Lithium-Ion Battery Protection: A Fully Integrated Solution

Lithium-ion (Li-Ion) rechargeable batteries are quickly gaining popularity over nickel based rechargeables due to their superior energy density, higher cell voltage and low self-discharge rate. These characteristics make them perfectly suited to today’s high performance portable products. However, safety concerns related to overcharging and short circuit protection have driven the industry to include battery protection circuits within the Li-Ion battery pack. Their purpose is to monitor the cell voltage(s) and prevent over-charge or over-discharge by opening the current path if a cell is out of the normal operating voltage range. By monitoring the discharge current, overload and short circuit protection is also provided. The protector control circuitry is usually implemented using a dedicated protector IC and two external back-to-back power MOSFETs to open the circuit, preventing charge and/or discharge current. Two MOSFETs are required to control current in both directions due to the MOSFET’s internal body diode. 

Systems Issues
The entire system, including parasitic elements, must be considered in designing the battery protection circuit. This includes the battery cell(s), the protection circuitry and the load (or charger) that the user applies to the battery pack terminals. Parasitics include the inductance and capacitance associated with the load, the switch on-resistance and the battery’s internal ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance). At normal steady-state currents of a few amperes or less, the parasitics do not play a major role in the operation of the system. However, during high transient currents of I OA or more, such as those encountered when a large capacitor or a short is applied to the battery pack, the parasitics have a major effect on the operation and robustness of the system.

Current Sensing
In some cases, the series MOSFETs are used as the current sensing elements. One of the drawbacks to this approach is the inaccuracy of the over-current sense due to the tolerance and temperature coefficient of the MOSFET on-resistance. If both MOSFET’s are used as the sensing element, a problem arises. During an over- charge condition, when one MOSFET is turned off to prohibit charge current, the discharge current must go through its body diode. The voltage drop across the body then causes an over-current trip, turning off the other MOSFET as well. To solve this problem, some manufacturer’s disable the overcurrent protection while in over-charge; precisely the wrong time. In any case, the current must be monitored continu- ously, or at a sufficient rate such that an overload doesn’t go undetected for too long. A delay of 1 Omsec may be long enough for the MOSFET’s to be destroyed during a short circuit. 

MOSFET Turn-off and Inductive Effects
When the protection circuit interrupts the current in response to a short or overload, inductance associated with the load (parasitic or otherwise) causes voltage overshoot in response to the rapid di/dt. For example, a 20A short circuit current interrupted in 1 (sec would result in a 20V overshoot if there was 1 (H of total circuit inductance. This overshoot can exceed the breakdown voltage of the MOSFET used to interrupt the current. Guarding against inductive kick is critical to surviving a short circuit condition, when the current is at its highest. An effective way to reduce overshoot when inter- rnpting the current is to increase the MOSFET turn-off time to tens of microseconds by raising the gate drive impedance, limiting the di/dt. The turn-off time cannot be made too long, however, or the MOSFET junction temperature rise may be excessive. 

An Integrated Solution
To deal with these real world applications issues, most Li-Ion protector IC’s require a significant number of external components, in addition to the two MOSFETs. These components raise the cost of the battery pack and pose packaging problems, especially in the tight confines of a single cell pack. To minimize cost and board area, a fully integrated protection circuit was designed for single cell Li-Ion applications. The UCC3958 includes a single integrated MOSFET switch, eliminating the two external MOSFETs normally used in protector circuits. A block diagram of the IC is shown in Fig. 1. The following sections describe the operation of each major circuit block within the IC, and how a single MOSFET does the job of two. 


Figure 1. Simplified Block Diagram of Integrated Single Cell Battery Protector IC


Cell Voltage Monitoring
This block consists of a precision temperature compensated, trimmed voltage reference and a multi- tap resistor divider. The divider is made to accommodate a number of different over-voltage and under-voltage thresholds as mask options. Each time the battery is sampled (at a rate of approximately 200Hz), the resistor is connected across the battery and a specific tap is compared to the reference. The sampling cycle only lasts 400(sec out of every 5msec. During this time, the reference, sampling resistor and a voltage comparator are turned on. At all other times, these blocks are shut off to conserve power. 

System Clock Generator
The system clock generator is used to sequence theturn-on and turn-off of the battery sampling circuits, as well as provide the clock for the MOSFET charge pump. This block is designed to run at a nominal frequency of 20kHz, with all the other frequencies and control signals derived from it. 

State Machine
The state machine is used to determine the condition of the battery and take appropriate action when an abnormal state has been detected (over-voltage or under-voltage). The output of the state machine is digitally filtered to ensure that a momentary noise spike or dip in battery voltage does not cause an erroneous fault. A voltage fault condition (OV or UV) must persist for at least two consecutive sampling cycles before the appropriate action is taken. The over and under voltage thresholds have approximately 300mV of hysteresis to prevent oscillation due to battery ESR. 

