ADP3810AR-16.8 Analog Devices Inc, ADP3810AR-16.8 Datasheet - Page 8

• Manufacturer Part Number: ADP3810AR-16.8
• Manufacturer: Analog Devices Inc
• Type: Battery Chargerr

Specifications of ADP3810AR-16.8

• Output Voltage: 16.8V
• Operating Supply Voltage (min): 2.7V
• Operating Supply Voltage (max): 16V
• Operating Temp Range: -40C to 85C
• Package Type: SOIC N
• Mounting: Surface Mount
• Pin Count: 8
• Operating Temperature Classification: Industrial
• Lead Free Status / Rohs Status: Not Compliant

-300 mV:t 15 mY. This results in a charge current range from
the absolute variation around the set point is increased (although
the percentage variation is the same).
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
-300 mV is guaranteed to be within IS mV of this value. This
tor. Thus, I % or better resistors are recommended.
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
charge current. The range ofVRcs is from -25 mV:t 5 mV to
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
is 5 IDA,which is much more than the typical I mA to 2 mA re-
quired in most applications.
Current Loop Accuracy Considerations
The accuracy of the current loop is dependent on several factors
tio of the internal 80 k.Qcompared to the external 20 k.Qresis-
tor, and the accuracy of Res. The specification for current loop
accuracy states that the full-scale current sense voltage, VRcs,of
assumes an exact 20 k,Qresistor for R3. Any errors in this resis-
tor will result in further errors in the charge current value. For
example, a 5% error in resistor value will add a 5% error to the
charge current. The same is true for Res, the current sense resis-
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
such as the offset ofGMI, the offset of the VCTRL buffer, the ra-
tegral in the compensation of the reference and is therefore re-
iOOmV or less. The 0.11lF capacitor on the reference pin is in-
~nceis guaranteed to source 5 mA with a dropout voltage of
mtput transistor for low dropout operation. Figure 3 shows a
ypical graph of dropout voltage versus load current. The refer-
IIIaccurate voltage is needed. The reference employs a pnp
he voltage and current loops, but it is also available externally if
l1e internal band gap reference is not only used internally for
'REF Output
Jut higher efficiency) as discussed below.
:ross Res, and the penalty of decreasing Res is lower accuracy
Icreasing R3 is lower efficiency due to the larger voltage drop
large current levels can be obtained by either reducing the
Ilue of Res or increasing the value of R3. The main penalty of
DP3810/ADP3811
=
0.1 .Qillustrates its impact on the accuracy of the
=
-8-
The detailed operation and design of the primary side PWM is
here. However, the following explanation should make clear the
wide applications. Add to that the additional requirement of
have a wide range. This charger achieves these ranges while
maintaining stable feedback loops.
widely described in the technical literature and is not detailed
reasons for the primary side component choices. The PWM fre-
quency is set to around 100 kHz as a reasonable compromise
ADP38 I I for NiCad and NiMH batteries, component count is
no longer need to compromise charging performance and bat-
tery life to achieve a cost effective system.
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
0% to 100% current control, and the PWM duty cycle must
The ADP3810 and ADP38 I I are ideal for use in isolated charg-
feedback of the control signal across an isolation barrier is a
with isolation provided by the flyback transformer and the
optocoupler. The essential operation of the circuit is not much
different from the simplified circuit described in Figure I. The
GMI loop controls the charge current, and the GM2 loop con-
trols the final battery voltage. The dc-dc converter block is
former, and the control signal passes through the optocoupler.
The circuit in Figure 23 incorporates all of the features neces-
sary to assure long battery life with rapid charging capability.
By using the ADP3810 for charging uIon batteries, or the
minimized, reducing system cost and complexity. With the cir-
cuit as presented or with its many possible variations, designers
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
nally. Thus, the input to GM2 never sees much more than 2.0 V,
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
ers. Because the output stage can directly drive an optocoupler,
simple task. Figure 23 shows a complete flyback battery charger
comprised of a primary side PWM circuit and flyback trans-
has a large impact on the final voltage accuracy, and I % or bet-
ter is recommended.
Supply Range
the ADP3810 is specified with respect to the final battery volt-
age. This is tested in a full feedback loop so that the single ac-
curacy specification given in the specification table accounts for
all of the errors mentioned above. For the ADP3811, the resis-
tors are external, so the final voltage accuracy needs to be deter-
mined by the designer. Certainly, the tolerance of the resistors
16.8 V is divided down by the thin film resistors to 2.0 V inter-
REV. 0