The MAX77757 is a highly integrated USB Type-C Charger with autonomous configuration. The MAX77757 can operate from an input range of 4.5V to 13.7V to support 5V, 9V, and 12V AC adapters as well as USB input. The fast-charge current is up to 3.15A and the max input current limit is 3.0A.
The MAX77757 can run BC1.2 and USB Type-C CC detection upon input insertion and configure input source to max power option and charger input current limit to max power.
Fast-charge current and top-off current threshold can be programmed with an external resistor. The input voltage regulation feature (AICL) even allows users to use weak AC adapters without preventing a charge.
The power path design provides system power even when the battery is fully discharged, and it supplements current from the battery and charge input automatically when the system demands a higher current.
A reverse boost from the battery can be enabled by the ENBST pin to allow 5.1V/1.5A OTG to VBUS.
- Complete Li+/LiPoly/LiFePO4 Battery Charger
- Prequalification, Constant Current, Constant Voltage
- 55mA Precharge Current
- 300mA Trickle Charge Current for Charge Termination Voltage from 4.1V to 4.5V. For the 3.6V/3.7V Termination Voltage Options, Trickle Charge Current is Disabled
- Resistor Adjustable Constant Current Charge
- 500mA to 3.15A
- Resistor Adjustable Charge Termination Threshold
- 50mA to 150mA
- Battery Regulation Voltage
- 3.60V, 4.20V, 4.35V, and 4.40V
- -0.9/+0.3% Accuracy at +25°C
- -1/+0.5% Accuracy from 0°C to +85°C
- Synchronous Switch-Mode Based Design
- Smart Power Selector
- Optimally Distributes Power Between the Charge Adapter, System, and Main Battery
- When Powered by a Charge Adapter, the Main Battery can Provide Supplemental Current to the System
- The Charge Adapter can Support the System without the Main Battery
- No External MOSFETs Required
- Single Input Operation
- Reverse Leakage Protection (Prevents the Battery from Leaking Current to the Inputs)
- VCHGIN_OVLO = 13.7V
- Supports AC-to-DC Wall Adapters
- Automatic Input Current Limit Selection After Charger Type Detection
- 500mA, 1A, 2A, 2.5A, and 3A
- Charge Safety Timer
- 6 Hours
- Die Temperature Monitor with Thermal Foldback Loop
- Die Temperature Thresholds: 130°C
- Input Voltage Regulation Allows Operation from High-Impedance Sources (AICL)
- BATT to SYS Switch is 20mΩ Typical
- Capable of 4.5A Steady-State Operation from BATT to SYS
- Short Circuit Protection
- BATT to SYS Overcurrent Threshold: 6A
- SYS Short-to-Ground
- Buck Operates with Input Current Limit to 200mA when VSYS < VSYSPU
The MAX77757 is a switch-mode charger for a one-cell lithium-ion (Li+), lithium polymer (Li-polymer), or LiFePO4 battery. The current limit for CHGIN input is configured automatically allowing the flexibility to connect to either an AC-to-DC wall charger or a USB port, as shown in Figure 2.
The synchronous switch-mode DC-DC converter utilizes a high 1.3MHz switching frequency, which is ideal for portable devices since it allows the use of small components while eliminating excessive heat generation. The DC-DC converter has both a buck and a boost mode of operation. When charging, the main battery converter operates in buck mode. The DC-DC buck operates from a 4.3V to 13.7V source and delivers up to 3.15A to the battery. The battery charge current is programmable from 500mA to 3.15A with an external resistor.
As a boost converter, the DC-DC uses energy from the main battery to boost the voltage at BYP. The boosted BYP voltage is used to supply the USB OTG voltage which is fixed to 5.1V.
Maxim Integrated’s Smart Power Selector architecture makes the best use of the limited adapter power and the battery’s power at all times to supply up to buck current limit from the buck to the system. (Additionally, supplement mode provides additional current from the battery to the system up to B2SOVRC.) Adapter power that is not used for the system is used to charge the battery. All power switches for charging and switching the system load between the battery and adapter power are included on-chip—no external MOSFETs are required.
