While a valid input source is present, the battery charger attempts to charge the battery with a fast-charge current determined by the resistance from IFAST to GND. Top-off current matches to the fast-charge current. Table 3 shows resistance values which correspond to target IFAST and ITOPOFF values.
| Resistance (kΩ) | IFAST (mA) | ITOPOFF (mA) |
|---|---|---|
| 24.9 | 3150 | 150 |
| 22.6 | 3000 | 150 |
| 20.5 | 2800 | 125 |
| 18.7 | 2500 | 125 |
| 16.9 | 2400 | 125 |
| 15.4 | 2200 | 100 |
| 14 | 2000 | 100 |
| 12.4 | 1800 | 75 |
| 11 | 1500 | 75 |
| 9.53 | 1400 | 75 |
| 8.2 | 1200 | 50 |
| 6.65 | 1000 | 50 |
| 5.23 | 800 | 50 |
| 3.6 | 600 | 50 |
| 2.4 | 500 | 50 |
USB D+/D- lines, which are used for the detection of BC1.2 and proprietary Travel Adaptors (TAs), 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 MAX77757 and the MCU as show in Figure 12. Switchers are required for each D+ and D- line to guarantee Hi-Z for the connections to MCU to avoid wrong detections of TAs (see Figure 12). It is recommended to connect the INOKB of the MAX77757 to the MCU in this configuration so that the MAX77757 can signal the completion of the detection to the MCU. Once the MCU receives a valid INOKB signal, it can switch the D+/D- lines from the MAX77757 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 the sufficient voltage rating and derated capacitance at max operating voltage conditions. Table 4 shows proper capacitors after considering the derating and operating voltage.
| PIN | TYPE |
|---|---|
| CHGIN Capacitor | 2.2µF/16V |
| BYP Capacitor | 10µF + 22µF/16V |
| SYS Capacitor | 2x10µF/10V |
| BATT Capacitor | 10µF/10V |
| VDD Capacitor | 2.2µF/10V |
| PVL Capacitor | 2.2µF/10V |
| BST Capacitor | 0.1μF/6.3V |
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 unconnected. In this case, the input current to MAX77757 is limited to 3A.
Place all bypass capacitors for CHGIN, BYP, SYS, VDD, and BATT as close as possible to the IC. 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 MAX77757 has two kinds of ground pins, PGND and GND. Use caution when connecting PGND since it is the switching node ground of the Charger Buck; it should be tied to the ground of the SYS and BYP capacitor and connected to the ground plane directly without sharing the other ground. The GND can be connected to the ground plane.
Figure 13 is a recommended placement and layout guide.
The MAX77757's control scheme requires an external inductor from 0.47μH to 1μH for proper operation.
| MANUFACTURER | PART NUMBER | INDUCTANCE (μH) | ISAT(TYP) (A) | IRMS(TYP) (A) | DCR (TYP) (mΩ) | SIZE (L x W x T) (mm) |
|---|---|---|---|---|---|---|
| SEMCO | CIGT252008LMR47MNE | 0.47 | 5.5 | 4.5 | 24 | 2.5 x 2.0 x 0.8 |
| SEMCO | CIGT252010LMR47MNE | 0.47 | 6 | 4.5 | 24 | 2.5 x 2.0 x 1.0 |
| SEMCO | CIGT201610EHR47MNE | 0.47 | 5.9 | 5 | 18 | 2.0 x 1.6 x 1.0 |
| CYNTEC | HTEH25201T-R47MSR-63 | 0.47 | 6.6 | 5.6 | 16.5 | 2.5 x 2.0 x 1.0 |
Input Status (INOKB)
INOKB is an open-drain and active-low output that indicates 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 I/O voltage.
INOKB can also be used as a LED indicator driver by adding a current limit resistor and a LED to SYS.
Charging Status Output (STAT)
STAT is an open-drain and active-low output that indicates charge status. STAT status changes as shown in Table 6.
| CHARGING STATUS | STAT | LOGIC STATE | CHARGE STATUS LED |
|---|---|---|---|
| No input | High impedance | High | Off |
| Trickle, precharge, fast charge | Repeat low and high impedance with 1Hz, 50% duty cycle | After an external diode and a capacitor rectifier, high | Blinking with 1Hz, 50% duty cycle |
| Top-off and done | Low | Low | Solid on |
| Faults | High impedance | High | Off |
STAT can be used as a logic output for the system processor by adding a 200kΩ pullup resistor to the system I/O voltage and a rectifier (a diode and a capacitor).
STAT also can be used as a LED indicator driver by adding a current limit resistor and a LED to SYS.