adp3811 Analog Devices, Inc., adp3811 Datasheet - Page 13

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

amplifiers are represented by voltage controlled current sources,
the optocoupler by a current controlled current source, and the
error amplifier by a voltage controlled voltage source.
Design Criteria
Charging a 6 cell NiCad battery.
Max Battery Stack Voltage: V
Max Charge Current:
R
Pick a value for R1:
Calculated Current Sense
Calculated Voltage Sense
Output Filter Cap:
2nd Filter Cap:
Gain of Each Block
ADP3810/ADP3811
ADP3810/ADP3811
ADP3810/ADP3811
Optocoupler:
Voltage Error Amplifier:
Power Stage (General):
Power Stage
Power Stage
The gains for the ADP3810/ADP3811 GM amplifiers are based
on typical measurements of the IC’s open-loop gain, and they
are expressed in units of milliamps per volt. The dc voltage gain
REV. 0
S
Divider:
V
V
Output Buffer:
(Voltage Loop):
Resistor:
(Current Loop):
Fixed Value:
CS
SENSE
Input:
Input:
A
V2
VOLTAGE ERROR
= 0.33V/V
AMPLIFIER
R
V
V
X
C
2R
GM3 = 6 mA/V
ITX
A
GM4 =
GM4 = 0.091 A/V
GM4 = 1.0 A/V
I
R
R1 = 80.6 k
R
R2 = 20 k
C
C
GM1 = 8.3 mA/V
GM2 = 2.1 mA/V
OMAX
V2
OMAX
S
CS
F1
F2
= 20 k
= V
oc
= 1 mF (ESR = 0.1 )
= 200 F (ESR = 0.2 )
= 0.25
= 0.36 mA/mA
COMP
V
Figure 31. Block Diagram of the Linearized Feedback Model
= 1 A
FB
= 6
C
POWER
STAGE
/V
I
OMAX
R
3.3k
X
V
F
+5V
1.67 V = 10 V
= 0.333
C
GM4
ITX
OPTO COUPLER
OC
C
1nF
= 0.36mA/mA
I
F
OMAX
V
OMAX
V
CTRL
R
1.0V
LOAD
OUT
C
1nF
F1
–13–
6mA/V
80k
GM3
of these stages is the value of GM times the load resistance. At the
COMP pin, the internal load resistance, R5, is typically 400 k .
The optocoupler gain is the typical value taken from the
MOC8103 data sheet. The voltage error amplifier gain is due
to the resistor divider internal to the 3845 only. V
put of the internal amplifier, as labeled in Figure 31. The actual
op amp is assumed to have sufficient open-loop gain and band-
width compared to the system bandwidth; as a result, it can be
considered an ideal transimpedance amplifier. The pole created
by the 1 nF capacitor in parallel with R
quency to not affect the compensation.
The power stage gain equation is linearized based on primary
side current mode control with the flyback transformer operat-
ing with discontinuous inductor current.
mum change in output current, which is equal to I
Since the minimum current is 0.0 A, I
maximum change in control voltage is set by internal circuitry
within the 3845 to V
ferent for the voltage and current loop cases. For the voltage
loop without the battery, the effective load is R4, but for the
current loop, the effective load is R
voltage limit has not been reached, so the maximum output
voltage is equal to the maximum output current times the load
resistor. Thus, the entire expression under the square root re-
duces to 1.0. Substituting these values into the general equation
for the power stage yields the specific gain values shown for
GM4.
When calculating the loop gain for the voltage loop and the cur-
rent loop, there are two main differences. First, GM2 applies
only to the voltage loop, and GM1 applies only to the current
loop. Use the appropriate GM input stage for the particular
loop calculations. Second, there are three battery conditions to
consider. For the current loop, the battery is present and un-
charged. Thus, the battery is modeled as a very large capaci-
tance (greater than 1 Farad). For the voltage loop, the battery is
R4
1.2k
20k
R3
V
300
CS
R
R6
200
C2
0.25
8.3mA/V
R
GM1
CS
0.2µF
C
C2
R5
400k
ADP3810/
ADP3811
C
220µF
F2
C
= 1 V. The load resistor, R
COMP
R
C
C1
C1
V
R1
80.6k
R2
20k
BAT
ADP3810/ADP3811
2.0V
2.1mA/V
CS
V
SENSE
GM2
. In the current loop, the
OMAX
F
is high enough in fre-
BATTERY
I
OMAX
= I
OMAX
X
is the maxi-
OMAX
is the out-
LOAD
= 1 A. The
–I
, is dif-
OMIN
.