Detailed Description

• Complete Li+/LiPoly Battery Charger
  • Prequalification, Constant Current, Constant Voltage
  • 55mA Precharge Current
  • 300mA Trickle Charge Current
  • Adjustable Constant Current Charge
    • 100mA to 5.5A in 50mA steps
  • Adjustable Charge-Termination Threshold
    • 150mA to 850mA in 50mA Steps
  • Adjustable Battery Regulation Voltage
    • 4.15V to 4.46V in 10mV Steps
    • -0.8/+0.2% accuracy from 0°C to +85°C
    • Remote Differential Sensing
• Synchronous Switch-Mode Based Design
• Smart Power Selector
  • Optimally distributes power between the charge adapter, system, and battery.
  • When powered by a charge adapter, the battery can provide supplemental current to the system.
  • The charge adapter can support the system with a dead battery or without a battery.
• No External MOSFETs Required for Switcher
• CHGIN Input
  • Adjustable Input Current Limit
    • 100mA to 3.20A in 50mA steps (CHGIN_ILIM)
    • Default is set to 500mA
  • Supports AC-to-DC Wall Adapters
  • V_{CHGIN_OVLO} = 19V
  • Reverse-Leakage Protection Prevents the Battery Leaking Current to the Inputs
• Charge Safety Watchdog Timer
  • Selectable: 3hr to 10hr, plus a Disable Setting
• Die Temperature Monitor with Thermal Foldback Loop
  • Selectable Die-Temperature Thresholds (°C):  +85°C to +130°C in +5°C steps
• Input Voltage Dropout Control Allows Operation from High-Impedance Sources (AICL)
• BATT to SYS Switch is 7.7mΩ Typical
  • Capable of 10A Steady-State Operation from BATT to SYS
• Short-Circuit Protection
  • DISIBS Bit Allows the Host to Disable the Battery to System Discharge Path to Protect Against a Short-Circuit
  • SYS Short to Ground
    • Buck current is limited by switcher current limit and disabling of the synchronous rectifier.

The MAX77975/MAX77976 includes a full-featured switch-mode charger for a one-cell lithium-ion (Li+) or lithium-polymer (Li-polymer) battery. The current limit for CHGIN input is independently programmable from 100mA to 3.2A in 50mA steps 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/2.6MHz 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 main-battery the converter operates as a buck. The DC-DC buck operates from a 4.3V to 19.5V source. The battery charge current is programmable from 100mA to 5.5A.

As a boost converter, the DC-DC uses energy from the main-battery to boost the voltage at BYP. The BYP supplies the USB OTG voltage (5.1V) and USB Type-C^® PD Source Voltages (5V to 12V). The programmable boost output current limit range is from 0.5A to 3.1A with a 0.1A step.

Maxim’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.)  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.

A multitude of safety features ensures reliable charging. Features include a charge timer, watchdog, junction thermal regulation, over/under voltage protection, and short circuit protection.

The SPS architecture is a network of internal switches and control loops that distribute energy between external power sources CHGIN, BYP, SYS, and BATT.

The Functional Diagram shows a detailed arrangement of the Smart Power Selector switches and gives them the following names: Q_CHGIN, Q_HS, Q_LS, and Q_BATT.

Switch and Control Loop Descriptions

• CHGIN Input Switch: The input switch is either completely on or completely off. As shown in the Functional Diagram, there are SPS control loops that monitor the current through the input switches as well as the input voltage.
• DC-DC Switches:  Q_HS and Q_LS 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:  Q_BATT controls the battery charging and discharging. Additionally, Q_BATT allows the battery to be isolated from the system (SYS). An SPS control loop monitors the Q_BATT current.

Control Bits

• MODE configures the Smart Power Selector
• V_BYPSET sets the BYP regulation voltage target

Energy Distribution Priority

• With a valid external power source:
  • The external power source is the primary source of energy
  • The main-battery is the secondary source of energy
  • Energy delivery to BYP is the highest priority
  • Energy delivery to SYS is the second priority
  • Any energy that is not required by BYP or SYS is available to the main-battery charger
• With no power source available at CHGIN:
  • The main-battery is the primary source of energy
  • Energy delivery to BYP (if boost mode is selected) and SYS share the same priority
    • BYP includes CHGIN if boost OTG mode is selected, itself limited by OTG_ILIM threshold

BYP Regulation Voltage

• When the DC-DC is off or in one of its buck modes and there is a valid power source at CHGIN, V_BYP = V_CHGIN - I_CHGIN x RCHGIN2BYP.
• When the DC-DC is off and there is no valid power source at CHGIN, BYP is connected to LX through the high-side switch’s body diode.

