adp3811 Analog Devices, Inc., adp3811 Datasheet - Page 6

• Manufacturer Part Number: adp3811
• Description: Secondary Side, Off-line Battery Charger Controllers
• Manufacturer: Analog Devices, Inc.

ADP3810/ADP3811
APPLICATIONS SECTION
Functional Description
The ADP3810 and ADP3811 are designed for charging NiCad,
NiMH and LiIon batteries. Both parts provide accurate voltage
sense and current sense circuitry to control the charge current
and final battery voltage. Figure 1 shows a simplified battery
charging circuit with the ADP3810/ADP3811 controlling an
external dc-dc converter. The converter can be one of many
different types such as a Buck converter, Flyback converter or a
linear regulator. In all cases, the ADP3810/ADP3811 maintains
accurate control of the current and voltage loops, enabling the
use of a low cost, industry standard dc-dc converter without
compromising system performance. Detailed realizations of
complete circuits including the dc-dc converter are included
later in this data sheet.
The ADP3810 and ADP3811 contain the following blocks
(shown in Figure 1):
• Two “GM” type error amplifiers control the current loop
• A common COMP node is shared by both GM amplifiers
• A precision 2.0 V reference is used internally and is available
• A current limited buffer stage (GM3) provides a current out-
• An amplifier buffers the charge current programming volt-
• An UVLO circuit shuts down the GM amplifiers and the
• A transient overshoot comparator quickly increases I
Figure 20. Output Gain (V
Distribution
(GM1) and the voltage loop (GM2).
such that an RC network at this node helps compensate both
control loops.
externally for use by other circuitry. The 0.1 F bypass ca-
pacitor shown is required for stability.
put, I
put can directly drive an optocoupler in isolated converter
applications. The dc-dc converter must have a control scheme
such that higher I
not the case, a simple, single transistor inverter can be used
for control phase inversion.
age, V
output when the supply voltage (V
protects the charging system from indeterminate operation.
when the voltage on the “+” input of GM2 rises over 120 mV
above V
quickly recover from overvoltage transients and protect ex-
ternal circuitry.
240
200
160
120
80
40
0
5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0
OUT
CTRL
V
T
R
CC
A
L
REF
OUTPUT GAIN (V
= +25 C
= 1k
, to control an external dc-dc converter. This out-
= +10V
, to provide a high impedance input.
. This clamp shuts down the dc-dc converter to
OUT
OUT
results in lower duty cycle. If this is
/V
COMP
OUT
) – V/V
/V
COMP
CC
) falls below 2.7 V. This
)
Figure 21. Output Gain (V
vs. V
CC
8
7
3
6
5
4
0
R
V
OUT
OUT
L
= 1k
3
= +1.0V
T
T
A
A
= –40 C
= +85 C
6
–6–
V
CC
– Volts
9
Description of Battery Charging Operation
The IC based system shown in Figure 1 charges a battery with a
dc current supplied by a dc-dc converter, which is most likely a
switching type supply but could also be a linear supply where
feasible. The value of the charge current is controlled by the
feedback loop comprised of R
converter and a dc voltage at the V
charge current is set by the voltage, V
upon the choice for the values of R
formula below:
Typical values are R
in a charge current of 1.0 A for a control voltage of 1.0 V. The
80 k resistor is internal to the IC, and it is trimmed to its ab-
solute value. The positive input of GM1 is referenced to
ground, forcing the V
The resistor R
V
at V
1.0 V, V
grammed level (i.e., the charge current increases), the negative
input of GM1 goes slightly below ground. This causes the out-
put of GM1 to source more current and drive the COMP node
high, which forces the current, I
decreases the drive to the dc-dc converter, reducing the charg-
ing current and balancing the feedback loop.
As the battery approaches its final charge voltage, the voltage
loop takes over. The system becomes a voltage source, floating
the battery at constant voltage thereby preventing overcharging.
The constant voltage feature also protects the circuitry that is
actually powered by the battery from overvoltage if the battery is
removed. The voltage loop is comprised of R1, R2, GM2 and
the dc-dc converter. The final battery voltage is simply set by
the ratio of R1 and R2 according to the following equation
(V
If the battery voltage rises above its programmed voltage,
V
current, raising the COMP node voltage and I
RCS
SENSE
REF
T
A
12
RCS
= +25 C
, and it is this voltage that GM1 is regulating. The voltage
OUT
= 2.000 V):
is pulled above V
is equal to –(R3/80 k ) V
RCS
/V
15
COMP
equals –250 mV. If V
CS
18
)
I
converts the charge current into the voltage at
CHARGE
V
CS
BAT
CS
= 0.25
pin to a virtual ground.
REF
Figure 22. V
2.000V
R
. This causes GM2 to source more
0.25
0.20
0.15
0.10
0.05
1
CS
CS
0
–50
, R3, GM1, the external dc-dc
OUT
and R3 = 20 k , which result
V
I
80 k
CTRL
LOAD
RCS
CC
CTRL
CS
R3
–25
, to increase. A higher I
= +10V
CTRL
= 5mA
and R3 according to the
R2
. When V
falls below its pro-
R1
TEMPERATURE – C
SAT
input. The actual
V
0
, and is dependent
CTRL
1
vs. Temperature
25
OUT
CTRL
. As with the
50
equals
75
REV. 0
OUT
100