Abstract:
A battery charger for a portable electronic device includes a linear charger to generate a substantially constant current for charging the battery and a switching voltage regulator to convert power supplied by an external adapter to a supply voltage for the linear charger. A feedback circuit controls operation of the switching voltage regulator so that the voltage supplied to the linear charger is substantially equal to the combination of the battery voltage and the drain-to-source voltage of the linear charger. In this way, power dissipation by the linear charger is minimized without requiring the use of a high accuracy current limited adapter.

Description:
[0001]    In battery powered portable solutions, due to increased integration and features, the demand and requirement for higher current power sources is increasing. To accommodate this increased demand, larger and more efficient rechargeable batteries are being used. Among these, Lithium Ion (Li or Li Ion) based batteries are perhaps the most important type because of their high power density both in terms of volume and in terms of weight. 
         [0002]    Efficiency, power dissipation, fast charge rate, and signal noise are some of the key concerns in charging batteries designed for portable applications. Two types of chargers are most commonly used: linear chargers and switching chargers. Of the two, linear chargers provide the least noise and can be configured to produce accurately regulated charging voltages. Switching chargers tends to produce more noise, but offer higher efficiencies and the ability to provide a boosted (increased) charging voltage. 
         [0003]    As shown in  FIG. 1 , a typical charging sequence for a Lithium Ion based battery includes pre-charge, constant current and constant voltage phases. During the pre-charging phase, the battery is charged using a relatively low, fixed current (typically less that 1/10 of the battery&#39;s fast charge rate). This is followed by the regulated current phase where the charging current is fixed at a higher magnitude while the battery voltage continues to ramp. Once the battery voltage has reached its charged level, the constant voltage phase is initiated where current is regulated to maintain the battery&#39;s charged voltage. 
         [0004]    In typical portable applications, an external adapter is used in series with an internal charger. If V in  is the input voltage for the charger (and the output voltage of the adapter) the power dissipation requirement across the charger during each of these phases is equal to: 
         [0000]        P   diss =( V   in   −V   bat )* I   chrg    
         [0005]    In the constant current phase, where the charge current (I chrg ) is constant, power dissipation is proportional to voltage difference between the input voltage and battery voltage (i.e., V in −V bat ). If it assumed that the input voltage is constant (i.e., linear charging), the power dissipation will be vary as function of increasing battery voltage (V bat ). 
         [0006]    For example, if a typical adapter is used, an input voltage to the charger of 5.5V is common (i.e., V in =5.5V). If a LiIon battery is depleted to 3.0 volts and fully charged at 4.2 volts and a 1.5 Amp current is used for the constant current charging phase then the power dissipation will be 1.5 A*(5.5V−3.0V)=3.75 W at the beginning of charge and 1.5 A*(5.5V−4.2V)=1.95 W at the end of charge. 
         [0007]    In general, this relatively high power dissipation presents certain challenges for designers of portable electronic devices. This is increasingly true because there is continuous pressure to reduce the size of internal charging systems which can severely limit their ability to dissipate heat generating during the charging process. One solution has been to use an external high accuracy current limited adapter in series with an internal charger to transfer the power dissipation from the charger to adapter. As shown in  FIG. 2  shows, adapters of this type provide a relatively fixed output voltage over a wide range of output currents. Once the adapter current limit has been exceeded, however the output voltage decays rapidly. By operating the adapter at or near its current limit, the voltage at the input of the charger becomes: 
         [0000]    
       