Maxim Integrated’s proprietary process technology allows for low-RDSON devices in a small solution size. The total dropout resistance from the adapter power input to the battery is 165mΩ (typ), assuming that the inductor has 0.04Ω of ESR. This 165mΩ typical dropout resistance allows for charging a battery up to 3.0A from a 5V supply. The resistance from the BATT-to-SYS node is 20mΩ, allowing for low power dissipation and long battery life.
A multitude of safety features ensures reliable charging. Features include a charge timer, junction thermal regulation, over/undervoltage protection, and short circuit protection.
The BATT-to-SYS switch has overcurrent protection (see the Main Battery Overcurrent Protection During System Power-Up section for more information).
The SPS architecture is a network of internal switches and control loops that distribute energy between external power sources CHGIN, BYP, SYS, and BATT.
Figure 1 shows a simplified arrangement for the smart power selector’s power steering switches. Figure 2 shows a more detailed arrangement of the smart power selector switches with the following names: QCHGIN, QHS, QLS, and QBAT.
Switch and Control Loop Descriptions
- CHGIN Input Switch: QCHGIN provides the input overvoltage protection of +16V. The input switch is either completely on or completely off. As shown in Figure 2, there are SPS control loops that monitor the current through the input switches as well as the input voltage.
- DC-DC Switches: QHS and QLS are the DC-DC switches that can operate as a buck (step-down) or a boost (step-up). When operating as a buck, energy is moved from BYP to SYS. When operating as a boost, energy is moved from SYS to BYP. SPS control loops monitor the DC-DC switch current, the SYS voltage, and the BYP voltage.
- Battery-to-System Switch: QBAT controls the battery charging and discharging. Additionally, QBAT allows the battery to be isolated from the system (SYS). An SPS control loop monitors the QBAT current.
SYS Regulation Voltage
- When DC-DC is enabled as a buck and the charger is enabled but in a non-charging state such as done, thermal shutdown, or timer fault, VSYS is regulated to VBATTREG and QBAT is off.
- When the DC-DC is enabled as a buck and charging in trickle-charge, fast-charge, or top-off modes, VSYS is regulated to VSYSMIN when the VPRECHG < VBATT < VSYSMIN. And, when the DC-DC is enabled as a buck and charging in precharge mode (VBATT < VPRECHG), VSYS is regulated to VBATTREG. In these modes, the QBAT switch acts as a linear regulator and dissipates power (P = (VSYS - VBATT) × IBATT). When VBATT > VSYSMIN, then VSYS = VBATT + IBATT × RBAT2SYS. In this mode, the QBAT switch is closed.
In all of the previous modes, if the combined SYS load exceeds the input current limit, then VSYS drops to VBATT – VBSREG, and the battery provides supplemental current.
The charger input is compared with several voltage thresholds to determine if it is valid. A charger input must meet the following three characteristics to be valid:
- CHGIN must be above VCHGIN_UVLO to be valid. Once CHGIN is above the UVLO threshold, the information (together with LIN2SYS, described as follows) is latched and only can be reset when the charger is in adaptive input current loop (AICL) and input current is lower than the IULO threshold of 60mA. Note that VCHGIN_REG is lower than their UVLO falling threshold, respectively.
- CHGIN must be below its overvoltage lockout threshold (VCHGIN_OVLO).
- CHGIN must be above the system voltage by VCHGIN2SYS.
INOKB pin is pulled down when CHGINOK = 1 and the switcher starts.
After the charger type detection is complete, the MAX77757 automatically configures the input current limit to the highest setting that the source can provide. If the input source is not a standard power source described by BC1.2, USB Type-C, or a proprietary charger type that the MAX77757 can detect, the MAX77757 sets the input current limit to 3A.
An input voltage regulation loop allows the charger to function well when it is attached to a poor-quality charge source. The loop improves performance with relatively high resistance charge sources that exist when long cables are used or devices are charged with non-compliant USB hub configurations.
