Polymer Shunt Battery Charger System

Published Date: 02 Nov 2017

The LTC4071 allows simple charging of Li-Ion/Polymer batteries from very low current, intermittent or continuous charging sources. A near-zero current low battery latching disconnect function protects even the lowest capacity batteries from deep discharge and potentially irreparable damage. The 550nA to 50mA operating current makes charging possible from previously unusable sources. With its low operating current the LTC4071 is well suited to charge low capacity Li-Ion or thin film batteries in energy harvesting applications. The unique architecture of the LTC4071 allows for an extremely simple battery charger solution, requiring just one external resistor.

The LTC4071 offers a pin selectable float voltage with + or - 1% accuracy. The integrated thermal battery qualifier extends battery lifetime and improves reliability by automatically reducing the battery float voltage at NTC thermistor temperatures above 40°C. The LTC4071 also provides two pin selectable low battery disconnect levels and a high battery status output.

The device is offered in two thermally enhanced packages, a compact low profile (0.75mm) 8-lead (2mm × 3mm) DFN and an 8-lead MSOP package.

Features of LTC4071:

Charger Plus Pack Protection in One IC

Low Operating Current (550nA)

Near Zero Current (<0.1nA) Low Battery Disconnect Function to Protect Batteries from Over-Discharge

Pin Selectable Low Battery Disconnect Level: 2.7V or 3.2V

1% Float Voltage Accuracy Over Temperature

50mA Maximum Internal Shunt Current

Pin Selectable Float Voltage Options: 4.0V, 4.1V, 4.2V

Ultralow Power Pulsed NTC Float Conditioning for Li-Ion/Polymer Protection

Suitable for Intermittent, Continuous and Very Low Power Charging Sources

High Battery Status Output

Thermally Enhanced, Low Profile (0.75mm) 8-Lead (2mm × 3mm) DFN and MSOP Packages.

Application of LTC4071:

Low Capacity, Li-Ion/Polymer Battery Back-Up

Thin Film Batteries

Energy Scavenging/Harvesting

Solar Power Systems with Back-Up

Memory Backup

Embedded Automotive

LTC4071 Typical Application:

C:\Users\PRAVEEN\Desktop\Project\LTC4071.jpg

PIN Configuration LTC4071:

C:\Users\PRAVEEN\Desktop\Project\Pincon 4071.jpg

Pin Functions:

NTCBIAS (Pin 1): NTC Bias Pin. Connect a resistor from NTCBIAS to NTC, and a thermistor from NTC to GND. Float NTCBIAS when not in use. Minimize parasitic capacitance on this pin.

NTC (Pin 2): Input to the Negative Temperature Coefficient Thermistor Monitoring Circuit. The NTC pin connects to a negative temperature coefficient thermistor which is typically co-packaged with the battery to determine the temperature of the battery. If the battery temperature is too high, the float voltage is reduced. Connect a low drift bias resistor from NTCBIAS to NTC and a thermistor from NTC to GND. When not in use, connect NTC to VCC. Minimize parasitic capacitance on this pin.

ADJ (Pin 3): Float Voltage Adjust Pin. Connect ADJ to GND to program 4.0V float voltage. Disconnect ADJ to program 4.1V float voltage. Connect ADJ to VCC to program 4.2V float voltage. The float voltage is also adjusted by the NTC thermistor

HBO (Pin 4): High Battery Monitor Output (Active High). HBO is a CMOS output that indicates that the battery is almost fully charged and current is being shunted away from VCC. This pin is driven high when VCC rises to within VHBTH of the effective float voltage, VFLOAT_EFF. The absolute value of this threshold depends on ADJ and NTC both of which affect the float voltage. HBO is driven low when VCC falls by more than (VHBTH + VHBHY) below the effective float voltage. Refer to Table 1 for the effective float voltage.

LBSEL (Pin 5): Low Battery Disconnect Select Pin. Connect LBSEL to GND to select a low battery disconnect level of 3.2V, connect LBSEL to VCC to select a low battery disconnect level of 2.7V. Do not float.

GND (Pin 6, Exposed Pad Pin 9): Ground. The exposed package pad has no internal electrical connection but must be connected to PCB ground for maximum heat transfer.

BAT (Pin 7): Battery Pin. Battery charge current is sourced from VCC through this pin when an external supply is present. BAT supplies current to VCC from this pin when no other source of power is available. If BAT falls below VLBD this pin disconnects the battery from VCC protecting the battery from discharge by the load when no external power supply is present.