      
       V 
       in 
       =V 
       bat 
       +V 
       ds 
       
         — 
       
       chrg  
      
     
         [0008]    where V ds     —     chrg  is the voltage drop over the charger. So, if it is assumed that V ds     —     chrg =1V and the charge current is 1.5 Amp, the power dissipation will be 1.5 A*(4.0V−3.0V)=1.5 W at the beginning of charge (assuming, again that the battery is depleted to 3.0V). At the end of constant current charge phase, the power dissipation will be 1.5 A*(5.2V−4.2V)=1.5 W. Obviously, this is an improvement in power dissipation throughout the charging process. 
         [0009]    In high volume applications, the cost of high accuracy current limited adapters is relatively high compare to standard adapters. And since most noise sensitive applications require linear chargers over switch mode charging, there is still a need for a relatively lower cost, low noise charging solution capable of supporting high charging currents (1.5 A to 2 A typical). 
         [0010]    The objective of the recommended solution below is to provide a relatively lower cost system side solution which can provide a low noise, high current charging solution which can use a standard adapter and yet will have much lower power dissipation than the industry standard method. 
       SUMMARY OF THE INVENTION  
       [0011]    The present invention includes a battery charger for portable electronic devices. For a typical implementation, the battery charger includes a linear charger and a switching voltage regulator. The linear charger typically includes a transistor with its source connected to supply power to the battery being charged. The switching regulator is connected to an external power supply, typically a wall adapter or similar device. The output of the switching voltage regulator controls the voltage at the drain of transistor in the linear charger. 
         [0012]    In use, the linear charger controls the gate of its transistor so that the battery is supplied with a constant charging current. As the battery is being charged, a feedback circuit controls the switching voltage regulator so that the voltage at the transistor drain is maintained at an optimal level. Typically, this means that this voltage is equal to (or slightly higher) than the sum of the battery voltage and the drain-to-source voltage of the transistor. In this way, the amount of power that is dissipated by the transistor is minimized without requiring the use of a high accuracy current limited adaptor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  is a prior art graph showing the voltage of a lithium ion battery as a function of time as the battery is charged from a deleted state. 
           [0014]      FIG. 2  is a graph showing output current as a function of voltage for a high-accuracy current limited adapted as provided by the prior art. 
           [0015]      FIG. 3  is a block diagram of an implementation of the battery charger of the present invention. 
           [0016]      FIG. 4  is a graph showing the voltage of a lithium ion battery and the voltage produced the switching portion of the battery charger of the present invention as a function of time as the battery is charged from a deleted state. 
           [0017]      FIG. 5  is schematic of first implementation of the switching/linear battery charger of the present invention. 
           [0018]      FIG. 6  is schematic of first implementation of the switching/linear battery charger of the present invention. 
           [0019]      FIG. 7  is schematic of a feedback circuit as used by the implementations of  FIGS. 5 and 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    The present invention includes an apparatus and method for efficiently charging batteries in portable electronic devices. As shown in  FIG. 3 , a representative implementation of the battery charging apparatus includes a switching regulator connected in series with a linear charger. The output of the linear charger is connected to a battery. For typically applications, the switching regulator, linear charger and battery will all be included in a portable electronic device such as a cellular telephone or portable music player. An external adapter is used to provide an input voltage to the switching regulator. 
         [0021]    The linear charger and switching regulator both receive a feedback voltage derived from the output voltage of the linear charger. As the battery is charged (and its voltage increases), the output voltage of the switching regulator is adjusted so that the input voltage to the linear charger is just enough to keep the keep the linear charger operating. This is shown, for example in  FIG. 4 . In this way, the power dissipation across the linear charger is reduced compared to traditional chargers without the expense of a high accuracy current limited adapter. 
         [0022]    In  FIG. 5 , the first of two implementations for the charger of  FIG. 3  is shown and generally designated  500 . As in  FIG. 5 , a switching/linear charger  500  as provided by the present invention includes a switching control circuit that is connected to drive two switches (S 1  and S 2 ). The switches S 1  and S 2  are connected in a half-bridge configuration between an input pin and an internal ground node. In an actual system, the input pin would be connected to a power source (typically a wall adapter) and the internal ground node would be connected, via a ground pin to ground. An LX pin is connected to the middle of the half bridge between the switches S 1  and S 2 . 
