ADP3810AR-8.4 Analog Devices, ADP3810AR-8.4 Datasheet - Page 9

• Manufacturer Part Number: ADP3810AR-8.4
• Manufacturer: Analog Devices

ADP3811 won't be present. Thus, the primary side current
power NMOS transistor, IRFBC30, and ground.
The current drive of the ADP3810/ADP3811's output stage di-
between inductive and capacitive component sizes, switching
The primary PWM-IC circuit derives its starting Vee through a
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 Vee level specified for
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 Vee for the IC
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, Vee to the ADP38101
limit is the only control of the charge current. As the secondary
side Vee rises above 2.7 V, the ADP3810/ADP3811 takes over
and controls the average current. The primary side current limit
is set by the 1.6 Q current sense resistor connected between the
rectly connects to the photo diode 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 kQ feedback resistor, RFB'setting the voltage at the
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
losses and cost.
cycle of the converter. Approximately 1 mA should be the
REV. 0
100 kQ resistor directly from the rectified ac input. After start-
.. TX1
" 1% TOLERANCE
f = 120kHz
LpA = 750llH
LSEC=7.5I1H
3.3kQ
3.3ka
RF
Figure
CF
1nF
23. ADP3810/ADP3811
.COMP
VFB
PWM
3845
Vcc
100kQ
ISENSE
VAEF
OPTO COUPLER
MOC81 03
Controlling
-9-
1.6Q
The diode also prevents the battery from back driving the
filters the transformer current, providing an average dc current
needed. This filter has an extra inverted zero due to Rez to im-
prove the phase margin of the loop. The 1 mF capacitor is con-
nected between VOUT and the 0.25 Q sense resistor. To provide
nected to VOUT' Output ripple voltage is not critical, so the out-
put capacitor was selected for lowest cost instead of lowest
The Vee source to the ADP3810/ADP3811 can come from a di-
voltage is less then 2.7 V (e.g., with a shorted battery, or a bat-
ADP3811 will be in Undervoltage Lock Out (UVLO) and will
not drive the optocoupler. In this condition, the primary PWM
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.
charger when input power is disconnected. A 1 mF capacitor
to charge the battery. The resistor, Res, senses the average cur-
rent which is controlled via the Ves input. In this case, the
charging current has high ripple due to the flyback architecture,
so a low-pass filter (R3 and Cez) on the current sense signal is
additional decoupling to ground, a 220
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.
rect connection to the battery as long as the battery voltage re-
mains below the specified 16 V operating range. If the battery
tery discharged below it's minimum voltage), the ADP38101
circuit will run at its designed current limit. The Vee of the
ADP3810/ADP3811 can be boosted using the circuit shown in
Figure 23. This circuit keeps Vee above 2.7 V as long as the
an Off-Line, Flyback Battery Charger
10nF
VCS
OUT
1N4148
20ka"
R3
O.2pF
CC2
+1 CFT
~
1mF
VAEF
3O0Q
ADP3810/ADP3811
100Q
Rc2
0.111F
RcS
0.25Q"
~
COMP
I
R4
1.2ka
0.111F
RC1
10ka
~~~
Vcc
ADP3810/ADP3811
3.3V I
VSENSE
GND
VCTAl
~
MAXIMUM
CHARGE
R1
80.6ka"
20ka"
R2
CF2
220llF
capacitor is also con-
CURRENT
IO"I1F
VOUT
r
,1
::
=
+10V
BATTERY
0.1A
VOUT
CHARGE
CURRENT
CONTROL
VOLTAGE
TO 1A