ADP3810/ADP3811

–9–

REV. 0

between inductive and capacitive component sizes, switching

losses and cost.

100 k

Ω 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

the 3845 under a shorted or discharged battery condition. There-

fore, a voltage doubler circuit was developed (as shown in Fig-

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-

ADP3811 won’t be present. Thus, the primary side current

limit is the only control of the charge current. As the secondary

and controls the average current. The primary side current limit

is set by the 1.6

Ω current sense resistor connected between the

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

Ω 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

9.1

Ω

3W

1A

L

N

AC

120/220V–

22nF

1N4148

13V

330pF 330

Ω

10k

Ω

10

Ω

1k

Ω

1.6

Ω

470pF

47µF

1nF

3.3k

Ω

3.3k

Ω

100k

Ω

IRFBC30

50µF/450V

TX1**

10nF

1N4148 100

Ω

22µF

1mF

R3

20k

Ω*

MURD320

0.25

Ω*

0.2µF

300

Ω

R4

1.2k

Ω

3.3V

0.1µF

3.3k

Ω

2.2nF

OPTO COUPLER

MOC8103

220µF

0.1µF

0.1µF

R1

80.6k

Ω*

R2

20k

Ω*

10k

Ω

1µF

OUT

COMP

GND

0.1µF

ADP3810/ADP3811

BATTERY

CHARGE

CURRENT

CONTROL

VOLTAGE

CHARGE CURRENT 0.1A TO 1A

* 1% TOLERANCE

** TX1

f = 120kHz

OUTPUT

COMP

RT/CT

GND

PWM

3845

Figure 23. ADP3810/ADP3811 Controlling an Off-Line, Flyback Battery Charger

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

charging current has high ripple due to the flyback architecture,

prove the phase margin of the loop. The 1 mF capacitor is con-

additional decoupling to ground, a 220

µF capacitor is also con-

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.

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

ADP3810/ADP3811 can be boosted using the circuit shown in