VCC (Pin 8): Input Supply Pin. Attach system load to this pin. The input supply voltage is regulated to 4.0V, 4.1V, or 4.2V depending on the ADJ pin state (see the ADJ pin description for more detail). This pin can sink up to 50mA in order to keep the voltage regulation within accuracy limits. Decouple to GND with a capacitor, CIN, of at least 0.1μF, use a larger decoupling cap to handle high peak load currents.

Block diagram of chip LTC4071:

C:\Users\PRAVEEN\Desktop\Project\blockdiagram 4071.jpg

Operation of LTC4071 IC:

The LTC4071 provides a simple, reliable, and high performance battery protection and charging solution by preventing the battery voltage from exceeding a programmed level. Its shunt architecture requires just one resistor from the input supply to charge and protect the battery in a wide range of battery applications. When the input supply is removed and the battery voltage is below the high battery output threshold, the LTC4071 consumes just 550nA from the battery. If the battery voltage falls below the programmable low battery disconnect level, the battery disconnects from VCC, protecting the battery from over-discharge either by the load connected to VCC or from the LTC4071 quiescent current. When an input supply is present the battery charges through the body diode of the internal disconnect PFET, MP1, until the battery voltage rises above the low battery connect threshold. Select an input voltage large enough for VCC to reach VLBC_VCC to ensure that MP1 turns on. The user may detect the connected state by observing periodic pulses at the NTCBIAS pin that only occur once VCC has risen above VLBC_VCC, and cease once VCC falls below VLBD. Depending on the capacity of the battery and the input decoupling capacitor, the VCC voltage generally falls to VBAT when MP1 turns on; rather than VBAT rising to VCC. The internal PFET then reconnects the battery to VCC and the charge rate is determined by the input voltage, the battery voltage, and the input resistor:

As the battery voltage approaches the float voltage, the LTC4071 shunts current away from the battery thereby reducing the charge current. The LTC4071 can shunt up to 50mA. The shunt current limits the maximum charge current. In cases where the input supply may be shorted to GND when not supplying power, for example with a solar cell, add a diode in series with RIN to prevent the input from loading the battery. For more information, refer to the photovoltaic charger example in the Applications Information section.

Adjustable Float Voltage, VFLOAT

A built-in 3-state decoder connected to the ADJ pin provides three programmable float voltages: 4.0V, 4.1V, or 4.2V. The float voltage is programmed to 4.0V when ADJ is tied to GND, 4.1V when ADJ is floating (disconnected), and 4.2V when ADJ is tied to VCC. The state of the ADJ pin (and NTC pins) is sampled for about 36μs about once every 1.2 seconds when HBO is high, and when HBO is low the sampling rate reduces to about once every 3.6 seconds with the same duty cycle. If VCC falls below VLBD, the sampling stops. When it is being sampled, the LTC4071 applies a relatively low impedance voltage at the ADJ pin. This technique prevents low level board leakage from corrupting the programmed float voltage.

NTC Qualified Float Voltage, ΔVFLOAT(NTC)

The NTC pin voltage is compared against an internal resistor divider tied to the NTCBIAS pin. This divider has tap points that are matched to the NTC thermistor resistance/temperature conversion table for a Vishay curve 2 thermistor at temperatures of 40°C, 50°C, 60°C, and 70°C. The curve 2 thermistor is also designated by a B25/85 value of 3490. Battery temperature conditioning adjusts the float voltage down to VFLOAT_EFF when the NTC thermistor indicates that the battery temperature is too high. For a 10k curve 2 thermistor and a 10k NTCBIAS resistor, each 10°C increase in temperature above 40°C causes the float voltage to drop by a fixed amount, ΔVFLOAT(NTC), depending on ADJ. If ADJ is at GND, the float voltage steps down by 50mV for each 10°C temperature increment. If ADJ is floating, the step size is 75mV. And if ADJ is at VCC, the step size is 100mV. Refer to Table for the range of VFLOAT_EFF programming.

Table 2: NTC Qualified Float Voltage:

C:\Users\PRAVEEN\Desktop\Project\table 2.jpg

For all ADJ pin settings the lowest float voltage setting is:

3.8V = VFLOAT_MIN = VFLOAT – 4 • ΔVFLOAT (NTC).