SYS Regulation Voltage

• When the DC-DC is enabled as a buck and the charger is disabled, Q_BATT is off and V_SYS is regulated to V_{SYSREG_TRK_MIN} when the V_BATT < V_SYSMIN or V_{SYSREG_TRK} when the V_BATT ≥ V_SYSMIN.
• When the DC-DC is enabled as a buck and the charger is enabled but in a non-charging state such as done, thermistor suspend, watchdog suspend or timer fault, Q_BATT is off and V_SYS is regulated to V_{SYSREG_TRK_MIN} when the V_BATT < V_SYSMIN or V_{SYSREG_TRK} when the V_BATT ≥ V_SYSMIN.
• When the DC-DC is enabled as a buck and charging in prequalification, fast-charge, or top-off modes, V_SYS is regulated to V_SYSMIN when the V_BATT < V_SYSMIN; in this mode, the Q_BATT switch acts as a linear regulator and dissipates power [P = (V_SYSMIN - V_BATT) x I_BATT]. When V_BATT > V_SYSMIN, then V_SYS = V_BATT + I_BATT x R_BAT2SYS; in this mode, the Q_BATT switch is closed.
• In all of the above modes, if the combined SYS and BYP loading exceeds the input current limit, then V_SYS drops to V_BATT - V_BSREG and the battery provides supplemental current.
• When the DC-DC is enabled as a boost, then the Q_BATT switch is closed, and V_SYS = V_BATT - I_BATT x R_BAT2SYS.

The charger input is compared with several voltage thresholds to determine if it is valid. A charger input must meet the following four characteristics to be valid:

• CHGIN must be above V_{CHGIN_UVLO} to be valid. Once CHGIN is above the UVLO threshold, the information (together with IN2SYS, described below) is latched and can only be reset when the charger is in adaptive input current loop (AICL) and input current is lower than the IULO_DET threshold.
• CHGIN must be below its overvoltage-lockout threshold (V_{CHGIN_OVLO}).
• CHGIN must be above the system voltage by IN2SYS drop out.
• CHGIN input generates a CHGIN_I interrupt when its status changes. The input status can be read with CHGIN_OK and CHGIN_DTLS. Interrupts can be masked with CHGIN_M.

The default settings of the CHGIN_ILIM and MODE control bits are such that when a charge source is applied to CHGIN, the IC turns its DC-DC converter on in BUCK mode, limits V_SYS to V_{SYSREG_TRK}, and limits the charge source current to I_INLIMIT. All control bits are reset on global shutdown.

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 non-compliant USB hub configurations. Additionally, this input voltage regulation loop improves performance with current limited adapters. If the ICs input current limit is programmed above the current-limit threshold of a given adapter, the input voltage loop allows the IC to regulate at the current limit of the adapter. Finally, the input-voltage regulation loop allows the IC to perform well with adapters that have poor transient load response times.

The input voltage regulation loop automatically reduces the inductor average current to keep the input voltage at V_{CHGIN_REG}. If the input current is reduced to I_{ULO_DET} and the input voltage is below V_{CHGIN_REG}, then the charger input is turned off. V_{CHGIN_REG} is programmable with V_{CHGIN_REG}[1:0].

After operating with the input voltage regulation loop active, a AICL_I interrupt is generated, AICL_OK sets to 0. To optimize input power when working with a current limited charge source, monitor the AICL_OK status while decreasing the input current limit. When the input current limit is set below the limit of the adapter, the input voltage rises. Although the input current limit is lowered, more power can be extracted from the input source when the input voltage is allowed to rise.

Example 1. Optimum use of the Input Voltage Regulation Loop along with a current limited adapter.

