Detailed Description
The MAX77751 is a highly integrated USB Type-C Charger with autonomous configuration. The MAX77751 can operate at an input range from 4.5V to 13.7V to support a 5V, 9V, and 12V AC adapter and USB input. It is capable of supplying a fast-charge current up to 3.15A and the maximum input current limit is 3.0A.
The MAX77751 can run BC1.2 and USB Type-C CC detection when USB input is plugged in and can configure the input source to the maximum power option and the charger input current limit to maximum power.
The fast-charge current and top-off current threshold can be programmed with an external resistor. The input voltage regulation feature with adaptive input current limit (AICL) allows charging to continue even with a weak adapter by preventing it from collapsing or folding back.
The power path design provides system power even when the battery is fully discharged, and it supplements current from the battery and CHGIN input automatically when the system demands higher current.
Reverse boost from the battery can be enabled by the ENBST pin to allow 5.1V/1.5A OTG to VBUS.
- Complete Li-ion/Li-Polymer Switching Charger
- Prequalification, Constant Current, Constant Voltage Charging
- 55mA Precharge, 300mA Trickle Charge Current
- 500mA to 3.15A Resistor-Adjustable Fast-Charge Current
- 100mA to 350mA Resistor-Adjustable Charge Termination Threshold
- 4.1V to 4.5V Battery Regulation Voltage (see the Ordering Information section)
- Smart Power Selector
- Optimally Distributes Power Between the Charge Adapter, System, and 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 a Battery
- 20mΩ BATT to SYS Switch with up to 4.5A Continuous Discharge Capability
- No External MOSFETs Required
- 4.5V to 13.7V Input Operating Voltage
- Reverse Leakage Protection Prevents the Battery Current Leaking to the Input
- Automatic Detection of USB Type-C and BC1.2 Adapters
- 500mA to 3A Automatic Input Current Limit Selection After USB Charger Type Detection
- Supports Non-USB Sources
- 6-Hour Charge Safety Timer
- Die Temperature Monitor with Thermal Foldback Loop (130°C Threshold)
- Input Voltage Regulation Allows Operation from High-Impedance Sources (AICL)
- Short Circuit Protection
- BATT to SYS Overcurrent Threshold: 6A
- SYS Short-to-Ground
The MAX77751 includes a full-featured switch-mode charger for a one-cell lithium ion (Li+) or lithium polymer (Li-polymer) battery. As shown in Figure 2, the current limit for the CHGIN input is automatically configured allowing the flexibility for connection to either an AC-to-DC wall charger or a USB port.
The synchronous switch-mode DC-DC converter utilizes a high 1.3MHz switching frequency, which is ideal for portable devices because it allows the use of small components while eliminating excessive heat generation. The DC-DC has both a buck and a boost mode of operation. When charging the battery, the converter operates as a buck. 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 battery to boost the voltage at BYP. The boosted BYP voltage supplies the power to CHGIN as USB OTG voltage, which is fixed to 5.1V.
Maxim’s Smart Power Selector architecture makes the best use of the limited adapter power and the battery power at all times to supply up to the Buck Current Limit from the buck to the system (supplement mode also provides additional current from the battery to the system up to B2SOVRC). Adapter power that is not used for the system goes to charging 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’s proprietary process technology allows for low-RDSON devices in a small solution size. The total dropout resistance from 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 Battery Overcurrent Protection section for more information).
The Smart Power Selector (SPS) architecture is a network of internal switches and control loops that distributes energy between 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 and gives them the following names: QCHGIN, QHS, QLS, and QBAT.
Switch and Control Loop Descriptions
- CHGIN Input Switch: QCHGIN provides 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 which 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 the DC-DC is enabled as a buck and the charger is enabled but in a non-charging state (i.e., 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 is charging in trickle-charge, fast-charge, or top-off modes, VSYS is regulated to VSYSMIN when the VPRECHG < VBATT < VSYSMIN. Additionally, when the DC-DC is enabled as a buck and is charging in precharge mode (VBATT < VPRECHG), VSYS is regulated to VBATTREG. In these modes, the QBAT switch acts like 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.
For the above modes, if the combined SYS loading exceeds the input current limit, then VSYS drops to VBATT – VBSREG, and the battery provides supplemental current.
