adp3811 Analog Devices, Inc., adp3811 Datasheet - Page 9

• Manufacturer Part Number: adp3811
• Description: Secondary Side, Off-line Battery Charger Controllers
• Manufacturer: Analog Devices, Inc.

between inductive and capacitive component sizes, switching
losses and cost.
The primary PWM-IC circuit derives its starting V
100 k resistor directly from the rectified ac input. After start-
up, a conventional bootstrapped sourcing circuit from an auxil-
iary flyback winding wouldn’t work, since the flyback voltage
would be reduced below the minimum V
the 3845 under a shorted or discharged battery condition. There-
fore, a voltage doubler circuit was developed (as shown in Fig-
ure 23) that provides the minimum required V
across the specified ac voltage range even with a shorted battery.
While the signal from the ADP3810/ADP3811 controls the av-
erage charge current, the primary side should have a cycle by
cycle limit of the switching current. This current limit has to be
designed so that, with a failed or malfunctioning secondary cir-
cuit or optocoupler, the primary power circuit components (the
FET and transformer) won’t be overstressed. In addition, dur-
ing start-up or for a shorted battery, V
ADP3811 won’t be present. Thus, the primary side current
limit is the only control of the charge current. As the secondary
side V
and controls the average current. The primary side current limit
is set by the 1.6
power NMOS transistor, IRFBC30, and ground.
The current drive of the ADP3810/ADP3811’s output stage di-
rectly connects to the photodiode of an optocoupler with no ad-
ditional circuitry. With 5 mA of output current, the output stage
can drive a variety of optocouplers. An MOC8103 is shown as
an example. The current of the photo-transistor flows through
the 3.3 k feedback resistor, R
3845’s COMP pin, thus controlling the PWM duty cycle. The
controlled switching regulator should be designed as shown so
that more LED current from the optocoupler reduces the duty
cycle of the converter. Approximately 1 mA should be the
REV. 0
120/220V–
AC
CC
N
** TX1
L
* 1% TOLERANCE
rises above 2.7 V, the ADP3810/ADP3811 takes over
L
L
f = 120kHz
PR
SEC
1A
= 750µH
= 7.5µH
9.1
3W
current sense resistor connected between the
3.3k
3.3k
R
F
Figure 23. ADP3810/ADP3811 Controlling an Off-Line, Flyback Battery Charger
50µF/450V
C
1nF
F
FB
2.2nF
, setting the voltage at the
COMP
V
RT/CT
FB
PWM
3845
CC
3.3k
V
CC
CC
to the ADP3810/
OUTPUT
100k
I
47µF
SENSE
GND
level specified for
V
REF
CC
13V
for the IC
CC
1N4148
10
470pF
OPTO COUPLER
through a
0.1µF
MOC8103
22nF
10k
1k
–9–
IRFBC30
1.6
330pF 330
TX1**
maximum current needed to reduce the duty cycle to zero. The
difference between the 5 mA drive and the 1 mA requirement
leaves ample margin for variations in the optocoupler gain.
Secondary Side Considerations
For the lowest cost, a current-mode flyback converter topology
is used. Only a single diode is needed for rectification
(MURD320 in Figure 23), and no filter inductor is required.
The diode also prevents the battery from back driving the
charger when input power is disconnected. A 1 mF capacitor
filters the transformer current, providing an average dc current
to charge the battery. The resistor, R
rent which is controlled via the V
charging current has high ripple due to the flyback architecture,
so a low-pass filter (R3 and C
needed. This filter has an extra inverted zero due to R
prove the phase margin of the loop. The 1 mF capacitor is con-
nected between V
additional decoupling to ground, a 220 F capacitor is also con-
nected to V
put capacitor was selected for lowest cost instead of lowest
ripple. Most of the ripple current is shunted by the parallel bat-
tery, if connected. If needed, high frequency ringing caused by
circuit parasitics can be damped with a small RC snubber across
the rectifier.
The V
rect connection to the battery as long as the battery voltage re-
mains below the specified 16 V operating range. If the battery
voltage is less then 2.7 V (e.g., with a shorted battery, or a bat-
tery discharged below it’s minimum voltage), the ADP3810/
ADP3811 will be in Undervoltage Lock Out (UVLO) and will
not drive the optocoupler. In this condition, the primary PWM
circuit will run at its designed current limit. The V
ADP3810/ADP3811 can be boosted using the circuit shown in
Figure 23. This circuit keeps V
MURD320
10nF
CC
V
OUT
source to the ADP3810/ADP3811 can come from a di-
CS
1N4148 100
R3
20k *
OUT
0.2µF
C
C2
. Output ripple voltage is not critical, so the out-
C
1mF
V
F1
300
REF
ADP3810/ADP3811
OUT
R
C2
0.1µF
R
0.25 *
CS
COMP
and the 0.25
R4
1.2k
22µF
0.1µF
R
10k
C
1µF
C1
C1
V
CC
C2
ADP3810/ADP3811
) on the current sense signal is
CC
CS
3.3V
above 2.7 V as long as the
V
input. In this case, the
GND
SENSE
CS
sense resistor. To provide
V
, senses the average cur-
MAXIMUM V
CHARGE CURRENT 0.1A TO 1A
CTRL
R1
80.6k *
R2
20k *
C
220µF
F2
OUT
0.1µF
CC
= +10V
BATTERY
of the
C2
V
CHARGE
CURRENT
CONTROL
VOLTAGE
OUT
to im-