adp3811 Analog Devices, Inc., adp3811 Datasheet - Page 7

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

current loop, the higher I
converter and causes the battery voltage to fall, balancing the
feedback loop.
Each GM stage is designed to be asymmetrical so that each am-
plifier can only source current. The outputs are tied together at
the COMP node and loaded with an internal constant current
sink of approximately 100 A. Whichever amplifier sources
more current controls the voltage at the COMP node and there-
fore controls the feedback. This scheme is a realization of an
analog “OR” function where GM1 or GM2 has control of the
dc-dc converter and the charging circuitry. Whenever the cir-
cuit is in full current limiting or full voltage limiting, the respec-
tive GM stage sources an identical amount of current to the
fixed current sink. The other GM stage sources zero current
and is out of the loop. In the transition region, both GM stages
source some of the current to comprise the full amount of the
current sink. The high gains of GM1 and GM2 ensure a
smooth but sharp transition from current control to voltage con-
trol. Figure 24 shows a graph of the transition from current to
voltage mode, that was measured on the circuit in Figure 23 as
detailed below. Notice that the current stays at its full pro-
grammed level until the battery is within 200 mV of the final pro-
grammed voltage (10 V in this case), which maintains fast
charging through almost all of the battery voltage range. This
improves the speed of charging compared to a scheme that re-
duces the current at lower battery voltages.
The second element in a battery charging system is some form
of a dc-dc converter. To achieve high efficiency, the dc-dc con-
verter can be an isolated off-line switching power supply, or it
can be an isolated or nonisolated Buck or other type of switch-
ing power supply. For lower efficiency requirements, a linear
regulator fed from a wall adapter can be used. In the above dis-
cussion, the current, I
supply; but in the case of the linear regulator, I
pass transistor drive. Examples of these topologies are shown
later in this data sheet. If an off-line supply such as a flyback
converter is used, and isolation between the control logic and
the ADP3810/ADP3811 is required, an optocoupler can be in-
serted between the ADP3810/ADP3811 output and the control
input of the primary side PWM.
Charge Termination
If the system is charging a LiIon battery, the main criteria to de-
termine charge termination is the absolute battery voltage. The
ADP3810, with its accurate reference and internal resistors, ac-
complishes this task. The ADP3810’s guaranteed accuracy
specification of
LiIon battery will not be overcharged. This is especially impor-
tant with LiIon batteries because overcharging can lead to cata-
strophic failure. It is also important to insure that the battery be
charged to a voltage equal to its optimal final voltage (typically
4.2 V per cell). Stopping at less than 1% of full-scale results in
a battery that has not been charged to its full mAh capacity,
reducing the battery’s run time and the end equipment’s operat-
ing time.
The ADP3810/ADP3811 does not include circuitry to detect
charge termination criteria such as – V/ t or T/ t, which are
common for NiCad and NiMH batteries. If such charge termi-
nation schemes are required, a low cost microcontroller can be
added to the system to monitor the battery voltage and tempera-
ture. A PWM output from the microcontroller can subsequently
REV. 0
1% of the final battery voltage ensures that a
OUT
OUT
, controls the duty cycle of a switching
reduces the duty cycle of the dc-dc
OUT
controls the
–7–
program the V
impedance of V
tegrate a PWM output into a dc control voltage.
Compensation
The voltage and current loops have significantly different natu-
ral and crossover frequencies in a battery charger application, so
the two loops most likely need different pole/zero feedback com-
pensation. Figure 1 shows a single RC network from the
COMP node to ground. This is primarily for low frequency
compensation (f
COMP node is shared by both GM stages, this compensation
also affects the current loop. The internal 200
change the zero location of the compensation for the current
loop with respect to the voltage loop. To provide a separate
higher frequency compensation (f
series RC may be needed. A detailed calculation of the com-
pensation values is given later in this data sheet.
ADP3810 and ADP3811 Differences
The main difference between the ADP3810 and the ADP3811
is illustrated in Figure 1. The resistors R1 and R2 are external
for the ADP3811 and internal for the ADP3810. The ADP3810
is specifically designed for LiIon battery charging, and thus, the
internal resistors are precision thin-film resistors laser trimmed
for LiIon cell voltages. Four different final voltage options are
available in the ADP3810: 4.2 V, 8.4 V, 12.6 V, and 16.8 V.
For slightly different voltages to accommodate different LiIon
chemistries, please contact the factory. The ADP3811 does not in-
clude the internal resistors, allowing the designer to choose any
final battery voltage by appropriately selecting the external resis-
tors. Because the ADP3810 is specifically for LiIon batteries,
the reference is trimmed to a tighter accuracy specification of
V
The voltage on the V
level. This input is buffered by an internal single supply ampli-
fier (labeled BUFFER) to allow easy programmability of V
For example, for a fixed charge current, V
resistor divider from the reference output. If a microcontroller
is setting the charge current, a simple RC filter on V
the voltage to be set by a PWM output from the micro. Of
course, a digital-to-analog converter could also be used, but the
high impedance input makes a PWM output the economical
choice. The bias current of V
flows out of the pin.
The guaranteed input voltage range of the buffer is from 0.0 V
to 1.2 V. When V
put of the internal amplifier is fixed at 0.1 V. This corresponds
to a charge current of 100 mA for R
The graph of charge current versus V
relationship. Figure 1 shows a diode in series with the buffer’s
output and a 1.5 M resistor from V
diode prevents the amplifier from sinking current, so for small
input voltages the buffer has an open output. The 1.5 M
resistor forms a divider with the internal 80 k resistor to fix the
output at 0.1 V, i.e., about 10% of the maximum current. This
corresponds to the typical trickle charge current level for NiCad
batteries. When V
current and the output follows the input. The total range of
V
from 100 mA to 1.2 A (for R
1% instead of 2% for the ADP3811.
CTRL
CTRL
Input and Charge Current Programming Range
from 0.0 V. to 1.2 V results in a charge current range
CTRL
CTRL
C
< 100 Hz) of the voltage loop. Since the
CTRL
CTRL
input to set the charge current. The high
enables the inclusion of an RC filter to in-
CTRL
is in the range of 0.0 V to 0.1 V, the out-
rises above 0.1 V, the buffer sources
input determines the charge current
CS
ADP3810/ADP3811
CTRL
= 0.25 , R3 = 20 k ). Larger
C
is typically 25 nA, which
~ 1 kHz–10 kHz), a second
CS
REF
CTRL
= 0.25 , R3 = 20 k .
to this output. The
CTRL
in Figure 7 shows this
can be set by a
resistor does
CTRL
enables
CTRL
.