure 5). When the input voltage comes back to a safe value, the device
waits for t_ON(OVP) then switches on Q1 and goes through the soft-start routine (see Figure 7).
Figure 6 describes graphically the behavior of the OUT pin over the entire range of input voltage variation.
Input Overcurrent Protection
The device can supply load current up to I_OCP continuously. If the load current tries to exceed this threshold, the
current is limited to I_OCP for a maximum duration of t_BLANK(OCP). If the load current returns to less than I_OCP before
t_BLANK(OCP) times out, the device continues to operate (see Figure 9). However, if the overcurrent situation
persists for t_BLANK(OCP), FET Q1 is turned off for a duration of t_REC(OCP). It is then turned on again and the current
is monitored all over again (see Figure 10 and Figure 8).
To prevent the input voltage from spiking up due to the inductance of the input cable, Q1 is not turned off rapidly
in an overcurrent fault condition. Instead, the gate drive of Q1 is reduced slowly, resulting in a “soft-stop”, as
shown in Figure 11.
Battery Overvoltage Protection
The battery overvoltage threshold BV_OVP is internally set to 4.35V. If the battery voltage exceeds the BV_OVP
threshold for longer than t_DGL(BOVP), FET Q1 is turned off (see Figure 12). This switch-off is also a soft-stop. Q1 is
turned ON (soft-start) once the battery voltage drops to BV_OVP – V_HYS-BOVP.
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