Sequence of Events:

1. V_BATT = 3.2V, the system is operating normally.
2. MODE = 0x04, CHGIN_ILIM = 100mA, CHG_CV_PRM = 4.2V, V_{CHGIN_REG} = 4.5V, CHG_CC_TOT = 2.0A.
3. A 5.0V 1.2A current limited dedicated USB charger is applied to CHGIN.
4. The DC-DC buck regulator turns on, V_SYS is regulated to V_BATTREG (4.2V) and the input is allowed to provide 100mA to the system.
5. The system detects that the charge source is a dedicated USB charger and enables the battery charger (MODE = 0x05) and programs an input current limit to 1.8A (CHGIN_ILIM = 1.8A).
6. The input current limit starts to ramp up from 100mA to 1.8A, but at the input current limit of the adapter (1.2A), the adapter voltage collapses. The ICs input voltage regulation loop prevents the adapter voltage from falling below 4.5V (V_{CHGIN_REG} = 4.5V). A AICL_I interrupt is generated and AICL_OK sets to 0.
7. With the input-voltage regulation loop active, the adapter provides 1.2A at 4.5V which is a total of 5.4W being delivered to the system.
8. The system software detects that the input voltage regulation loop is active and it begins to ramp down the programmed input current limit. When the current limit ramps down to 1.175A, the adapter is no longer in current limit, and the adapter voltage increases from 4.5V to 5.0V.
9. With the adapter operating just below its current limit, it provides 1.175A at 5.0V which is a total of 5.88W to the system. This is 440mW more than when the adapter was in current limit.

To ensure a timely, complete, repeatable, and reliable reset behavior when the system has no power, the ICs actively discharge the SYS nodes when Q_BATT and switcher are disabled and V_SYS is less than V_SYSUVLO. As shown in Figure 1, the SYS discharge resistor is 600Ω.

Example 1. Basic System Self-Discharge

Initial Conditions: No charger adapter is present at CHGIN, the BAT-to-SYS switch is closed, C_BAT = 100µF, C_SYS = 200µF, V_BATT = 3.6V, and V_SYSUVLO falling is SYS_UVLOB_F.

Sequence of Events:

1. With the system in its normal operating mode it is drawing 1A.
2. The main battery is removed.
3. The system continues to draw 1A until V_SYS falls below V_SYSUVLO. This takes 480µs ((3.6V-2.0V)/1A x 300µF).
4. When the system voltage falls below V_SYSUVLO, the system turns off leakage current. To facilitate discharging C_BAT and C_SYS the IC engages its 600Ω discharge resistors.

The MAX77975/MAX77976 transitions between power states as input/battery and load conditions dictate; see Figure 2.

The IC provides five (5) power modes and one (1) no power mode (MODE detailed description is at register CHG_CNFG_00 [3:0]). Under power limited conditions, the PowerPath^TM feature maintains SYS load at the expense of battery charge current. Also, the battery supplements the input power when required. As shown, transitions between power states are initiated by detection/removal of valid power sources, OTG events, and undervoltage conditions. Details of the SYS voltage and BATT current are provided for each state. There are six main usage modes:

1. NO INPUT POWER, MODE = undefined: No input adapter or battery is detected. The charger and system are off. The battery is disconnected and the charger is off.

2. BATTERY-ONLY, MODE = any modes: Adapter is invalid and outside the input voltage operating range (Q_CHGIN = OFF). The battery is connected to power the SYS load (Q_BATT = ON).

3. NO CHARGE-BUCK, MODE = 0x04: Adapter is valid, buck supplies power to SYS. The battery is disconnected (Q_BATT = OFF) when SYS load is less than the power that buck can supply.

When SYS load is larger than the power that buck can supply, the battery is reconnected (Q_BATT = ON) and supplements extra SYS load.

4. CHARGE-BUCK, MODE = 0x05: Adapter is valid, buck supplies power to SYS, and charges battery with I_BATT.

5. BATTERY-BOOST (FLASH), MODE = 0x09: OTG is inactive (Q_CHGIN = OFF). Battery is connected to support SYS and BYP loads (Q_BATT = ON), and charger is operating in boost mode (Boost = ON).

6. BATTERY-BOOST (OTG), MODE = 0x0A: OTG is active (Q_CHGIN = ON). Battery is connected to support SYS and OTG loads (Q_BATT = ON), and charger is operating in boost mode (Boost = ON).

The ICs utilize several charging states to safely and quickly charge batteries as shown in Figure 3. The figure 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. It shows a complete charging state transition process with four states: prequalification, fast-charge, top-off, and done.