The input voltage regulation loop automatically reduces the input current limit in order to keep the input voltage at VCHGIN_REG. If the input current limit is reduced to ICHGIN_REG_OFF (50mA, typ) and the input voltage is below VCHGIN_REG, then the charger input is turned off.
To ensure that a rapid removal and reinsertion of a charge source always results in a charger input interrupt, the charger input presents loading to the input capacitor to ensure that when the charge source is removed the input voltage decays below the UVLO threshold in a reasonable time (tINSD). The input self-discharge is implemented with a 44kΩ resistor (RINSD) from CHGIN input to ground.
Li+/Li-Poly Battery
The MAX77757 utilizes several charging states to safely and quickly charge Li+/Li-Poly batteries as shown in Figure 5 and Figure 6. Figure 5 shows an exaggerated view of the Li+/Li-Poly battery progressing through the following charge states when there is no system load and the die and battery are close to room temperature: precharge → trickle → fast-charge → top-off → done.
As for the LiFePO4 battery, the MAX77757 skips the trickle charge state and directly enters the fast-charge state after the precharge state. Figure 7 and Figure 8 presents the LiFePO4 battery charge profile and state machine: precharge → fast-charge → top-off → done.
From any state shown in Figure 6 except thermal shutdown, the “INIT” state is entered whenever the charger inputs CHGIN is invalid or the charger timer is suspended.
While in the “INIT” state, the charger current is 0mA, the charge timer is forced to 0, and the power to the system is provided by the battery.
To exit the “INIT” state, the charger input must be valid.
In the buck state, battery charging is disabled while the charger input CHGIN is valid. Entering or leaving the buck state is controlled by the voltage of the THM pin. If the voltage of this pin is pulled down by an external device (e.g., MCU) under VCHGR_EN, the chip goes to the buck state from any state if CHGIN is valid as shown in Figure 6. Charging is disabled in the buck state, which means QBAT is off unless it is in supplement mode. If the voltage of this pin is over VCHGR_EN, the chip leaves the buck state and resumes charging. It should be noted that it is only when CHGIN is valid that charging can be enabled or disabled. Therefore, the external device (e.g., MCU) should check the INOKB signal if CHGIN is valid before trying to enable or disable charging.
As shown in Figure 6, the precharge state occurs when the main battery voltage is less than VPRECHG. In the precharge state, charge current into the battery is IPRECHG.
The following events cause the state machine to exit this state:
- The main battery voltage rises above VPRECHG and the charger enters the next state in the charging cycle, trickle charge.
- If the battery charger remains in this state for longer than tPQ, the charger state machine transitions to the timer fault state.
Note that the precharge state works with battery voltages down to 0V. The low 0V operation typically allows this battery charger to recover batteries that have an “open” internal pack protector. Typically a pack internal protection circuit opens if the battery has seen an overcurrent, undervoltage, or overvoltage. When a battery with an “open” internal pack protector is used with this charger, the precharge mode current flows into the 0V battery—this current raises the pack’s terminal voltage to the point where the internal pack protection switch closes.
Note that a normal battery typically stays in the precharge state for several minutes or less; therefore, a battery that stays in the precharge state for longer than tPQ might be experiencing a problem.
The trickle charge mode descripted below is for Li-ion and Li-poly batteries only, with charge termination voltage from 4.1V to 4.5V.
The trickle charge state occurs when VBATT > VPRECHG and VBATT < VTRICKLE, as shown in Figure 6.
When the MAX77757 is in its trickle charge state, the charge current in the battery is less than or equal to ITRICKLE.
Charge current might be less than ITRICKLE/IFC for any of the following reasons:
- The charger input is under input current limit
- The charger input voltage is low
- The charger is in thermal foldback
- The system load is consuming adapter current. Note that the system load always gets priority over the battery charge current.
The following events cause the state machine to exit this state:
- When the main battery voltage rises above VTRICKLE, the charger enters the next state in the charging cycle, fast-charge constant current (CC).
- If the battery charger remains in this state for longer than tPQ, the charger state machine transitions to the timer fault state.
Note that a normal battery typically stays in the trickle charge state for several minutes or less; therefore, a battery that stays in trickle charge for longer than tPQ might be experiencing a problem.
