Applications Information

When a valid input source is present, the battery charger attempts to charge the battery with a fast-charge current determined by the resistance from I_FAST to GND. Table 3 shows the resistance values which correspond to the target I_FAST values.

The top-off charging current is programmed by an external resistor connected from ITOPOFF to GND (RTOPOFF). Table 4 shows the resistance values which correspond to the target ITOPOFF values.

The USB D+/D- lines, which are used for BC1.2 and proprietary adapter detection, can be used for data communication. If an MCU handles this communication in the target system, the D+/D- lines can be connected to the MAX77751 and the MCU, as shown in Figure 11. The switchers are required for each D+ and D- lines to guarantee high impedance state for the MCU connections to avoid incorrect adapter detection. It is recommended to connect the MAX77751’s INOKB to the MCU in this configuration so that the MAX77751 can signal that detection is complete to the MCU. When the MCU receives a valid INOKB signal, it can switch the D+/D- lines from the MAX77751 to the MCU for data communication.

All capacitors should be X5R dielectric or better. Be aware that multi-layer ceramic capacitors have large voltage coefficients. Before selecting capacitors, check for sufficient voltage rating and derated capacitance at the max operating voltage condition. Table 5 shows the capacitors to select after considering the derating and operating voltage.

If the system that includes the MAX77751 can read the battery temperature, the temperature can be controlled by adjusting the battery current through enabling switching and disabling charging. As described in the Buck State section, charging is disabled by pulling the ITOPOFF pin low. Figure 12 shows a recommended system diagram where the MCU has an ADC to sense the temperature, a GPIO output connected to ITOPOFF to enable or disable charging, and a GPIO input connected to INOKB to check the presence of a valid charger. Note that the GPIO output should be an open-drain type. In this system configuration, if the MCU judges the temperature is too high, the MCU should pull the GPIO output low to disable charging. It is important to check if a valid charger is present through the INOKB signal before disabling charging. Pulling ITOPOFF low when the INOKB signal is high (i.e., inactive) could affect the TOPOFF current sensing. Meanwhile, if the temperature goes back to normal, the MCU should make the GPIO be high impedance state to enable charging.

If the system does not have an MCU which can get the INOKB signal from the MAX77751 and provide the enable or disable signal to ITOPOFF of the MAX77751, the circuit depicted in Figure 13 can provide the same function which the MCU does in Figure 12. The Hot and Cold Temperature Detection portion of the circuit detects if the temperature is greater than the hot threshold or less than the cold threshold. If the temperature is hot or cold, it provides the disable signal (Active High) to the INOKB Control portion of the circuit, which can control the disable signal according to the INOKB status. If INOKB is high (i.e., the charger input is not valid), the disable signal from the temperature detection part is blocked. If INOKB is low (i.e., the charger input is valid), the disable signal passes through the INOKB Control part and reach the ITOPOFF pin of the MAX77751. If the output of the temperature detection part is high (i.e., the disable signal) when INOKB is low, ITOPOFF is pulled low, and battery charging through the MAX77751 is disabled.

In an application where the power source is not USB, all the USB related pins such as CC1, CC2, DP, and DN should be left not connected (NC). In this case, the input current to the MAX77751 is limited to 3A.

Place all bypass capacitors for CHGIN, BYP, SYS, V_DD, and BATT as close as possible to the IC. Connect the battery to BATT as close as possible to the IC to provide accurate battery voltage sensing. Provide a large copper ground plane to allow the PGND pad to sink heat away from the device. Use wide and short traces for high current connections such as CHGIN, BYP, SYS, and BATT to minimize voltage drops. The MAX77751 has two kinds of ground pins: PGND and GND. Carefully connect PGND because it is a switching node ground of the Charger Buck. It should be tied to ground of the SYS and BYP capacitors and connected to the ground plane directly without sharing other ground. The GND can be connected to the ground plane.

Figure 14 is a recommended placement and layout guide.

The MAX77751's control scheme requires an external inductor from 0.47μH to 1μH for proper operation.

INOKB is an open-drain and active low output that indicates the input status. If a valid input source is inserted and the buck converter starts switching, INOKB pulls low. When the reverse boost is enabled, INOKB pulls low to indicate 5V output from CHGIN.

INOKB can be used as a logic output for the system processor by adding a 200kΩ pullup resistor to the system IO voltage.

INOKB can also be used as an LED indicator driver by adding a current limit resistor and an LED to SYS.

STAT is an open-drain and active low output that indicates charge status. Table 7 shows the STAT status changes.

STAT can be used as a logic output for the system processor by adding a 200kΩ pullup resistor to the system IO voltage and a rectifier (a diode and a capacitor).

STAT also can be used as an LED indicator driver by adding a current limit resistor and an LED to SYS.