ADP3810AR-8.4 Analog Devices, ADP3810AR-8.4 Datasheet - Page 12

• Manufacturer Part Number: ADP3810AR-8.4
• Manufacturer: Analog Devices

is approximately 2 V for a 0.5 A charge current.
The ADP3810/ADP3811 uses two transconductance error am-
plifiers with "merged" output stages to create a shared compen-
explained previously. Since the voltage and current loops have
significantly different natural crossover frequencies in a battery
N-channel MOSFET on. In this case, the dropout voltage is
realizations of the pass element are shown in Figure 30. In case
a lower dropout voltage so that VBAT can be within a few hun-
dred millivolts of VIN. In case (b), a Darlington configuration of
two npn transistors is used. The dropout voltage of this circuit
STABIUZATION OF FEEDBACK LOOPS
sation point (COMP) for both the current and voltage loops as
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
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
approximately 2.2 V for a 0.5 A output current. Two alternative
Figure 30. Alternative Pass Transistor for Linear Regulator
(a), the pass transistor is a P-channel MOSFET. This provides
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 (Vrn-VBAT) x IeHARGE. S ince the circuit is powered
ADP3811
a. P-Channel MOSFET
VREF
V'N
VeAT
ADP3811
OUT
VREF
VCTRL &
VCTRL
+V'N'
VREF
RTN
RTN
V'N,
80.61ill
201ill
ADP3811
R1
R2
OUT
Figure 29. ADP3811 Controlling a Linear Battery Charger
V'N
b. NPN Darlington
0.1pF
0.25>2'
VREF
VSENSE
VCTRL
2N5058
GND
ADP3811 vcs
VCC
0.1pF
CaMP
VeAT
q
OUT
2000
CCI
RCI
1pF
-12-
The primary side in Figure 23 is represented here by the "Power
The "Voltage Error Amplifier" block is the internal error ampli-
fier of the 3845 PWM-IC (RF
followed by an internal resistor divider. The optocoupler is
ues of all the blocks are defined below.
This linear model makes the calculation of compensation values
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
other provides a separate high frequency (fe
compensation to the current loop. In addition, the current loop
input requires a ripple reduction filter on the Ves pin to filter
out switching noise. Instead of placing both RC networks on the
CaMP pin, the current loop network is placed between Ves and
ground as shown in Figure 23 (Cez and Rcz). Thus, it performs
two functions, ripple reduction and loop compensation.
Loop Stability Criteria for Battery Charger Applications
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). GMI and GM2
are the internal GM amplifiers of the ADP3810/ADP3811, and
GM3 is the buffered output stage that drives the optocoupler.
Stage," which is modeled as GM4, a linear voltage controlled
current source model of the flyback transformer and switch.
modeled as a current controlled current source as shown. Its
output current develops a voltage, Vx, across RF. The gain val-
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
(typical fe < 100 Hz) compensation to the voltage loop, and the
1. The voltage loop has to be stable when the battery is
560>2
RC2
CC2
220nF
201ill'
removed or floating.
voltage range.
charged within its specified charge current range.
'!:L
;1PF
R8
11ill
VeAT=2.0V(!!!+1)
101ill
250>2
220pF
IRF7201
R2
11ill
=
veAT
3.3 k.Qin Figure 23), and it is
~
BATTERY
-
1 kHz-10kHz)
REV, 0