This occurs at NTC thermistor temperatures above 70°C, or if the NTC pin is grounded. To conserve power in the NTCBIAS and NTC resistors, the NTCBIAS pin is sampled at a low duty cycle at the same time that the ADJ pin state is sampled.

High Battery Status Output: HBO

The HBO pin pulls high when VCC rises to within VHBTH of the programmed float voltage, VFLOAT_EFF, including NTC qualified float voltage adjustments assuming VCC has risen above VLBC_VCC. If VCC drops below the float voltage by more than VHBTH + VHBHY the HBO pin pulls low to indicate that the battery is not at full charge. The input supply current to the LTC4071 drops to less than 550nA (typ) as the LTC4071 no longer shunts current to protect the battery. And the NTCBIAS sample clock slows to conserve power. For example, if the NTC thermistor requires the float voltage to be dropped by 100mV

(ADJ = VCC and 0.29•VNTCBIAS < VNTC < 0.36•VNTCBIAS) then the HBO rising threshold is detected when VCC rises past:

VFLOAT – ΔVFLOAT (NTC) – VHBTH = 4.2V – 100mV – 40mV = 3.96V.

Low Battery Disconnect/Connect: LBD/LBC

The low battery disconnect (VLBD) and connects (VLBC) voltage levels are programmed by the LBSEL pin. As shown in the Block Diagram the battery disconnects from VCC by shutting off MP1 when the BAT voltage falls below VLBD. This disconnect function protects Li-Ion batteries from permanent damage due to deep discharge. If the voltage of a Li-Ion cell drops below a certain level, the cell may be permanently damaged. Disconnecting the battery from VCC prevents the load at VCC as well as the LTC4071 quiescent current from further discharging the battery. Once disconnected the VCC voltage collapses towards ground. When an input supply is reconnected the battery charges through the internal body diode of MP1. The input supply voltage should be larger than VLBC_VCC to ensure that MP1 is turned on. When the VCC voltage reaches VLBC_VCC, MP1 turns on and connects VCC and BAT. While disconnected, the BAT pin voltage is indirectly sensed through MP1’s body diode. Therefore VLBC varies with charge current and junction temperature. Please see the Typical Performance Characteristics section for more information.

Low Battery Select: LBSEL

The low battery discharge cut off voltage level is programmed by the LBSEL pin. The LBSEL pin allows the user to trade-off battery runtime and maximum shelf life. A lower battery disconnect threshold maximizes run time by allowing the battery to fully discharge before the disconnect event. Conversely, by increasing the low battery disconnect threshold more capacity remains following the disconnect event which extends the shelf life of the battery. For maximum run time, tie LBSEL to VCC so that the battery disconnects at VCC = 2.7V. For extended shelf life, tie LBSEL to GND so that the battery disconnects at VCC = 3.2V. If a high peak current event is expected, users may temporarily select the lower disconnect threshold. This avoids disconnecting the battery too early when the load works against the battery series resistance and temporarily reduces VCC.

General Charging Considerations of LTC4071:

The LTC4071 uses a different charging methodology from previous chargers. Most Li-Ion chargers terminate the charging after a period of time. The LTC4071 does not have a discrete charge termination. Extensive measurements on Li-Ion cells show that the cell charge current drops to very low levels with the shunt charge control circuit effectively terminating the charge. For improved battery lifetime choose 4.0V or 4.1V float voltage. The battery disconnect function requires some care in selecting the input supply compliance for charging a battery while powering a load at VCC. The internal battery disconnect switch remains off while charging the battery through the body diode of the internal switch until VCC exceeds VLBC_VCC. If the source voltage compliance is not greater than VLBC_VCC, then the battery will never re-connect to VCC and the system load will not be able to run on battery power. Users may detect that the battery is connected by monitoring the NTCBIAS pin as it will periodically pulse high once VCC has risen above VLBC_VCC, and stops pulsing once VCC falls below VLBD. The simplest application of the LTC4071 is shown in Figure below. This application requires only an external resistor to program the charge/shunt current. Assume the wall adapter voltage (VWALL) is 12V and the maximum charge current is calculated as:

= 54mA

C:\Users\PRAVEEN\Desktop\Project\LTC4071 single.jpg

Care must be taken in selecting the input resistor. Power dissipated in RIN under full charge current is given by the following equation:

= = 0.48W

The charge current decreases as the battery voltage increases. If the battery voltage is 40mV less than the programmed float voltage the LTC4071 consumes only 550nA of current, and all of the excess input current flows into the battery. As the battery voltage reaches the float voltage, the LTC4071 shunts current from the wall adapter and regulates the battery voltage to VFLOAT = VCC. The more shunt current the LTC4071 sinks, the less charge current the battery gets. Eventually, the LTC4071 shunts all the current flowing through RIN; up to the maximum shunt current. The maximum shunt current in this case, with no NTC adjustment is determined by the input resistor and is calculated as:

= = 49mA

At this point the power dissipated in the input resistor is 388mW.