         [0023]    Switching/linear charger  500  also includes a linear charge control circuit that is connected to drive a third switch S 3 . The switch S 3  is connected between a V chg  pin and a V bat  bin of the switching/linear charger  500 . 
         [0024]    A feedback control circuit is connected to provide a feedback voltage representative of the voltage at the V bat  pin to the linear charge control circuit and the switching control circuit. A current sense circuit is connected to provide a current sense voltage representative of the current passing from the input pin and the switch S 1  to the linear charge control circuit and the switching control circuit. 
         [0025]    In use, the input pin is connected to a power source such as a wall adapter. An inductor and reservoir capacitor are connected in series between the LX pin and the V chg  pin. The V bat  pin is connected to the battery to be charged. The switching control circuit operates switches SI and S 2  as a buck switching regulator. Switch S 1  is turned ON (and switch S 2  is turned OFF) during a charging phase. This causes current to flow from the input pin through the inductor to charge the reservoir capacitor and store energy in the inductor in the form of a magnetic field. The charging phase is followed by a discharge phase where switch S 1  is turned OFF and the switch S 2  is turned ON. During the discharge phase current flows from the inductor to the capacitor and ground. The charging phase and the discharging phase are repeated to maintain the voltage at the V chg  pin at a desired level. 
         [0026]    Using the voltage at the V chg  pin as its input, the linear charge control circuit operates the switch S 3  as a linear charger. This means that the linear charge control circuit modulates the drive to switch S 3  to control the current and voltage supplied to the battery being charged. During constant current mode, the linear charge control circuit modulates the drive to switch S 3  so that a constant current is delivered to the battery being charged. The magnitude of the constant current is typically preset to a value such as 1.5 A and is measured by the current sense circuit. 
         [0027]    In  FIG. 6 , a second of two implementations for the charger of  FIG. 3  is shown and generally designated  600 . Switching/linear charger  600  is similar to the first implementation just described except that switching/linear charger  600  uses an asynchronous buck converter in place of the synchronous buck converter just described. Specifically, this means that the switching control circuit operates a single switch S 1  and that the switch S 2  is replaced with a diode. This simplifies the operation of the switching control circuit at the expense of somewhat lower efficiency (since there is a fixed voltage drop over the diode). 
         [0028]    The key to efficient operation of switching regulators  500  and  600  is making the input voltage to the linear charger (i.e., the voltage at the V_chg pin) just enough to keep the keep the linear battery charger ON while the output voltage (battery voltage) is increasing.  FIG. 7  shows an implementation  700  of a circuit that provides the necessary feedback for effective operation of the switching control circuit. As shown in  FIG. 7 , the feedback circuit includes resistors R 1  and R 2  coupled in series between the output voltage of the switching regulator (or the input voltage of the linear regulator) and ground. For the purposes of this description, it may be assumed that a node V 1  exists between the two resistors. 
         [0029]    The feedback circuit also includes a current mirror composed of transistors Q 1  and Q 2  along with resistors R 3 , R 4  and R 5 . Resistor R 5 , transistor Q 1  and resistor R 4  are connected in series between the battery input voltage (i.e., the output of the linear charger) and ground. Transistor Q 2  and resistor R 3  are connected in series between the node V 1  and ground. A bias current flows from the battery voltage through transistor Q 1  to ground. The bias current is mirrored by transistor Q 2  forcing the voltage at the node V 1  to be proportional to the voltage at the battery input. Since the voltage at V 1  functions as the feedback voltage for the buck regulator, the natural operation of the buck regulator maintains the voltage at its output at the level required to operate the linear charger as a function of battery voltage. 
         [0030]    More concretely, assuming that R 3 =R 4 , R 4 +R 5 =R 1 , and Q 1  and Q 2  are identical sizes, then 
         [0000]        V   buck   =V   bat   +[V   ref *( R 1+ R 2)/ R 2− V   be ] 
         [0000]    where V be  is the base-emitter junction voltage of Q 1 . 
         [0031]    So, it is further assumed that if the output of the switching regulator (V buck ) should be 300 mV higher than the battery voltage, the following component values may be used: 
         [0032]    V ref =600 mV 
         [0033]    V be =600 mV 
         [0034]    R 1 =3 k ohms 
         [0035]    R 2 =6 k ohms 
         [0036]    R 3 =R 4 =300 ohms 
         [0037]    R 5 =2.7 k ohms 
         [0000]        V   ref *( R 1+ R 2)/ R 2=1V 
         [0000]        V   buck   =V   bat +(1V−0.6V)= V   bat +300 mV