The huge increase of portable devices during the last 10-15 years has lead to numerous companies seeking (and succeeding) to make significant profits in this market. Many consumer applications are now powered by accumulators, ranging from the omnipresent cellular phones to media players, handheld gaming devices, and navigation systems (that is to name only a few categories). “Portable” means lightweight, low power and of course, battery or accumulator that needs to be charged. Charging an accumulator is not rocket science, but doing it in an energy efficient manner, with tiny circuits that provide provision for many types of batteries and charging modes, represents a design challenge accepted by quite a few semiconductor companies, one of which is Freescale.
Out of all portable devices, the most numerous are the mobile phones. Most of them feature Li-ion or Li-polymer accumulators and Freescale has a broad range of charger ICs dedicated to supporting all the phases of a complete recharge cycle. Generally speaking the charging of a mobile phone is performed by taking energy from:
a) from a wall outlet
b) from the USB port of a computer
c) from the 12V output of a vehicle
The purpose of the battery charger IC is to take energy from this wide range of sources and to deliver this energy in a controlled manner to the battery. The controlled manner means that the IC is capable to operate in all the necessary modes of charging a battery for portable device: trickle mode, constant current (CC) mode and constant voltage (CV) mode. As it may be seen from the block diagram below (of the MC3467x battery chargers family from Freescale), the central point in such a battery charger is a transistor which is backed up with sophisticated control and feedback circuitry:
The MC3467x battery chargers can take an input as high as 28V (with overvoltage protection) and as low as 2.6V; they deliver also overcurrent protection, together with thermal fold back (with up to +/-0.7% voltage accuracy), features which increase battery lifetime and ensure a full charge cycle of a battery, thus maximizing autonomous operation of the portable serviced device. All these are crammed into an 8-lead 2mmx3mmx0.65mm UDFN thermally enhanced package. In order to minimize PCB cost, every other additional required component has been added inside the ICs including the charging FET transistor, blocking diodes and sense resistors.
Although batteries seem common to anybody, charging them is not as straightforward as we would like it to be. To maximize life of the battery and to prevent any physical damage due to heating, a precise charging cycle has to be employed for totally depleted batteries and these Freescale battery chargers provide just the means to do that. The logic control logic packed in the small package of the MC3467x family ensures a minimal circuit diagram for the hardware developer, allowing for a battery charger to be made as a standalone unit, without any supervisory microcontroller or processor:
Below you may see the complete charging cycle, with the three states that have to be controlled by the circuit: trickle mode, CC mode and CV mode:
The charging current is illustrated (in black) and also the battery voltage (in pink). Also available in the diagram are the states of two pins of the charger, /CHG and /FAST, which indicate the state of the charging cycle.
In the beginning, when the battery is totally depleted, it has to be charged with a “trickle” current which is much lower than the normal charging current of the battery. In the case of the MC3467x family, the trickle current value is preset to 20% of the normal charging current which is used in the next step. Trickle current is supplied to the Li-ion or Li-polymer battery until it reaches a voltage of 2.7V.
This threshold is detected by the battery charger (which flags the event by setting the /FAST pin in a LOW state), and from now on the chip will control the charging current to its nominal constant current value. In case of these Freescale ICs, the constant current value is set with the help of an external resistor, which provides flexibility in design. Using this resistor (R_ISET), the current for the MC34671 can be set for up to 600mA, for the MC34673 up to 1.2A and for the MC34675 up to 1A.
When battery voltage reaches 4.2V, the MC3467x switches to CV mode, and regulates not the current through its internal FET but rather its output voltage. This is regulated to 4.2V and the charging current gradually drops until it reaches a threshold called EOC (End-Of-Charge). This threshold is preset to 10% of the nominal constant current value used in the previous phase. When this point in time is reached, the /CHG pin of the charger turns high, in order to indicate to a potential controller the fact that battery charging has completed.
Although charging is complete, the charger still monitors the state of the battery. Therefore, if a load occurs in parallel to the battery, the charger will supply current to the load, rather than the battery. In case the current required by the load will exceed the value for the constant current which was set with the external resistor, the battery will have to supply the additional current. In case the battery voltage drops below a specific “recharge level” the battery charger will return to the constant current charge mode, which is being indicated by the /CHG pin.
Taking into account these successive charging phases, the charger can be also regarded as a state machine, its function being governed by the following diagram (useful when you would want to incorporate it in a functional simulation of a finished product):
One important aspect of battery charger ICs is the accuracy. Freescale excels at this chapter, offering with its MC467x family:
+/-0.7% output voltage accuracy over -20 Celsius to +70 Celsius
+/-0.4% output voltage accuracy at room temperature
+/-6% charge current accuracy over -40 Celsius to +85 Celsius (+/-5% for the MC34671)
In addition, the MC34675 offer an extended -40 Celsius to +85 Celsius for the +/-0.7% accuracy of the output voltage and, what is even more interesting, it incorporates an 4.85V/10mA linear voltage regulator which may be used to power only parts of the circuit (an USB transceiver, for example). The output of the linear regulator is turned on when the input voltage is above the PowerOnReset but under the OvervoltageProtection thresholds. It is independent and it is not connected to any other signals of the charger.
Source: Power Management
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