Input Validation
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, as shown in Figure 3) is latched and can only be reset when the charger is in the adaptive input current loop (AICL) and the input current is lower than the IULO threshold of 60mA. Note that VCHGIN_REG is lower than the UVLO falling threshold.
- CHGIN must be below its overvoltage lockout threshold (VCHGIN_OVLO)
- CHGIN must be above the system voltage (VCHGIN2SYS)
If VCHGIN is greater than VCHGIN_UVLO, the USB Type-C CC and BC1.2 detection process starts. After the MAX77751 finishes USB detection, the switcher in the chip starts. The system can detect that a valid charger is present by the INOKB output signal, which is issued when the switcher starts and the VCHGIN_VLD signal is valid, as shown in Figure 4.
After the charger-type detection is done, the MAX77751 automatically configures the input current limit to the highest settings that the source can provide. If the input source is not a standard power source described by BC1.2 or USB Type-C or is not a proprietary charger type that the MAX77751 can detect, the MAX77751 sets the input current limit to 3A.
An input voltage regulation loop allows the charger to be well behaved 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 noncompliant 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.
When the charge source is removed, the input voltage decays below the UVLO threshold in time (tINSD). The input self-discharge is implemented with a 44kΩ resistor (RINSD) from the CHGIN input to ground.
The MAX77751 utilizes several charging states to safely and quickly charge batteries, as shown in Figure 5 and Figure 6. Figure 5 shows an exaggerated view of a 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.
From any state shown in Figure 6 except thermal shutdown, the INIT state is entered whenever the charger inputs that 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.
The chip has a state where battery charging is disabled while the charger input CHGIN is valid. The state is called buck state. Entering or leaving buck state is controlled by the voltage of the ITOPOFF pin. If the voltage of this pin is pulled down by an external device (i.e., MCU) under VCHGR_EN, the chip goes to the buck state from any state if CHGIN is valid, as shown in Figure 6. In the buck state, charging is disabled, which means QBAT is off, unless it is in supplement mode. If the voltage of this pin is over VCHGR_EN, the chip gets out of the buck state and resumes charging. Note that only when CHGIN is valid, charging can be enabled or disabled. Therefore, the external device (i.e., MCU) should check using 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 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:
- Battery voltage rises above VPRECHG, and the charger enters the next state in the charging cycle, trickle charge state.
- If the battery charger remains in this state for longer than tPQ, the charger state machine transitions to the timer fault state.
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, an internal pack 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 state current flows into the 0V battery—this current raises the pack’s terminal voltage to the voltage level 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.
As shown in Figure 6, the trickle charge state occurs when VBATT > VPRECHG and VBATT < VTRICKLE.
When the MAX77751 is in the trickle charge state, the charge current in the battery is less than or equal to ITRICKLE.
The charge current might be less than ITRICKLE for any of the following reasons:
- The charger input current is lower than the 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 state.
- 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 state for longer than tPQ might be experiencing a problem.
As shown in Figure 6, the fast-charge constant current (CC) state occurs when the 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. The charge current can be less than IFC for any of the following reasons:
- The charger input current is lower than the 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 battery voltage rises above VBATTREG, the charger enters the next state in the charging cycle, fast-charge constant voltage state.
- 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 CC 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, the 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 current is lower than the 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 state.
- 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 current is lower than the 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 state.
- If VBATT < VBATTREG – VRSTRT, the charger goes back to the fast-charge CC state
As shown in Figure 6, the battery charger enters its done state after the charger has been in the top-off state for tTO.
The state machine exits this state if VBATT < VBATTREG – VRSTRT and the charger goes back to the fast-charge CC state
In the done state, the charge current into the battery (ICHG) is 0A, and 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 state 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 its prequalification states is tPQ. The time that the charger is allowed to remain in the fast-charge CC and fast-charge 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 TREG, the charger folds back the charge current to 0A so that the charger is effectively off (see the Thermal Foldback section).
In the thermal shutdown state, the charger is off.
The DC-DC converter topology of the MAX77751 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, the DC-DC converter operates in reverse boost mode allowing it to source current to BYP and CHGIN. It is commonly referred to as OTG mode or a source role.
The current through the BYP to CHGIN switch is limited to 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 tries to retry 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 approximately 300ms OFF.
