Table 1. Design Parameters

9.2.2.1 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 device, 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 device and can be hazardous. Connecting the VBAT pin through R_BAT prevents a large current from flowing into the battery in case of failure of the device. In the interests of safety, R_BAT should have a very 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 BV_OVP threshold. This error is over and above the tolerance on the nominal 4.35-V BV_OVP threshold.

Choosing R_BAT in the range from 100 KΩ to 470 kΩ is a good compromise. In the case of a device failure, with R_BAT equal to 100 kΩ, the maximum current flowing into the battery would be (30 V – 3 V) ÷ 100 kΩ = 270 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 compared to the internal tolerance of 50 mV on the BV_OVP threshold.

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

9.2.2.2 Selection of R_CE

The CE pin can be used to enable and disable the device. If host control is not required, the CE pin can be tied to ground or left unconnected, 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 previous discussion), 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 bq24308 device's CE pin. The drop across the resistor is given by R_CE × I_IH.

9.2.2.3 Selection of Input and Output Bypass Capacitors

The input capacitor C_IN in Figure 12 and Figure 13 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 1 μF be used at the input of the device. It should be located in close proximity to the IN pin.

C_OUT in Figure 12 and Figure 13 is also important: If a very 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 device.

Figure 12. Overvoltage, Overcurrent, and Battery Overvoltage Protection

Figure 13. OVP, OCP, BATOVP With Input Reverse-Polarity Protection

9.2.2.4 Selection of the PGATE External MOSFET

The PGATE output drives the gate of an external MOSFET to protect the device from reverse polarity input voltages. The MOSFET must be sized to handle the expected current in the application. Additionally, the impedance of the MOSFET is in series with the internal FET of the bq24308, so that the overall acceptable system resistance must be taken into account. Ensure the MOSFET VDS maximum rating exceeds the worst-case expected reverse voltage in the application. The bq24308 withstands up to –30 V, so a 30 V rating on the MOSFET is a good target. The maximum VGS of the MOSFET must be greater than –17 V to ensure operation up to 30 V inputs.