9.1.1.1 Selection of R_ILIM

The overcurrent threshold is programmed by a resistor, R_ILIM, connected from the ILIM pin to VSS. Figure 4 shows the OCP threshold as a function of R_ILIM, and may be approximated by the following equation:

Choose a I_OCP between 50 mA and 300 mA and apply the above equation to select a R_ILIM resistor value from 500 kΩ to 83.3 kΩ respectively. However, at lower OCP limits, approaching 50 mA, the precision of the current protection circuit decreases the achievable accuracy of the OCP threshold.

9.1.1.2 Selection of R_BAT

It is strongly recommended that the battery not be tied directly to the VBAT pin of the device, as under some failure modes of the IC, the voltage at the IN pin may appear on the VBAT pin. This voltage can be as high as 30 V, and applying 30 V to the battery in case of the failure of the bq24311 can be hazardous. Connecting the VBAT pin through R_(BAT) prevents a large current from flowing into the battery in case of a failure of the IC. In the interests of safety, R_BAT should have a high value. The problem with a large R_(BAT) is that the voltage drop across this resistor because of the VBAT bias current I_(VBAT) causes an error in the V_(BOVP) threshold. This error is over and above the tolerance on the nominal 4.35V V_(BOVP) threshold.

Choosing R_BAT in the range 100 kΩ to 470 kΩ is a good compromise. In the case of an IC failure, with R_BAT equal to 100kΩ, the maximum current flowing into the battery would be (30 V – 3 V) ÷ 100 kΩ = 246 μA, which is low enough to be absorbed by the bias currents of the system components. R_(BAT) equal to 100 kΩ would result in a worst-case voltage drop of R_(BAT) × I_(VBAT) = 1 mV. This is negligible to compared to the internal tolerance of 50mV on V_(BOVP) threshold.

If the Bat-OVP function is not required, the VBAT pin should be connected to VSS.

9.1.1.3 Selection of R_(CE), R_(FAULT), and R_(PU)

The CE pin can be used to enable and disable the IC. If host control is not required, the CE pin can be tied to ground or left un-connected, permanently enabling the device.

In applications where external control is required, the CE pin can be controlled by a host processor. As in the case of the VBAT pin (see Selection of Rbat), the CE pin should be connected to the host GPIO pin through as large a resistor as possible. The limitation on the resistor value is that the minimum V_OH of the host GPIO pin less the drop across the resistor should be greater than V_IH of the bq24311 CE pin. The drop across the resistor is given by R_(CE) × I_IH.

The FAULT pin is an open-drain output that goes low during OV, OC, battery-OV, and OT events. If the application does not require monitoring of the FAULT pin, it can be left unconnected. But if the FAULT pin has to be monitored, it should be pulled high externally through R_(PU), and connected through R_(FAULT) to the host. R_(FAULT) prevents damage to the host controller if the bq24311 fails (see Selection of Rbat). The resistors should be of high value, in practice values between 22 kΩ and 100 kΩ should be sufficient.

9.1.1.4 Selection of Input and Output Bypass Capacitors

The input capacitor C_IN in Figure 11 is for decoupling, and serves an important purpose. Whenever there is a step change downwards in the system load current, the inductance of the input cable causes the input voltage to spike up. C_IN prevents the input voltage from overshooting to dangerous levels. It is strongly recommended that a ceramic capacitor of at least 1μF be used at the input of the device. It should be located in close proximity to the IN pin.

C_OUT in Figure 11 is also important: If a fast (< 1 μs rise time) overvoltage transient occurs at the input, the current that charges C_OUT causes the device’s current-limiting loop to kick in, reducing the gate-drive to FET Q1. This results in improved performance for input overvoltage protection. C_OUT should also be a ceramic capacitor of at least 1 μF, located close to the OUT pin. C_OUT also serves as the input decoupling capacitor for the charging circuit downstream of the protection IC.

9.1.2.1 Powering Accessories

In some applications, the equipment that the protection IC resides in may be required to provide power to an accessory (that is, a cellphone may power a headset or an external memory card) through the same connector pins that are used by the adapter for charging. Figure 12 and Figure 13 illustrate typical charging and accessory-powering scenarios:

Figure 12. Charging - The Red Arrows Show the Direction of Current Flow

Figure 13. Powering an Accessory - The Red Arrows Show the Direction of Current Flow

In the second case, when power is being delivered to an accessory, the bq24311 device is required to support current flow from the OUT pin to the IN pin.

If V_OUT > V_(UVLO) + 0.7 V, FET Q1 is turned on, and the reverse current does not flow through the diode but through Q1. Q1 will then remain ON as long as V_OUT > V_(UVLO) – V_(HYS-UVLO) + R_DS(on) x I_(ACCESSORY). Within this voltage range, the reverse current capability is the same as the forward capability, 0.5 A. It should be noted that there is no overcurrent protection in this direction.

Figure 14.