On-chip MOSFET
A single internal MOSFET acts as a bi-directional switch for charging and discharging the battery. Because the voltage across the MOSFET can be of either polarity, the substrate of the device is always switched to its most negative terminal. The substrate switch guarantees that the MOSFET will never conduct current through its substrate diode. By eliminating the need for two back-to-back MOSFETs, a lower on- resistance is achieved. There is also no time at which current is conducted through a MOSFET body diode. This greatly reduces losses and power dissipation during the over-charge and under-charge modes, when one of the external MOSFETs would be turned off. The gate of the MOSFET is driven by an internal charge-pump to a voltage approximately four times the battery voltage. This assures the lowest possible on- resistance, even at low cell voltages. The gate drive impedance was chosen to provide a MOSFET turn-off time of about 20(sec, minimizing di/dt. Another advantage to having the MOSFET switch integrated onto the die is the ability to sense die temperature and turn off the MOSFET in the event that the steady state current has caused excessive temperature rise.

Charge Pump
This block provides the gate drive voltage for the MOSFET. The charge pump circuitry is the largest noise generator in the circuit. To improve measurement accuracy, the charge pump is turned off when the battery is being sampled. During this short interval, the MOSFET drive voltage remains virtually unchanged. 

Over-Current Protection
The over-current protection block guarantees that the steady-state discharge current cannot exceed 5A nominal. This threshold was chosen to allow adequate design margin over the maximum required current draw of 2A in most single cell applications. The current sensing scheme utilizes metallization resistance around the MOSFET and is temperature compensated for the shift in metal resistance. There is a fixed internal delay of 360(sec from the time the overcurrent is detected to the time the MOSFET is shut off. The user can extend this time by adding a small external capacitor. This feature allows charging of large bypass capacitors without tripping the overcurrent protection. If the overcurrent condition persists, the MOSFET is turned off and remains off until the load is removed. A bypass capacitor, connected to the cell by an internal resistor, keeps the IC powered during a hard short, when the cell voltage may fall very low. This feature assures proper operation of the IC and its overcurrent protection, even under the worst short circuit conditions. 

Over and Under-Voltage Operation
When the cell voltage is out of its normal operating voltage range (typically 2.3V to 4.2V), the direction of battery current must be controlled to prohibit charge current if the voltage is too high, or discharge current if the voltage is too low. This is done using a linear control loop, which regulates the polarity of the differential voltage across the internal MOSFET.

When an over-voltage condition is sensed, the MOSFET gate voltage is controlled to only allow discharge current by regulating the differential voltage across the MOSFET’s drain to source terminals to lOOmV. The polarity of the l00mV offset is such that the direction of current must be out of the battery pack (discharge only). As the load varies, the impedance of the MOSFET is adjusted to maintain the l00mV drop. In this manner, the MOSFET can be mlide to function as a diode with a forward voltage drop of 100mV, guaranteeing that battery current is in only one direction. The offset of l00mV was chosen to be much larger than all other internal offsets. The MOSFET voltage drop in this mode is much lower than that of the conventional solution of two back-to-back MOSFETs, where the body diode of one MOSFET is conducting. Once the battery voltage has dropped 300mV below the over-charge threshold, normal operation is resumed and the MOSFET is fully enhanced to minimize voltage drop.

If an under-voltage condition is sensed, the load is disconnected from the battery pack (by turning off the MOSFET) to prevent further discharge. Most circuit functions are put into a sleep mode to minimize current drain form the battery. Several key circuits remain powered to detect the application of a charger. Once the charging voltage is detected, the linear control loop is again activated, and only charge current is allowed. The polarity of the 100mV control circuit is reversed, since the desired direction of battery current is opposite that of the discharge condition. Afain the MOSFET acts like a diode with a low forward drop, but the polarity has been reversed. This linear control mode of operation remains in effect util the battery voltage is 300 mV above the under voltage threshold, at which time normal operation resumes.


Fig. 2 Single Cell Protector Application Circuit


A complete application schematic of a single cell Lithium-Ion protector is shown in figure 2. by incorporation the MOSFET into the low current BiCMOS IC, only one small external capacitor is required. This capacitor (Cl) provides ample energy storage to power the IC during a hard short. Capacitor C2 is optional, and extends the over-current delay time for charging large capacitive loads. Without an external cap, the built-in delay of 360(sec allows charging up to 1 ,000(F of load capacitance.

The controlled MOSFET turn-off time of 20(sec results in little or no voltage overshoot. No snubber capacitors or clamp diodes are necessary on the pack output or across the Li-Ion cell.
The typical on-resistance is 45m( (from pin-to-pin of the IC) at room temperature. A plot of the on-resistance as a function of cell voltage is shown in Fig. 3. Because of the internal charge pump, the on-resistance remains very low, even at cell voltages below 2.6V.


Fig 3. Total on-resistance vs. Cell voltage


Other features incorporated in the UCC3958 are: a Chip Enable pin to turn the pack output off, a Low Power Warn output to warn of a low battery condition, and an output which indicates if the protector is in an over or undervoltage condition.
The total current draw is just 5(A in normal mode, and only 1 (A during under-voltage (sleep) mode. Note that the normal self-discharge rate of 6%/month for a typical 1.2 Amp-Hour Li-Ion cell corresponds to a current of 103(A. Therefore, the added current draw of the protection circuit in normal operation represents an increase of only 5% in the self-discharge rate of the battery. In sleep mode, when the cell is in under-voltage, the self-discharge rate is increased by less than 1%. 

David Salerno (salerno@unitrode.com) is a Principal Applications Engineer for portable products at Unitrode Corporation, a manufacturer of linear and mixed signal integrated circuits, located in Merrimack, NH.

Source: Battery Power, March 1st, 1998