and applications note

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