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 

DP3810/ADP3811

large current levels can be obtained by either reducing the

the ADP3810 is specified with respect to the final battery volt-

Ilue of Res or increasing the value of R3. The main penalty of age. This is tested in a full feedback loop so that the single ac-

Icreasing R3 is lower efficiency due to the larger voltage drop

:ross Res, and the penalty of decreasing Res is lower accuracy

curacy specification given in the specification table accounts for

all of the errors mentioned above. For the ADP3811, the resis-

Jut higher efficiency) as discussed below.

tors are external, so the final voltage accuracy needs to be deter-

'REFOutput

l1e internal band gap reference is not only used internally for

he voltage and current loops, but it is also available externally if

IIIaccurate voltage is needed. The reference employs a pnp

mined by the designer. Certainly, the tolerance of the resistors

has a large impact on the final voltage accuracy, and I % or bet-

ter is recommended.

Supply Range

mtput transistor for low dropout operation. Figure 3 shows a

ypical graph of dropout voltage versus load current. The refer-

The supply range is specified from 2.7 V to 16 V. However, a

final battery voltage option for the ADP38 I0 is 16.8 V. The

~nceis guaranteed to source 5 mA with a dropout voltage of

16.8 V is divided down by the thin film resistors to 2.0 V inter-

iOOmV or less. The 0.11lF capacitor on the reference pin is in- nally. Thus, the input to GM2 never sees much more than 2.0 V,

OBSOLETE tegral in the compensation ofthe referenceand isthereforere-

quired for stable operation. If desired, a larger value of capacitance

can also be used for the application, but a smaller value should

not be used. This capacitor should be located close to the VREF

pin. Additional reference performance graphs are shown in Fig-

ures 2 through 6.

Output Stage

The output stage performs two important functions. It is a

buffer for the compensation node, and as such, it has a high im-

pedance input. It is also a GM stage. The OUT pin is a current

output to enable the direct drive of an optocoupler for isolated

applications. The gain from the COMP node to the OUT pin is

approximately 5 mAN. With a load resistor of I ill, the voltage

gain is equal to five as specified in the data sheet. A different

load resistor results in a gain equal to RLx (5 rnNV). Figures

20 and 21 show how the gain varies from part to part and versus

the supply voltage, respectively. The guaranteed output current

is 5 IDA,which is much more than the typical I mA to 2 mA re-

which is well below the Vcc voltage limit. In fact, Vcc can be

fixed to 2.7 V and the ADP3810 will still control the charging of

a 16.8 V battery stack. The ADP38 I I, with external resistors,

can charge batteries to voltages well in excess of its supply volt-

age. However, if the final battery voltage is above 16 V, Vcc

cannot be supplied directly from the battery as it is in Figure I.

Alternative circuits must be employed as will be discussed later.

Decoupling capacitors should be located close to the supply pin.

The actual value of the capacitors depends on the application,

but at the very least a 0.11lF capacitor should be used.

OFF-UNE, ISOLATED, FLYBACK BATIERY CHARGER

The ADP3810 and ADP38 I I are ideal for use in isolated charg-

ers. Because the output stage can directly drive an optocoupler,

feedback of the control signal across an isolation barrier is a

simple task. Figure 23 shows a complete flyback battery charger

with isolation provided by the flyback transformer and the

optocoupler. The essential operation of the circuit is not much

quired in most applications.

different from the simplified circuit described in Figure I. The

GMI loop controls the charge current, and the GM2 loop con-

Current Loop Accuracy Considerations

trols the final battery voltage. The dc-dc converter block is

The accuracy of the current loop is dependent on several factors comprised of a primary side PWM circuit and flyback trans-

such as the offset ofGMI, the offset of the VCTRLbuffer, the ra- former, and the control signal passes through the optocoupler.

tio of the internal 80 k.Qcompared to the external 20 k.Qresis-

tor, and the accuracy of Res. The specification for current loop The circuit in Figure 23 incorporates all of the features neces-

accuracy states that the full-scale current sense voltage, VRcs,of sary to assure long battery life with rapid charging capability.

-300 mV is guaranteed to be within IS mV of this value. This

By using the ADP3810 for charging uIon batteries, or the

assumes an exact 20 k,Qresistor for R3. Any errors in this resis- ADP38 I I for NiCad and NiMH batteries, component count is

tor will result in further errors in the charge current value. For

minimized, reducing system cost and complexity. With the cir-

example, a 5% error in resistor value will add a 5% error to the

cuit as presented or with its many possible variations, designers

charge current. The same is true for Res, the current sense resis- no longer need to compromise charging performance and bat-

tor. Thus, I % or better resistors are recommended.

tery life to achieve a cost effective system.

As mentioned above, decreasing the value of Res increases the

charge current. Since it is VRcsthat is specified, the actual

value of Res is not accounted for in the specification.An example

= where Res 0.1 .Qillustrates its impact on the accuracy of the

charge current. The range ofVRcs is from -25 mV:t 5 mV to

-300 mV:t 15 mY. This results in a charge current range from

250 mA:t 50 mA to 3 A:t 150 IDA,as opposed to a charge cur-

= rent range of 100 mA:t 20 mA to 1.2 A:t 60 mA for Res

0.25.Q. Thus, not only is the minimum current changed, but

the absolute variation around the set point is increased (although

the percentage variation is the same).

Primary Side Considerations

A typical current-mode flyback PWM controller was chosen for

the primary control circuit for several reasons. First and most

importantly, it is capable of operating from very small duty

cycles to near the maximum designed duty cycle. This makes it

a good choice for a wide input ac supply voltage variation re-

quirement, which is usually between 70 V-270 V ac for world

wide applications. Add to that the additional requirement of

0% to 100% current control, and the PWM duty cycle must

have a wide range. This charger achieves these ranges while

maintaining stable feedback loops.

Voltage Loop Accuracy Considerations

The accuracy of the voltage loop is dependent on the offset of

GM2, the accuracy of the reference voltage, the bias current of

GM2 through Rl and R2, and the ratio of RlIR2. For the de-

manding application of charging uIon batteries, the accuracy of

The detailed operation and design of the primary side PWM is

widely described in the technical literature and is not detailed

here. However, the following explanation should make clear the

reasons for the primary side component choices. The PWM fre-

quency is set to around 100 kHz as a reasonable compromise

-8-

REV. 0

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