Based on the characteristic of the LiFePO4 battery, the trickle charge state of the MAX77757 3.6V option is disabled. After the precharge state, when VPRECHG < VBATT < VBATTREG, the MAX77757 enters the fast-charge constant current state to improve the charger efficiency.
As shown in Figure 6, the fast-charge constant current (CC) state occurs when the main-battery voltage is greater than the trickle threshold and less than the battery regulation threshold (VTRICKLE < VBATT < VBATTREG).
In the fast-charge CC state, the current into the battery is less than or equal to IFC. Charge current can be less than IFC for any of the following reasons:
- The charger input is under input current limit
- The charger input voltage is low
- The charger is in thermal foldback
- The system load is consuming adapter current. Note that the system load always gets priority over the battery charge current.
The following events cause the state machine to exit this state:
- When the main battery voltage rises above VBATTREG, the charger enters the next state in the charging cycle, fast-charge constant voltage (CV).
- If the battery charger remains in this state for longer than tFC, the charger state machine transitions to the timer fault state.
The battery charger dissipates the most power in the fast-charge constant current state. This power dissipation causes the internal die temperature to rise. If the die temperature exceeds TREG, IFC is reduced. See the Thermal Foldback section for more information.
As shown in Figure 6, the fast-charge constant voltage (CV) state occurs when the battery voltage rises to VBATTREG from the fast-charge CC state.
In the fast-charge CV state, the battery charger maintains VBATTREG across the battery and the charge current is less than or equal to IFC. As shown in Figure 5, charger current decreases exponentially in this state as the battery becomes fully charged.
The smart power selector control circuitry might reduce the charge current lower than the battery can otherwise consume for any of the following reasons:
- The charger input is under input current limit
- The charger input voltage is low
- The charger is in thermal foldback
- The system load is consuming adapter current. Note that the system load always gets priority over the battery charge current.
The following events cause the state machine to exit this state:
- When the charger current is below ITO for tTERM, the charger enters the next state in the charging cycle, top off.
- If the battery charger remains in this state for longer than tFC, the charger state machine transitions to the timer fault state.
As shown in Figure 6, the top-off state can only be entered from the fast-charge CV state when the charger current decreases below ITO for tTERM. In the top-off state, the battery charger tries to maintain VBATTREG across the battery and typically the charge current is less than or equal to ITO.
The smart power selector control circuitry might reduce the charge current lower than the battery can otherwise consume for any of the following reasons:
- The charger input is under input current limit
- The charger input voltage is low
- The charger is in thermal foldback
- The system load is consuming adapter current. Note that the system load always gets priority over the battery charge current.
The following events cause the state machine to exit this state:
- After being in this state for the top-off time (tTO), the charger enters the next state in the charging cycle, done.
- If VBATT < VBATTREG – VRSTRT, the charger goes back to the fast-charge (CC) state
As shown in Figure 6, the battery charger enters the done state after the charger has been in the top-off state for tTO.
The following event causes the state machine to exit this state:
- If VBATT < VBATTREG – VRSTRT, the charger goes back to the fast-charge (CC) state
In the done state, the charge current into the battery (ICHG) is 0A. In the done state, the charger presents a very low quiescent current to the battery. If the system load presented to the battery is low (<100μA), then a typical system can remain in the done state for many days. If left in the done state long enough, the battery voltage decays below the restart threshold (VRSTRT), and the charger state machine transitions back into the fast-charge CC state. There is no soft-start (di/dt limiting) during the done-to-fast-charge state transition.
The battery charger provides a charge timer to ensure safe charging. As shown in Figure 6, the charge timer prevents the battery from charging indefinitely. The time that the charger is allowed to remain in each of the prequalification states is tPQ. The time that the charger is allowed to remain in the fast-charge CC and CV states is tFC. Finally, the time that the charger is in the top-off state is tTO. Upon entering the timer fault state, STAT becomes Hi-Z.
In the timer fault state, the charger is off. The charger input can be removed and re-inserted to exit the timer fault state (See the “any state” bubble in the lower left of Figure 6).
