8.3.3.1 Input Source Connected – Adapter or USB

With a source connected, the power-path management circuitry of the bq2423x monitors the input current continuously. The OUT output 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 bq2423x has internal selectable current limits of 100 mA (USB100) and 500 mA (USB500) for charging from USB ports, as well as a resistor-programmable input current limit. See Table 1 for EN1, EN2 setting.

The bq2423x is USB-IF compliant for the inrush current testing. The USB spec allows up to 10 μF to be hard-started, which establishes 50 μF as the maximum inrush charge value when exceeding 100 mA. The input current limit for the bq2423x prevents the input current from exceeding this limit, even with system capacitances greater than 10 μF. Note that the input capacitance to the device must be selected small enough to prevent a violation (<10 μF), as this current is not limited.

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 1:

The input current limit is adjustable up to 500 mA. The valid resistor range is 2.75 kΩ to 8.4 kΩ.

When the IN source is connected, priority is given to the system load. The DPPM and Battery Supplement modes are used to maintain the system load. Figure 12 illustrates examples of the DPPM and supplement modes. These modes are explained in detail in the following sections.

Figure 12. bq2423x DPPM And Battery Supplement Modes
(V_OREG = 4.4 V, V_BAT = 3.6 V, I_LIM=400 mA, I_CHG = 150 mA)

8.3.3.2 Input Source Not Connected

When no source is connected to the IN input, OUT is powered strictly from the battery. During this mode, the current into OUT is unregulated, similar to Battery Supplement Mode; however, the short-circuit circuitry is active. If the OUT voltage falls below the BAT voltage by 250 mV for longer than t_DGL(SC2), OUT is turned off. The short-circuit recovery timer then starts counting. After t_REC(SC2), OUT turns on and attempts to restart. If the short-circuit remains, OUT is turned off and the counter restarts. This ON/OFF cycle continues until the overload condition is removed.

8.3.5.1 Modifying / Extending the Allowable Temperature Range for Charging

The nominal temperature range to allow charging is 0°C to 50°C when using a typical 103AT-2 type thermistor. However, the user can increase the range by adding two external resistors. See Figure 14 for the circuit. The values for Rs and Rp are calculated using the following equations:

Equation 2.

Equation 3.

Where:

R_TH: Thermistor Hot Trip Value found in thermistor data sheet
R_TC: Thermistor Cold Trip Value found in thermistor data sheet
V_H: Hot Trip Threshold of the IC = 0.3 V nominal
V_C: Cold Trip Threshold of the IC = 2.1 V nominal
I_TS: Output Current Bias of the IC = 75 µA nominal
NTC Thermsitor Semitec 103AT-2 Type or equivalent

Table 2 provides examples of the thermistor resistance at different temperatures and suggested typical Rs and Rp values, using 1% tolerance resistors that can extend the allowable temperature range beyond the standard 0°C – to – 50° C window.

RHOT and RCOLD are the thermistor resistance at the desired hot and cold temperatures, respectively. Note that the temperature window cannot be tightened more using the thermistor connected to TS, it can only be extended.

Figure 14. Extended TS Temperature Thresholds

8.4.1.1 Charge Current Translator

When the charger is enabled, internal circuits generate a current proportional to the charge current at the ISET input. The current out of ISET is 1/400 (±10%) of the charge current. This current, when applied to the external charge current programming resistor, R_ISET, generates an analog voltage that can be monitored by an external host to calculate the current sourced from BAT.

8.4.1.2 Battery Detection and Recharge

The bq2423x automatically detects if a battery is connected or removed. Once a charge cycle is complete, the battery voltage is monitored. When the battery voltage falls below V_RCH, the battery detection routine is run. The detection routine first applies I_BAT(DET) for t_DET to see if V_BAT drops below V_LOWV. If not, it indicates that the battery is still connected, but has discharged. If CE is low, the charger is turned on again to top off the battery. During this recharge cycle, the CHG output remains high-impedance as recharge cycles are not indicated by the CHG pin. If the BAT voltage falls below V_LOWV during the battery detection test, it indicates that the battery has been removed or the protector is open. Next, the precharge current is applied for t_DET to close the protector if possible. If the battery voltage does not rise above V_RCH, it indicates that the protector is closed, or a battery has been inserted, and a new charge cycle begins. If the voltage rises above V_RCH, the battery is determined missing and the detection routine continues. The battery detection runs until a battery is detected.

8.4.1.3 Termination Disable (TD Input, bq24230)

The bq24230 contains a TD input that allows termination to be enabled/disabled. Connect TD to a logic high to disable charge termination. When termination is disabled, the device goes through the precharge, fast-charge, and CV phases, then remains in the CV phase. During the CV phase, the charger maintains the output voltage at BAT equal to V_BAT(REG), and charging current does not terminate. BAT sources currents up to I_CHG or I_IN-MAX, whichever is less. Battery detection is not performed. The CHG output is high impedance once the current falls below I_TERM and does not go low until the input power or CE are toggled. When termination is disabled, the precharge and fast-charge safety timers are also disabled. Battery pack temperature sensing (TS pin functionality) is also disabled if the TD pin is high and the TS pin is unconnected.

8.4.1.4 Adjustable Termination Threshold (ITERM Input, bq24232)

The termination current threshold for the bq24232 is user-programmable. Set the termination current by connecting a resistor from ITERM to VSS. For USB100, mode (EN1 = EN2 = VSS), the termination current value is calculated as:

In the other input current limit modes (EN1 ≠ EN2), the termination current value is calculated as:

The termination current is programmable up to 50% of the fast-charge current. The R_ITERM resistor must be less than 15 kΩ. Leave ITERM unconnected to select the default internally set termination current.

8.4.1.5 Dynamic Charge Timers (TMR Input)

The bq2423x devices contain internal safety timers for the precharge and fast-charge phases to prevent potential damage to the battery and the system. The timers begin at the start of the respective charge cycles. The timer values are programmed by connecting a resistor from TMR to VSS. The resistor value is calculated using the following equation:

Leave TMR unconnected to select the internal default timers. Disable the timers by connecting TMR to VSS. Reset the timers by toggling CE pin.

Note that timers are suspended when the device is in thermal shutdown, and the timers are slowed proportionally to the charge current when the device enters thermal regulation. For the bq24230, the timers are disabled when TD is connected to a high logic level.

During the fast-charge phase, several events increase the timer durations.

1. The system load current activates the DPPM loop which reduces the available charging current
2. The input current is reduced because the input voltage has fallen to V_IN(LOW)
3. The device has entered thermal regulation because the IC junction temperature has exceeded T_J(REG)

During each of these events, the internal timers are slowed down proportionately to the reduction in charging current. For example, if the charging current is reduced by half for two minutes, the timer clock is reduced to half the frequency and the counter counts half as fast resulting in only one minute of counted time.

8.4.1.6 Status Indicators (PGOOD, CHG)

The bq2423x contains two open-drain outputs that signal its status. The PGOOD output signals when a valid input source is connected. PGOOD is low when (V_BAT + V_IN(DT)) < V_IN < V_OVP. When the input voltage is outside of this range, PGOOD is high impedance.

The CHG output signals when a new charge cycle is initiated. After a charge cycle is initiated, CHG goes low once the battery is above the short-circuit threshold. CHG goes high impedance once the charge current falls below I_TERM. CHG remains high impedance until the input power is removed and reconnected or the CE pin is toggled. It does not signal subsequent recharge cycles.