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ADP3810AR-126

Secondary Side, Off-Line Battery Charger Controllers

Analog Devices 

ADP3810/ADP3811

L

AC

120/220V–

N

9.1Ω

3W

1A

RF

3.3kΩ

3.3kΩ

* 1% TOLERANCE

** TX1

f = 120kHz

LPR = 750µH

LSEC = 7.5µH

50µF/450V

100kΩ

1N4148 22nF

TX1**

10nF 1N4148 100Ω

22µF

CF1

1mF

R4

1.2kΩ

3.3V

CF2

220µF

R1

80.6kΩ*

VOUT

BATTERY

13V

47µF

VCC

COMP

CF

1nF

VFB

OUTPUT

PWM

3845

ISENSE

VREF

RT/CT

GND

3.3kΩ

2.2nF

10Ω

10kΩ

1kΩ

470pF

0.1µF

MURD320

330pF 330Ω

IRFBC30

R3

20kΩ*

RCS

0.25Ω*

CC2

0.2µF

RC2

300Ω

0.1µF 0.1µF

1.6Ω

VCS

OUT

VREF

VCC

ADP3810/ADP3811

COMP

CC1

1µF

RC1

10kΩ

R2

20kΩ*

MAXIMUM VOUT = +10V

CHARGE CURRENT 0.1A TO 1A

VSENSE

VCTRL

GND

0.1µF

CHARGE

CURRENT

CONTROL

VOLTAGE

OPTO COUPLER

MOC8103

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

between inductive and capacitive component sizes, switching

losses and cost.

The primary PWM-IC circuit derives its starting VCC through a

100 kΩ 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 VCC 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 VCC 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, VCC to the ADP3810/

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

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

side VCC 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 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 kΩ 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

cycle of the converter. Approximately 1 mA should be the

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, RCS, senses the average cur-

rent which is controlled via the VCS input. In this case, the

charging current has high ripple due to the flyback architecture,

so a low-pass filter (R3 and CC2) on the current sense signal is

needed. This filter has an extra inverted zero due to RC2 to im-

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

nected between VOUT and the 0.25 Ω sense resistor. To provide

additional decoupling to ground, a 220 µF capacitor is also con-

nected to VOUT. Output ripple voltage is not critical, so the out-

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 VCC source to the ADP3810/ADP3811 can come from a di-

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 VCC of the

ADP3810/ADP3811 can be boosted using the circuit shown in

Figure 23. This circuit keeps VCC above 2.7 V as long as the

REV. 0

–9–

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