9.2.2.2 Detailed Design Procedure

The primary constraint for this application is choosing the correct device to monitor the battery supply voltage. The TPS3840 can monitor any voltage between 1.6 V and 10 V and is available in 0.1 V increments. Depending on how far away from the nominal voltage rail the user wants the voltage supervisor to trigger determines the correct voltage supervisor variant to choose. In this design example, the TPS3840DL30 is chosen for both the undervoltage and overvoltage monitoring. For undervoltage monitoring, the undervoltage fault occurs when the 3.3-V rail falls to 3 V and for the overvoltage monitoring, the overvoltage fault occurs when the 2.8-V rail rises above the 3-V threshold (V_IT-) plus 100mV hysteresis (V_HYS). It's important to note that in the undervoltage application, the TPS3840 RESET output is logic high during normal conditions whereas in the overvoltage application, the TPS3840 RESET output is logic low during normal conditions which is the reason a single device can be used for either type of monitoring depending on the logic required at the output. The opposite RESET output logic is offered in the push-pull, active-high device TPS3840PH noted with the RESET output. The secondary constraint for this application is the battery temperature monitoring accomplished by the TMP303A. Typical Lithium Ion battery discharge temperature range is 0°C to 60°C which is accomplished by the 'A' variant of TMP303A. The TMP303A triggers a fault to the MR pin of the TPS3840 or directly to the battery charger whenever the temperature is outside of the temperature range. The TMP303A offers 1°C resolution to meet the high resolution requirement. Because the undervoltage monitor design uses TMP303A, a push-pull active-high output device, an additional inverter is required before the MR pin because during normal operation, the TMP303 output is low but the MR pin must be logic high during normal operation. If using two TPS3840 devices for both undervoltage and overvoltage monitoring on the same battery, only one single temperature monitoring device is required. The last constraint is the RESET/RESET time delay set by C_CT. For applications with ambient temperatures ranging from –40°C to +125°C, C_CT can be calculated using R_CT and solving for C_CT in Equation 2. By choosing a standard 10% capacitor value of 10 µF ensures the RESET/RESET time delay will be at least 6 seconds. Note: active-low devices use the output label RESET and active-high devices use the output label RESET.

A 0.1-µF decoupling capacitor is connected to the VDD pin as a good analog design practice. The pull-up resistor is only required for the Open-Drain device variants and is calculated to maintain the RESET current within the ±5 mA limit found in the Recommended Operating Conditions: R_Pull-up = V_Pull-up ÷ 5 mA. For this design, a 1-MΩ pull-up resistor is selected to minimize current draw when RESET is asserted and to prevent the battery from unnecessary discharge. Keep in mind the lowering the pull-up resistor, increases V_OL and I_OUT. The MR pin is used for a second fault condition provided by the temperature switch.