Shunt Battery Charger System with Low Battery Disconnect Introducing the LTC4071 Shunt Battery Charger System with Low Battery Disconnect

Piezo-Electric Elements Emerging Markets Power management for nontraditional energy sources Thin film battery SuperCaps Organic Solar Cell A myriad of low current, continuous and intermittent power sources Piezo-Electric Elements Thermo-Electric Generator Micro Fuel Cell 2

LTC4071: What is It? Shunt Battery Charger + Integrated Pack Protection Charger System With Low Battery Disconnect Low Operating Current: 550nA 1% Float Voltage Accuracy Over Full Temperature and Shunt Current Range 50mA Maximum Internal Shunt Current Pin Selectable Float Voltage Options: 4.0V, 4.1V. 4.2V

LTC4071: Key Technical Features Integrated Pack Protection in One IC Overvoltage (zener) and Undervoltage (Low Battery Disconnect) Low Operating Current: 550nA Pin Selectable Low Battery Disconnect Level: 2.7V or 3.2V 1% Float Voltage Accuracy Over Full Temperature and Shunt Current Range 50mA Maximum Internal Shunt Current Pin Selectable Float Voltage Options: 4.0V, 4.1V. 4.2V Ultralow Power NTC Float Voltage Conditioning for Li-Ion/Polymer Protection Suitable for Very Low Power (Intermittent or Continuous) Charging Sources High Battery Status Output Thermally Enhanced, Low Profile 8-Lead DFN (2mm x 3mm x 0.75mm) and MSOP Packages

LTC4071: Features and Benefits The LTC4071 provides a simple, reliable, and high performance battery charging and protection solution by preventing the battery voltage from exceeding a programmed level. Its shunt architecture requires just one resistor between the input supply and the battery to handle 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. A low battery disconnect function is integrated. While the battery voltage is below the programmed float voltage, the charge rate is determined by the input voltage, the battery voltage, and the input resistor values, given by: ICHG = (VIN − VBAT) / RIN. 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 with a float voltage accuracy of ±1% over temperature. The shunt current limits the maximum charge current. The shunt charge control circuit effectively terminates the charge.

LTC4071: Features and Benefits - NTC (cont.) 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.

LTC4071: Features and Benefits – NanoPower (cont.) 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. This enables the device to draw or harvest power from previously unusable low current, intermittent or continuous power sources. 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. This enables the device to draw or harvest power from previously unusable low current, intermittent or continuous power sources.

LTC4071: Packaging * not to scale

LTC4071: Typical Application Circuits Simple Single Cell Li Charger

LTC4071: Typical Application Circuits (cont.) Photovoltaic (Solar) Charger 2-Cell Stacked Battery Charger

LTC4071: Positioning A summary of our portfolio positioning is shown in the table. Shaded areas show where the LTC4071 has performance differences compared to Linear’s other related battery chargers and shunt references; in most cases these are performance advantages for the LTC4071. Positioning-wise, the LTC4071 most resembles the LTC4070, with differences including integrated pack protection (low battery disconnect) but lower 50mA charge current capability, higher 550nA quiescent current and no LBO output. The low battery disconnect is a critical function required to protect low capacity batteries from damage due to self-discharge. While the LTC4070 can perform a low battery disconnect function with the LBO output and an external PFET, the IC will continue to consume full IQ (about 0.5uA) from the battery. Even this battery drain current can damage a low capacity battery overnight. The LTC4071 integrates a complete low battery disconnect which consumes nearly zero current from the battery when disconnected (<1nA at room temperature and <25nA at 125°C). To provide this function in the LTC4071, the LTC4070’s LBO and DRV pin have been eliminated. This makes the LTC4071’s maximum shunt current fixed at 50mA (LTC4070 is 50mA capable with 500mA possible with an external FET) and increases the IC’s quiescent current to 550nA (LTC4070 has 450nA). The LTC4065L charges 1-cell Li-Ion at 4.2V only, has higher charge current, but no NTC or load disconnect capability and is not battery stack compatible. Also, it cannot charge from low current intermittent or continuous input sources (solar, piezo, etc.) due to its 120uA operating current. The LT1389 can be set for 4.096V float voltage, but has high operating current and lacks the battery charger functionality (NTC, status signals) of the LTC4071, plus it is housed in a large SOIC package.

LTC4071: Competition A summary of the competitive landscape is also shown in the table. Shaded areas show where LTC4071 has performance advantages over competing solutions. Competition includes National Semi and other makers of shunt references, plus low cost linear battery chargers. An adjustable shunt reference could be programmed for the appropriate battery float voltage, but it will lack the NTC function of a battery charger.  More importantly, the required quiescent current will be sufficiently high such that battery charging from low power intermittent (solar, piezoelectric) or continuous sources will be impractical. Furthermore, there are no status signals either. Alternatively, a discrete shunt reference could be built from a zener diode, resistors, an NPN transistor and comparators for the NTC function. However, it will suffer from the same limitations as discussed above, additionally it will be cumbersome to implement and will consume more PCB area by comparison. Zero current disconnect can actually be done externally, but you need an LBO comparator, a pair of logic inverters, a FET and a large value resistor. It is very cumbersome due to the numerous external components. A typical competitive linear battery charger is usually an inexpensive solution; however it will suffer from high quiescent current, have no output disconnect option, and cannot charge stacks of cells.

LTC4071: End Markets/Applications Low Power Li-Ion/Polymer Battery Back-Up Energy Scavenging/Harvesting Solar Power Systems with Back-Up Memory Back-Up Embedded Automotive Thin Film Batteries