ADP3810AR-12.6 データシートの表示（PDF） - Analog Devices

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ADP3810AR-12.6

Secondary Side, Off-Line Battery Charger Controllers

Analog Devices 

100kQ

ADP3810/ADP3811

10nF 1N4148 100Q

+1 CFT

1mF

R4

1.2ka

R3

20ka"

RcS

0.25Q"

3.3V I

CF2

220llF

R1

80.6ka"

r

:: VOUT

BATTERY

R2

,1

20ka"

CC2 Rc2

Vcc

.COMP

O.2pF 3O0Q

~

MAXIMUM

CHARGE

= VOUT +10V

CURRENT 0.1A TO 1A

RF

CF

3.3kQ

1nF

0.111F 0.111F

3.3ka

~ " 1% TOLERANCE

.. TX1

f = 120kHz

OLpA = 750llH

B I LSEC=7.5I1H

VFB

PWM ISENSE

3845

1.6Q

VAEF

OPTO COUPLER

MOC81 03

VCS

VAEF

Vcc

VSENSE

ADP3810/ADP3811

VCTAl

OUT

COMP

GND

IO"I1F

~~~

RC1

10ka

CHARGE

CURRENT

CONTROL

VOLTAGE

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

L between inductive and capacitive component sizes, switching

losses and cost.

E The primary PWM-IC circuit derives its starting Vee through a

T 100 kQ resistor directly from the rectified ac input. After start-

E up, a conventional bootstrapped sourcing circuit from an auxil-

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

iary flyback winding wouldn't work, since the flyback voltage

is used. Only a single diode is needed for rectification

would be reduced below the minimum Vee level specified for

(MURD320 in Figure 23), and no filter inductor is required.

the 3845 under a shorted or discharged battery condition. There- The diode also prevents the battery from back driving the

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

charger when input power is disconnected. A 1 mF capacitor

ure 23) that provides the minimum required Vee for the IC

filters the transformer current, providing an average dc current

across the specified ac voltage range even with a shorted battery. to charge the battery. The resistor, Res, senses the average cur-

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

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

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

power NMOS transistor, IRFBC30, and ground.

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

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 VOUTand the 0.25 Q sense resistor. To provide

additional decoupling to ground, a 220 ~ 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 current drive of the ADP3810/ADP3811's output stage di-

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

cycle of the converter. Approximately 1 mA should be the

The Vee 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 ADP38101

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 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

REV. 0

-9-

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