A shunt reference is a current-fed, two terminal device that draws no current until the target voltage is reached. A shunt reference is used like a Zener diode and is often shown on a circuit schematic as a Zener diode. However, most shunt references are actually based on a bandgap reference voltage.
A shunt reference requires only a single external resistor to regulate its output voltage making it extremely easy to use. There is no maximum input voltage limit, and the minimum input voltage is set by the value of the reference voltage because some headroom is required for proper operation.
Further, shunt references have good stability over a wide range of currents. Many shunts are also stable with large or small capacitive loads.
Any solution to satisfy the battery charger IC design constraints outlined above would have to combine a shunt regulator’s characteristics and those of a battery charging IC with the ability to charge from low power continuous or intermittent sources. Such a device would also need to protect and extract the maximum performance from a lithium-ion/polymer battery, coin cell, or thin-film battery or battery pack.
Linear Technology has developed shunt-architecture battery chargers, the LTC4070 and the LTC4071, to address these applications. The LTC4070 is a tiny shunt battery charger system IC for Li-ion/polymer batteries, coin cells, or thin-film batteries. With its 450nA operating current, the IC protects batteries and charges them from previously unusable very low current, intermittent or continuous charging sources. The LTC4070’s charge current may be boosted from 50mA up to 500mA with the addition of an external PMOS shunt device. An internal thermal battery conditioner reduces the float voltage to protect Li-ion/polymer cells at elevated battery temperatures. Multiple-cell battery stacks can be charged and balanced by configuring several LTC4070s in series. Housed in a low profile (0.75mm) 8-lead 2mm x 3mm DFN package, the LTC4070 is an ultra-compact charger solution with a single external resistor required in series with the input voltage. The device’s feature set is suitable for both continuous and intermittent, lower power charging source applications including Lithium-Ion/Polymer battery backup, thin film batteries, coin cell batteries, memory backup, solar-powered systems, embedded automotive and energy harvesting.
With pin-selectable settings of 4.0V, 4.1V, and 4.2V, the LTC4070’s 1-percent accurate battery float voltage allows the user to make tradeoffs between battery energy density and lifetime. Independent low battery and high battery supervisory status outputs indicate a discharged or fully charged battery. In conjunction with an external P-FET in series with the load, the low battery status output enables a latch-off function that automatically disconnects the system load from the battery to protect the battery from deep discharge.
In addition to its compact 2mmx3mm 8-lead DFN package, the LTC4070 is also offered in an 8-lead MSOP package. The devices are rated for operation from -40°C to 125°C.
Figure 1: LTC4070 typical application circuit
The LTC4070 provides a high performance battery protection and charging 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 LTC4070 consumes 450nA from the battery.
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:
ICHG = (VIN - VBAT) / RIN
As the battery voltage approaches the float voltage, the LTC4070 shunts current away from the battery thereby reducing the charge current. The LTC4070 can shunt up to 50mA with a float voltage accuracy of ±1 percent over temperature. The shunt current limits the maximum charge current, but the 50mA internal capability can be increased by adding an external P-channel MOSFET; refer to Figure 1