ADP3810 Analog Devices, ADP3810 Datasheet - Page 8

• Manufacturer Part Number: ADP3810
• Description: Secondary Side/ Off-Line Battery Charger Controllers
• Manufacturer: Analog Devices

ADP3810/ADP3811
charge current levels can be obtained by either reducing the
value of R
increasing R3 is lower efficiency due to the larger voltage drop
across R
(but higher efficiency) as discussed below.
V
The internal band gap reference is not only used internally for
the voltage and current loops, but it is also available externally if
an accurate voltage is needed. The reference employs a pnp
output transistor for low dropout operation. Figure 3 shows a
typical graph of dropout voltage versus load current. The refer-
ence is guaranteed to source 5 mA with a dropout voltage of
400 mV or less. The 0.1 F capacitor on the reference pin is in-
tegral in the compensation of the reference and is therefore re-
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 V
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 mA/V. With a load resistor of 1 k , the voltage
gain is equal to five as specified in the data sheet. A different
load resistor results in a gain equal to R
20 and 21 show how the gain varies from part to part and versus
the supply voltage, respectively. The guaranteed output current
is 5 mA, which is much more than the typical 1 mA to 2 mA re-
quired in most applications.
Current Loop Accuracy Considerations
The accuracy of the current loop is dependent on several factors
such as the offset of GM1, the offset of the V
tio of the internal 80 k compared to the external 20 k resis-
tor, and the accuracy of R
accuracy states that the full-scale current sense voltage, V
–300 mV is guaranteed to be within 15 mV of this value. This
assumes an exact 20 k resistor 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 R
tor. Thus, 1% or better resistors are recommended.
As mentioned above, decreasing the value of R
charge current. Since it is V
value of R
where R
charge current. The range of V
–300 mV
250 mA
rent range of 100 mA
0.25 . 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).
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 R1 and R2, and the ratio of R1/R2. For the de-
manding application of charging LiIon batteries, the accuracy of
REF
Output
CS
CS
CS
CS
, and the penalty of decreasing R
= 0.1
50 mA to 3 A
15 mV. This results in a charge current range from
or increasing the value of R3. The main penalty of
is not accounted for in the specification. An example
illustrates its impact on the accuracy of the
20 mA to 1.2 A
CS
150 mA, as opposed to a charge cur-
. The specification for current loop
RCS
RCS
that is specified, the actual
is from –25 mV
CS
L
, the current sense resis-
60 mA for R
(5 mA/V). Figures
CS
CTRL
is lower accuracy
CS
increases the
buffer, the ra-
5 mV to
CS
RCS
=
REF
, of
–8–
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
has a large impact on the final voltage accuracy, and 1% or bet-
ter is recommended.
Supply Range
The supply range is specified from 2.7 V to 16 V. However, a
final battery voltage option for the ADP3810 is 16.8 V. The
16.8 V is divided down by the thin film resistors to 2.0 V inter-
nally. Thus, the input to GM2 never sees much more than 2.0 V,
which is well below the V
fixed to 2.7 V and the ADP3810 will still control the charging of
a 16.8 V battery stack. The ADP3811, 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, V
cannot be supplied directly from the battery as it is in Figure 1.
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 0.1 F capacitor should be used.
OFF-LINE, ISOLATED, FLYBACK BATTERY CHARGER
The ADP3810 and ADP3811 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
different from the simplified circuit described in Figure 1. The
GM1 loop controls the charge current, and the GM2 loop con-
trols the final battery voltage. The dc-dc converter block is
comprised of a primary side PWM circuit and flyback trans-
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 LiIon batteries, or the
ADP3811 for NiCad and NiMH batteries, component count is
minimized, reducing system cost and complexity. With the cir-
cuit as presented or with its many possible variations, designers
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
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.
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
CC
voltage limit. In fact, V
CC
can be
REV. 0
CC