As shown in Figure 6, the thermal shutdown state occurs when the battery charger is in any state and the junction temperature (TJ) exceeds the device’s thermal-shutdown threshold (TSHDN). When TJ is close to REG, the charger folds back the input current limit to 0A so that the charger and inputs are effectively off.
In the thermal shutdown state, the charger is off.
The DC-DC converter topology of the MAX77757 allows it to operate as a buck converter or as a reverse boost converter. The modes of the DC-DC converter are controlled by ENBST. When ENBST = high and CHGIN voltage is lower than 0.7V, the DC-DC converter operates in reverse boost mode allowing it to source current to BYP and CHGIN. This mode is commonly referred to as OTG mode or a source role.
The current through the BYP to CHGIN switch is limited to a 1.5A minimum. When the reverse boost mode is enabled, the unipolar CHGIN transfer function measures current going out of CHGIN.
The BYP to CHGIN switch automatically retries after 300ms if CHGIN loading exceeds the 1.5A current limit. If the overload at CHGIN persists, then the CHGIN switch toggles ON and OFF with approximately 60ms ON and 300ms OFF.
Under the reverse boost mode, the CC pins enter the low power source mode until the connection is established. Once Rd is detected, the MAX77757 enables the 180μA current source of the active CC pin, whereas the other CC pin stays high impedance.
The main battery overcurrent protection during system power-up feature limits the main battery to system current to ISYSPU if VSYS is less than VSYSPU. This feature limits the surge current that typically flows from the main battery to the device’s low-impedance system bypass capacitors during a system power-up. System power-up occurs anytime that energy from the battery is supplied to SYS when VSYS < VSYSPU. This “system power-up” condition typically occurs when a battery is hot-inserted into an otherwise unpowered device.
When “system power-up” occurs due to hot-insertion into an otherwise unpowered device, a small delay is required for this feature’s control circuits to activate. A current spike over ISYSPU might occur during this time.
The MAX77757 protects itself, the battery, and the system from potential damage due to excessive battery discharge current. Excessive battery discharge current can occur for several reasons such as exposure to moisture, a software problem, an IC failure, a component failure, or a mechanical failure that causes a short circuit.
When the main battery (BATT)-to-system (SYS) discharge current (IBATT) exceeds 6A for at least tBOVRC, then the MAX77757 disables the BATT-to-SYS discharge path (QBAT switch) and turns off the buck. Under OCP fault condition, when SYS is low (VSYS < VSYSUP) for tocp_retry, the MAX77757 restarts on its own and attempts to pull up SYS again. If the fault condition remains, the whole cycle repeats until this fault condition is removed.
The MAX77757 charger uses several thermal management techniques to prevent excessive battery and die temperatures.
Thermal foldback maximizes the battery charge current while regulating the MAX77757 junction temperature. As shown in Figure 9, when the die temperature exceeds the REGTEMP (TREG), a thermal limiting circuit reduces the battery charger’s target current by 5% of the fast-charge current per 1°C (ATJREG), which corresponds to 157.5mA/°C when the fast-charge current is 3.15A. For lower programmed charge currents such as 480mA, this slope is valid for charge current reductions down to 80mA; below 100mA, the slope becomes shallower but the charge current reduces to 0A if the junction temperature is 20°C above the programmed loop set point. The target charge current reduction is achieved with an analog control loop (i.e., not a digital reduction in the input current).
The thermistor input can be utilized to achieve functions that include charge suspension, JEITA-compliant charging, and disabling the charger.
The charger can be disabled by pulling the THM pin to ground. Figure 14 shows a recommended system diagram where the MCU has a GPIO output connected to THM 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.
The MAX77757J version safely charges batteries in accordance with JEITA specifications. The MAX77757J version monitors the battery temperature with an NTC thermistor connected at the THM pin and automatically adjusts the fast-charge current or charge termination voltage as the battery temperature varies.
The JEITA controlled fast-charge current is reduced to 50% of the detected fast charge current for TCOLD < T < TCOOL.
