ADP3810AR-42 AD [Analog Devices], ADP3810AR-42 Datasheet - Page 12

• Manufacturer Part Number: ADP3810AR-42
• Description: Secondary Side, Off-Line Battery Charger Controllers
• Manufacturer: AD [Analog Devices]

ADP3810/ADP3811
The trade-off between using a linear regulator as shown versus
using a flyback or buck type of charger is efficiency versus sim-
plicity. The linear charger in Figure 29 is very simple, and it
uses a minimal amount of external components. However, the
efficiency is poor, especially when there is a large delta between
the input output voltages. The power loss in the pass transistor
is equal to (V
from a wall adapter, efficiency may not be a big concern, but the
heat dissipated in the pass transistor could be excessive.
An important specification for this circuit is the dropout voltage,
which is the difference between the input and output voltage at
full charge current. There must be enough voltage to keep the
N-channel MOSFET on. In this case, the dropout voltage is
approximately 2.2 V for a 0.5 A output current. Two alternative
Figure 30. Alternative Pass Transistor for Linear Regulator
realizations of the pass element are shown in Figure 30. In case
(a), the pass transistor is a P-channel MOSFET. This provides
a lower dropout voltage so that V
dred millivolts of V
two npn transistors is used. The dropout voltage of this circuit
is approximately 2 V for a 0.5 A charge current.
STABILIZATION OF FEEDBACK LOOPS
The ADP3810/ADP3811 uses two transconductance error am-
plifiers with “merged” output stages to create a shared compen-
sation point (COMP) for both the current and voltage loops as
explained previously. Since the voltage and current loops have
significantly different natural crossover frequencies in a battery
ADP3811
a. P-Channel MOSFET
V
V
REF
IN
10k
1k
IN
IRF7205
–V
2N3904
BAT
IN
. In case (b), a Darlington configuration of
)
V
ADP3811
OUT
V
BAT
REF
V
CTRL
I
V
CHARGE
+V
V
RTN
CTRL
RTN
V
REF
IN
IN
&
80.6k
20k
ADP3811
. Since the circuit is powered
R2
R1
BAT
OUT
Figure 29. ADP3811 Controlling a Linear Battery Charger
V
IN
can be within a few hun-
10k
250
0.1µF
b. NPN Darlington
2N3904
0.1µF
0.25 *
2N3904
V
V
V
REF
2N5058
SENSE
CTRL
GND
ADP3811
V
CC
1k
0.1µF
COMP
V
BAT
OUT
V
C
1µF
R
200
CS
C1
–12–
C1
charger application, the two loops need different inverted zero
feedback loop compensations that can be accomplished by two
series RC networks. One provides the needed low frequency
(typical f
other provides a separate high frequency (f
compensation to the current loop. In addition, the current loop
input requires a ripple reduction filter on the V
out switching noise. Instead of placing both RC networks on the
COMP pin, the current loop network is placed between V
ground as shown in Figure 23 (C
two functions, ripple reduction and loop compensation.
Loop Stability Criteria for Battery Charger Applications
1. The voltage loop has to be stable when the battery is
2. The current loop has to be stable when the battery is being
3. Both loops have to be stable within the specified input source
Flyback Charger Compensation
Figure 31 shows a simplified form of a battery charger system
based on the off-line flyback converter presented in Figure 23.
With some modifications (no optocoupler, for example), this
model can also be used for converters such as a Buck Converter
(Figure 28) or a Linear Regulator (Figure 29). GM1 and GM2
are the internal GM amplifiers of the ADP3810/ADP3811, and
GM3 is the buffered output stage that drives the optocoupler.
The primary side in Figure 23 is represented here by the “Power
Stage,” which is modeled as GM4, a linear voltage controlled
current source model of the flyback transformer and switch.
The “Voltage Error Amplifier” block is the internal error ampli-
fier of the 3845 PWM-IC (R
followed by an internal resistor divider. The optocoupler is
modeled as a current controlled current source as shown. Its
output current develops a voltage, V
ues of all the blocks are defined below.
This linear model makes the calculation of compensation values
a manageable task. It also has the great benefit of allowing the
simulation of the ac response using a circuit simulator, such
as PSpice or MicroCap. For computer modeling, the GM
R
560
C
220nF
C2
20k *
C2
removed or floating.
charged within its specified charge current range.
voltage range.
1µF
C
< 100 Hz) compensation to the voltage loop, and the
R8
1k
V
2N3904
BAT
10k
250
220µF
= 2.0V
IRF7201
(
R1
–– + 1
R2
1k
F
)
V
= 3.3 k in Figure 23), and it is
BAT
C2
BATTERY
and R
X
, across R
C2)
C
. Thus, it performs
~ 1 kHz–10 kHz)
CS
F
. The gain val-
pin to filter
REV. 0
CS
and