The LTC4071 can also be used to regulate series-connected battery stacks as illustrated in Figure below. Here two LTC4071 devices are used to charge two batteries in series. A single resistor sets the maximum charge/shunt current.

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Figure of LTC4071 as 2-cell battery charger.

The GND pin of the top device is simply connected to the VCC pin of the bottom device. Care must be taken in observing the HBO status output pin of the top device as this signal is no longer ground referenced. Likewise for the control inputs of the top device; tie ADJ and LBSEL of the top device to the local GND or VCC pins. Also, the wall adapter must have a high enough voltage rating to charge both cells.

NTC Protection

The LTC4071 measures battery temperature with a negative temperature coefficient thermistor thermally coupled to the battery. NTC thermistors have temperature characteristics which are specified in resistance-temperature conversion tables. Internal NTC circuitry protects the battery from excessive heat by reducing the float voltage for each 10°C rise in temperature above 40°C (assuming a Vishay thermistor with a B25/85 value of 3490). The LTC4071 uses a ratio of resistor values to measure battery temperature. The LTC4071 contains an internal fixed resistor voltage divider from NTCBIAS to GND with four tap points; NTCTH1–NTCTH4. The voltages at these tap points are periodically compared against the voltage at the NTC pin to measure battery temperature. To conserve power, the battery temperature is measured periodically by biasing the NTCBIAS pin to VCC about once every 1.5 seconds. The voltage at the NTC pin depends on the ratio of NTC thermistor value, RNTC, and a bias resistor, RNOM. Choose RNOM equal to the value of the thermistor at 25°C. RNOM is 10k for a Vishay NTHS0402N02N1002F thermistor with a B25/85 value of 3490. RNOM must be connected from NTCBIAS to NTC. The ratio of the NTC pin voltage to the NTCBIAS voltage when it is pulsed to VCC is:

When the thermistor temperature rises, the resistance drops; and the resistor divider between RNOM and the thermistor lowers the voltage at the NTC pin. An NTC thermistor with a different B25/85 value may also be used with the LTC4071. However the temperature trip points are shifted due to the higher negative temperature coefficient of the thermistor. To correct for this difference add a resistor, RFIX, in series with the thermistor to shift the ratio:

Up to the internal resistive divider tap points: NTCTH1 through NTCTH4. For a 100k thermistor with a B25/85 value of 3950, e.g. NTHS0402N01N1003F, at 70°C (with RNOM = 100k) choose RFIX = 3.92k. The temperature trip points are found by looking up the curve 1 thermistor R/T values plus RFIX that correspond to the ratios for NTCTH1 = 36.5%, NTCTH2 = 29%, NTCTH3 = 22.8%, and NTCTH4 = 17.8%. Selecting RFIX = 3.92k results in trip points of 39.9°C, 49.4°C, 59.2°C and 69.6°C. Another technique may be used without adding an additional component. Instead decrease RNOM to adjust the NTCTH thresholds for a given R/T thermistor profile. For example, if RNOM = 88.7k (with the same 100k thermistor) then the temperature trip points are 41.0°C, 49.8°C, 58.5°C and 67.3°C. When using the NTC features of the LTC4071 it is important to keep in mind that the maximum shunt current increases as the float voltage, VFLOAT_EFF drops with NTC conditioning. Reviewing the single-cell battery charger application with a 12V wall adapter in Figure 2; the input resistor should be increased to 165Ω such that the maximum shunt current does not exceed 50mA at the lowest possible float voltage due to NTC conditioning, VFLOAT_MIN = 3.8V.

IC LTC4071 and LCT3588-1 can be used together to harvest piezoelectric energy and to store the energy in a battery. The simple schematic in Figure below illustrates a complete piezoelectric energy harvesting application using the LTC4071 to charge and protect Li-Ion cells along with the LTC3588-1 to rectify and regulate energy generated from a piezoelectric generator to a fixed 3.3V load.

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Piezoelectric Energy Harvester with Battery Backup