The battery overcurrent protection during system power-up feature limits the battery to system current to ISYSPU if VSYS is less than VSYSPU. This feature limits the surge current that typically flows from the battery to the device’s low-impedance system to bypass capacitors during a system power-up. System power-up is 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 MAX77751 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 battery (BATT)-to-system (SYS) discharge current (IBATT) exceeds 6A for at least tBOVRC, then the MAX77751 disables the BATT-to-SYS discharge path (QBAT switch) and turns off buck.
Under the OCP fault condition, when SYS is low (VSYS < VSYSUP) for tOCP_RETRY, the MAX77751 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.
Thermal foldback maximizes the battery charge current while regulating the MAX77751 junction temperature. As shown in Figure 7, 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. The target charge current reduction is achieved with an analog control loop (i.e., not a digital reduction in the input current).
VDD is the 1.8V power for the MAX77751 charger’s analog circuit. VDD is generated from the higher value between BATT and CHGIN as the power input source and generates the internal power supply. 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. ENBST has an internal 235kΩ pulldown resistor. When ENBST sets to high, the MAX77751 disconnects Rd from the CC line and provides a 180μA current source.
- D+/D- Charging Signature Detector
- USB BC1.2 Compliant SDP, DCP, and CDP Detection
- Proprietary Charger Types Detection
- Apple 500mA, 1A, 2A, 2.4A
- 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. In addition to the USB BC1.2 State Machine, the IC also detects a limited number of proprietary charger types (Apple and Samsung). The IC automatically sets the CHGIN input current limit based on the charger type detection results. If charger type detection results are an unknown charger type or D+/D- are found as open, the input current limits are set to 3A max.
| INPUT CURRENT LIMIT | CHARGER DETECTED |
|---|---|
| 500mA | SDP |
| 1.5A | CDP |
| 1.5A | DCP |
| INPUT CURRENT LIMIT | CHARGER DETECTED |
|---|---|
| 500mA | Apple |
| 1A | Apple |
| 2A | Apple |
| 2.4A | Apple |
| 2A | Samsung |
| 3A | All others |
USB Type-C CC Detection
- USB Type-C Sink Support
- CC Source Detection
- Automatic Set of the Input Current Limit According to Source Capability
- Source Role Support by ENBST Pin
The MAX77751 is sink compliant to the USB Type-C Rev 1.2 specifications. The USB Type-C functions are controlled by a logic state machine which follows the USB Type-C requirements. The MAX77751 sets the CHGIN input current limit based on the current advertised on the CC wires. The source role is enabled by the ENBST pin. When the 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 automatically detects the source advertised current (500mA, 1.5A, and 3.0A). Upon detection of a change in the advertised current, the MAX77751 automatically sets the input current limit.
The MAX77751 executes D+/D- detection and CC detection in parallel and takes the higher of the two input current limit values determined by D+/D- detection and by CC detection. No connection in the D+ and D- lines is considered as an unknown adapter, and therefore the final input current limit is always 3A, regardless of the CC detection result.
Some target systems could have other types of connectors such as Micro-B and barrel connectors other than USB Type-C connectors. Since the Micro-B connector does not have CC1 and CC2 pins and the barrel connector does not have CC1, CC2, D+ and D- pins, the recommended connections described in the following sections can be used in the target system.
In the case of a Micro-B connector, CC1 and CC2 can be floated as shown in Figure 8. When CC1 and CC2 are floating, the MAX77751 determines the input current limit solely based on D+/D- detection such as BC1.2 and some proprietary TA detection.
When a barrel connector is used in the application, there is no connection to determine the input current limit. Resistors can be connected to mimic BC1.2 detection and/or CC detection to configure the desired input current limit. If the capacity of the power source is 500mA, the MAX77751 can set the input current limit to 500mA by configuring resistors on DP/DN for BC1.2 SDP (Figure 9). R1 and R2 should have the same resistance with the recommended value of 20kΩ. Meanwhile, if the capacity of the power source is 1.5A, the MAX77751 can set the input current limit to 1.5A by connecting a resistor for BC1.2 DCP as shown in Figure 10. The recommended value of resistor R is 200Ω.
ENBST = high enables the MAX77751's source role. The MAX77751 disconnects Rd from the CC line and connects an 180μA current source to advertise the 5V/1.5A power source. The MAX77751 enables the reverse boost and supply 5.1V/1.5A through the CHGIN pin.