AP003 Designing Battery Chargers using the AQT105 The AQT105 is a secondary-side controller for implementing a constant current/constant voltage supply suitable for charging batteries to a final voltage at a fixed current. The AQT105 replaces the conventional TL431-type secondary error amplifier traditionally used for power supply control with a 5 pin SOT-23 device which adds the functionality of sensing limiting the output current The following application note makes no assumption about the specifics of the power supply primary side, which may use any topology and control mechanism suitable for use with a standard TL431 driving optocoupler feedback. Likewise, loop compensation remains unchanged from a more conventional design, and it is presumed that this compensation will remain unchanged when compared to a conventional constant-voltage power supply. Control Architecture. The AQT105 regulates a final voltage using a simple 1.25V bandgap reference Vr and an amplifier driving the external optocoupler, as shown in figure 1. A resistor Rs in the ground return generates a drop proportional to the current in the battery. The voltage across this resistor is compared to a 200mV nominal Vsns, and as the current across Rs exceeds Vsns, the value of the main reference voltage is reduced. A simple transconductance amp A2 with a current sinking output can reduce but not increase the voltage at Vr’. A fixed gain of 40 in this path causes the current to increase by about 15% as the error amp is progressively driven from 1.25 to zero volts. Note that the potential at the negative battery terminal is not in common with the negative connection to the power source. U1 Optocoupled Negative Feedback OPTO U2 R1 10k 1K R3 AQT105 Switching Power Converter Vr' 10k A1 R ref 1.25V VR Battery GM=100 m S A2 C1 200m V Vsns GM=4 m S R2 4.3k 1 Current Sense R6 Fig. 1 Basic AQT105 control scheme It is perhaps useful to contrast this circuit with the ubiquitous TL431. The COMP, FB and GND pins of the AQT105 are directly analogous to Cathode, Ref and Ground of the 431. The differences are that the reference in this case is 1.25V rather than 2.5V, and the use of a separate VCC pin allows COMP to swing low (Unlike a 431, where the cathode is both the output and the positive supply terminal that keeps the chip biased.). Of course the significant change from the 431 architecture is the addition of the CS pin, which will reduce the effective reference voltage when CS drops below –200mV. This architecture allows the voltage-control loop to dominate under all conditions, with the constant-current operation working by reducing the voltage to which the circuit regulates. There are however two interacting loops, so there is some concern about stability over all load conditions, to be discussed below. AQT105 Applications Figure 2 shows a simple application for a 7.2V NiMH battery pack. Throughout this application note we will presume that the goal is to charge this pack to a maximum of under 8.4V at a current of 1A. This output voltage is set by VCHG = VREF ∗ 11/15/2004 ( R1 + R 2 = 1.25V ∗ 1 + R1 R2 R2 Battery Charging with AQT105 ) 2 1K R3 VPOS 5.6k R1 OPTO U2 Compensation T1A 10u C4 100u C3 D2 C1 VC C CS AQT105 U1 C2 BATT C OMP R4 FB GN D 1K R2 VN EG 240m R6 Fig. 2 Simple Secondary-side circuit using AQT105 If the battery load to the charger is somewhat discharged, the voltage presented to the FB pin will be low and the drive to the optocoupler will be reduced, causing the primary side to increase the energy transfer to the secondary. As the current through the battery increases, the voltage drop across R6 will increase until it reaches the 200mV threshold of the CS pin. At that point, further increases in load current will decrease the AQT105’s internal reference voltage until a balance is reached with the FB. If the battery is charges fully, the divided output presented to the FB pin will turn the optocoupler on sufficiently to hold the primary side control off. At the knee between the constant voltage and constant current operation, the output current ICHG will be: I CHG = VSNS 200mV = R6 RSNS Based on the fixed gain of 40 between the CS pin and the internal reference, the drop across R6 increases to about 230mV in a short circuit condition. The V/I curve of this simple circuit is shown in figure 3. This finite gain improves stability, and this output impedance can be improved with the addition of an external network, as shown below. 11/15/2004 Battery Charging with AQT105 3 8 Battery Voltage / V 7 6 5 4 30 0.2 0.4 0.6 0.8 1 Charge Current/A 200mA /div Fig. 3 V/I curve of simple application One problem not addressed in this simple design is the condition of a shorted (or completely dead) battery – if the voltage across the battery is below 2.2V, then the AQT105 may not be able to control the output. This min Vcc specification for this chip is fairly coincident with the voltage at which we would also loose the ability to drive the optocoupler LED. In very low voltage conditions, it is also important to guarantee the behavior of the primary side circuit, which may also rely on a cross-regulated output to supply its control circuit. The application of figure 4 is more typical solution. Though it adds complication in a number of areas, it solves a variety of problems. 470 R6 1K R3 D4 12V 10u C4 LED D1 T1B VPOS D3 100u T1A C 3 5.6k R1 33k R 11 D2 C1 VC C CS 150 AQT105 C OMP 10u C6 R7 1 R8 1 10u C5 C2 R 10 R5 1 U1 5 V1 R4 FB R9 1 GN D 1K R2 VN EG Fig. 4 Typical Application for AQT105 11/15/2004 Battery Charging with AQT105 4 1. The low value R6 (250 mOhms) has been implemented with a parallel combination of resistors R6, R7, R8 and R9. Breaking this resistor into a parallel combination allows standard 1/4W resistors to be used with a reasonable derating. 2. The use of two transformer windings allows the main output to be driven as a flyback and the Vcc supply is a function of the forward voltage across a separate winding. Although the AQT105 can work well with over a 5:1 range of supply voltage, it may be advisable to use an appreciable series R in the RC filter shown and provide a zener clamp D4 to reduce the risk of worst-case stresses from line variations. (Additional tricks using a single winding are possible but are not very general). 3. In charge mode, depending on the battery impedance, (e.g. short circuit) the constant current loop may not be perfectly stable. A small signal oscillation on the charge current may be acceptable, but can be prevented by adding network R10/C6. Because the CS pin is not a high impedance input, the value of R3 must be kept fairly low; as shown R3 will have a drop of approximately 15 mV at the current limit threshold. This requires a small adjustment in the value of the current sense resistor. 4. R11 provides a positive feedback path which increases the output impedance to create a more constant charge current. Taking into account the impedance of the CS pin, a good rule of thumb for the selection of R11 is: R11 ≈ VCHG 35mV 75µA + R10 Choosing higher value allows some finite positive output impedance. Choosing a lower value will tend to give a negative output impedance. The overall effect on the V/I curve is shown in figure 5. After adding these various improvements, the calculation of the charge current becomes somewhat more complex. The combination of the addition of R3 and R10 shifts the value of the charge current: I CHG V ⎞ ⎛ 200mV + ⎜100uA + CHG ⎟ R3 R10 ⎠ ⎝ = RSNS Where Rsns is the parallel combination of R6 through R9, as applicable. The 100uA is the nominal current flowing out the IPROG pin at the threshold. 11/15/2004 Battery Charging with AQT105 5 8 7 Battery Voltage / V 6 5 4 3 2 1 00 0.2 0.4 0.6 Charge Current/A 0.8 1 200mA /div Fig. 5 V/I curve of typical application Additional Applications In principle, the AQT105 can be used in any supply application where a constant voltage/ constant current characteristic is desired. Additional circuitry can further give foldback limiting as the output droops. The overall architecture is sufficiently general purpose to allow for a wide variety of topologies to be addressed. 11/15/2004 Battery Charging with AQT105 6