Batteries & Fuel Cells
Getting clever
Steve Knoth looks at how a shunt charger system can harvest power from previously unusable low-current sources
S
hunt voltage references are simple to use; they have been around for many years and are in a myriad of
products. However, they cannot effectively charge a battery. To configure one to do such a task is extremely cumbersome. Moreover, the ability to accurately and safely charge a Lithium-Ion/Polymer, coin cell or a thin film battery from a low-current source or an intermittent harvested energy type source has been unachievable. Embryonic in the market place,
energy harvesting ICs can convert a transducer’s signal into an input appropriate for a battery charger device. A “growth” phase is imminent for this technology and the opportunities in terms of energy harvesting applications are widespread. On the battery side, although
technology has improved, portable electronic device batteries still require protection and conditioning to keep them running optimally. Lithium- Ion/Polymer batteries are a mature technology and a popular choice due to their high energy density, low self- discharge, low maintenance, and wide voltage range, among other features. Coin cells offer high energy density, stable discharge characteristics and low weight in a small form factor. Thin film batteries are an emerging technology with such benefits as a high number of
charge cycles and physical flexibility. However, some potential detrimental effects on these batteries exist if not properly charged and conditioned.
Design challenge
An adjustable shunt reference can be programmed for an appropriate battery float voltage, but it will lack the NTC function of a battery charger. More importantly, the required operating current is so high that battery charging from low power or intermittent sources is not practical. Alternatively, a discrete shunt reference can be built from a zener diode, resistors, an NPN transistor, and comparators for the NTC function. However, it will still suffer from the same limitations outlined above. Typical battery charger ICs require a
constant DC input voltage and cannot handle bursts of energy. However, intermittent energy harvesting sources such as indoor photovoltaic arrays or piezoelectric transducers provide bursts of power. A unique IC with sub-1µA quiescent/operating current is necessary to charge a battery from this type of energy source. Lithium-Ion/Polymer chemistry
batteries provide the high performance features necessary for portable electronic devices but must be treated
with care. For example, Lithium- Ion/Polymer cells can become unstable if charged over 100mV beyond their recommended float voltage. Further, simultaneous instances of high voltage and high temperatures adversely affect battery life, and in extreme cases can lead to their self-destruction.
Shunt basics & benefits
A shunt reference is a current-fed, two terminal device that draws no current until the target voltage is reached. It 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. It 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.
Simple solution
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 battery or cell. Linear Technology’s LTC4070 is an
Figure 1. LTC4070 application circuit operational modes
18 April 2010
Components in Electronics
easy-to-use, tiny shunt battery charger system IC. With its 450nA operating current, the IC protects batteries and charges them from previously unusable very low current, intermittent or continuous charging sources. The 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 ICs in series. The device’s feature set suits iboth 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% 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 PFET 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. The LTC4070 prevents 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, it consumes just 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, it 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% over temperature. The shunt current limits the maximum
www.cieonline.co.uk
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60