While in the “no input power or charger idle” state, the charge current is 0mA, the watchdog and charge timers are forced to 0, and the power to the system is provided by either the battery or the adapter. When both battery and adapter power is available, the adapter provides primary power to the system and the battery contributes supplemental energy to the system if necessary.

To exit the “no input power or charger idle” state, the charger input must be valid and the charger has to be enabled.

As shown in Figure 3, the precharge state occurs when the main-battery voltage is less than V_PRECHG. After being in this state for t_SCIDG, a CHG_I interrupt is generated only if CHG_OK was 0 previously, CHG_OK is set to 1, and CHG_DTLS is set to 0x00. In the precharge state, charge current into the battery is I_PRECHG.

The following events cause the state machine to exit this state:

• Main battery voltage rises above V_PRECHG and the charger enters the next state in the charging cycle: “Trickle Charge”.
• If the battery charger remains in this state for longer than t_PQ, the charger state machine transitions to the “Timer Fault” state.
• If the watchdog timer is not serviced (see the Watchdog Timer section), the charger state machine transitions to the “Watchdog Suspend” 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 pointer 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 for longer than t_PQ may be experiencing a problem.

As shown in Figure 3, the trickle charge state occurs when V_BATT > V_PRECHG and V_BATT < V_TRICKLE. After being in this state for t_SCIDG, a CHG_I interrupt is generated only if CHG_OK was 0 previously, CHG_OK is set to 1, and CHG_DTLS = 0x00.

With TKEN = 1 and the IC is in its trickle charge state, the current in the battery is less than or equal to ITRICKLE. When TKEN = 0, the battery current is less than or equal to I_FC.

Charge current may be less than ITRICKLE/I_FC for any of the following reasons:

• The charger input is in 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.

Typical systems operate with TKEN = 1. When operating with TKEN = 0, the system’s software usually sets I_FC to a low value such as 450mA and then monitors the battery voltage. When the battery exceeds a relatively low voltage such as 3.1V, then the system’s software usually increases I_FC.

The following events cause the state machine to exit this state:

• When the main battery voltage rises above VTRICKLE or the PQEN bit is cleared, the charger enters the next state in the charging cycle: “Fast Charge (CC)”.
• If the battery charger remains in this state for longer than t_PQ, the charger state machine transitions to the “Timer Fault” state.
• If the watchdog timer is not serviced, the charger state machine transitions to the “Watchdog Suspend” 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 t_PQ may be experiencing a problem.

As shown in Figure 3, the fast-charge CC state occurs when the main-battery voltage is greater than the low-battery prequalification threshold and less than the battery regulation threshold (V_TRICKLE < V_BATT < V_BATTREG). After being in the fast-charge CC state for t_SCIDG, a CHG_I interrupt is generated only if CHG_OK was 0 previously, CHG_OK is set to 1, and CHG_DTLS = 0x01.

In the fast-charge CC state, the current into the battery is less than or equal to I_FC. Charge current may be less than I_FC for any of the following reasons:

• The charger input is in 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 causes the state machine to exit this state:

• When the main battery voltage rises above V_BATTREG, the charger enters the next state in the charging cycle: “Fast Charge (CV)”.
• If the battery charger remains in this state for longer than t_FC, the charger state machine transitions to the “Timer Fault” state.
• If the watchdog timer is not serviced, the charger state machine transitions to the “Watchdog Suspend” 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 T_REG, I_FC is reduced. See the Thermal Foldback section for more information.

As shown in Figure 3, the fast-charge CV state occurs when the battery voltage rises to V_BATTREG from the fast-charge CC state. After being in the fast-charge CV state for t_SCIDG, a CHG_I interrupt is generated only if CHG_OK was 0 previously, CHG_OK is set to 1, and CHG_DTLS = 0x02.

In the fast-charge CV state, the battery charger maintains V_BATTREG across the battery and the charge current is less than or equal to I_FC. As shown in Figure 3, charger current decreases exponentially in this state as the battery becomes fully charged.

The smart power selector control circuitry may reduce the charge current lower than the battery may otherwise consume for any of the following reasons:

• The charger input is in 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 causes the state machine to exit this state:

• When the charger current is below I_TO for t_TERM, the charger enters the next state in the charging cycle: “TOP OFF” state.
• If the battery charger remains in this state for longer than t_FC, the charger state machine transitions to the “Timer Fault” state.
• If the watchdog timer is not serviced, the charger state machine transitions to the “Watchdog Suspend” state.