The charge termination voltage for TWARM < T < THOT is reduced to programmed termination voltage -150mV, as shown in Figure 11. Charging is suspended when the battery temperature is too cold or too hot (T < TCOLD or THOT < T).
The MAX77757H version disables the JEITA under warm and cool conditions and stops charging when the temperature is too hot or cold. See the Ordering Information for details.
Temperature thresholds (TCOLD, TCOOL, TWARM, and THOT) depend on the thermistor selection. See the Thermistor Input (THM) section for more details.
Since the thermistor monitoring circuit employs an external bias resistor from THM to PVL, the thermistor is not limited only to 10kΩ (at +25ºC); any resistance thermistor can be used if the value is equivalent to the thermistors +25ºC resistance. The thermistor installed on the evaluation kit is 10kΩ with a beta of 3435.
The general relation of thermistor resistance to temperature is defined by the following equation:
where
RT = The resistance in Ω of the thermistor at temperature T in Celsius
R25 = The resistance in Ω of the thermistor at +25ºC
β = The material constant of the thermistor, which typically ranges from 3000k to 5000k
T = The temperature of the thermistor in Celsius
VDD is the 1.8V power for the MAX77757 charger’s analog circuit. VDD chooses the higher value between the BATT and CHGIN as power input source. VDD has a bypass capacitance of 2.2µF.
ENBST is an input control signal for the reverse boost mode with an external logic signal. If ENBST is driven high, the reverse boost is enabled and the BYP to CHGIN path is closed. It has an internal 235kΩ pulldown resistor. When ENBST sets high, the MAX77757 disconnects Rd from the CC line and provides 180μA current source.
- D+/D- Charging Signature Detector
- USB BC1.2 Compliant
- SDP, DCP, and CDP Detection
- Detect Proprietary Charger Types
- Apple® 500mA, 1A, 2A, 12W
- Samsung® 2A
The USB charger detection is USB BC1.2 compliant with the ability to automatically detect some common proprietary charger types.
The Charger Detection State Machine follows USB BC1.2 requirements and detects SDP, CDP, and DCP types. The Charger Detection State Machine indicates if D+/D- were found as open but ChgTyp indicates SDP as required by BC1.2 specifications.
In addition to the USB BC1.2 State Machine, the IC also detects a limited number of proprietary charger types (Apple, Samsung, and generic 500mA). The UIC automatically sets the CHGIN input current limiting based on the charger type detection results. If the charger type detection results are from an unknown charger type or data contact detection timed out, the input current limits are set to a maximum of 3A.
| USB BC1.2 DETECTED CHARGER TYPE | |
|---|---|
| INPUT CURRENT LIMIT | CHARGER DETECTED |
| 500mA | The default setting before charger detection |
| 500mA | SDP |
| 1.5A | CDP |
| 1.5A | DCP |
| DETECTED PROPRIETARY CHARGER TYPE | |
|---|---|
| INPUT CURRENT LIMIT | CHARGER DETECTED |
| 500mA | Apple |
| 1A | Apple |
| 2A | Apple |
| 2.4A | Apple |
| 2A | Samsung |
| 3A | All others |
- USB Type-C sink support
- CC source detection and automatically set the input current limit according to source capability
- Source role is supported by ENBST pin
The MAX77757 works as a sink compliant to USB Type-C rev1.2. The USB Type-C functions are controlled by a logic state machine that follows the USB Type-C requirements. The MAX77757 sets the CHGIN input current limit based on the current advertised on the CC wires. Source role is enabled by the ENBST pin. When source role is enabled, Rd is removed and a 180μA current source is connected.
When a source is detected, the USB Type-C state machine auto-detects the active CC line. The state machine also auto-detects the source advertised current (500mA, 1.5A, and 3.0A). Upon detection of a change in advertised current, the MAX77757 automatically sets the input current limit.
ENBST = high enables the MAX77757's source role. The MAX77757 disconnects Rd from the CC line and connects a 180μA current source to advertise a 5V/1.5A power source. The MAX77757 enables the reverse boost and supplies 5.1V/1.5A through the CHGIN pin.