adp3811 Analog Devices, Inc., adp3811 Datasheet - Page 10

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

ADP3810/ADP3811
battery voltage is at least 1.5 V with a programmed charge cur-
rent of 0.1 A. For a higher programmed charge current, the
battery voltage can drop below 1.5 V, and V
above 2.7 V. This is because of the additional energy in the
flyback transformer, which transfers more energy through the
10 nF capacitor to V
stores the energy transferred through the 10 nF capacitor.
Secondary Side Component Calculations
Design Criteria:
Charging a 6 cell NiCad battery.
Max Individual Cell Voltage:
Max Battery Stack Voltage:
Max Charge Current:
Max Control Voltage:
R
Pick a Value for R1:
The voltage limit of 10 V is approximately 10% above the maxi-
mum fully charged voltage when – V/ t termination is used.
This limit gives a second level of protection without interfering
with – V/ t charge termination.
Component Value Calculations:
Current Sense Resistor:
Battery Divider, R2:
The final voltage and charge current accuracy is dependent
upon the resistor tolerances. Choose appropriate tolerances for
the desired accuracy. One percent accuracy is recommended.
Charger Performance Summary
The charger circuit properly executes the charging algorithm ex-
hibiting stable operation regardless of battery conditions, includ-
ing an open circuit load. The circuit can charge to other battery
voltages by modifying only the battery voltage sense divider. As
would be expected, circuit efficiency is best at high battery volt-
ages. Replacing the output blocking rectifier diode with a
Schottky would improve efficiency if the Schottky’s leakage
could be tolerated, and its reverse voltage rating met the appli-
cation requirement.
Figure 24. Charge Current vs. Battery Voltage at Four Set-
tings for the Flyback Charger in Figure 23
S
Fixed Value:
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
2
3
V
V
V
V
CTRL
CTRL
CTRL
CTRL
CC
4
. The 22 F bypass capacitor on V
= 1.0V
= 0.5V
= 0.25V
= 0.125V
R
= 0.25 W, 1%, 0.5 W
R2 = V
R2 = 2
20.15 k , Pick 20.0 k
5
CS
= V
V
V
I
V
R
R1 = 80.6 k
6
OMAX
CELLMAX
OMAX
CTRL
S
V
REF
OUT
= 20 k
CTRL
80.6 k /(10 V–2 V) =
7
= 1 A
= 1 V (for I
= 6 1.67 V = 10 V
/(4
R1/(V
= 1.67 V
8
CC
OMAX
I
is still maintained
9
OMAX
OMAX
–V
10
) = 1/(4
REF
= 1 A)
11
)
CC
1)
–10–
The Battery Charge Current vs. Battery Voltage characteristics
for four different charge current settings are given in Figure 24.
The high gain of the internal amplifiers ensures the sharp transi-
tion between current mode and voltage mode regardless of the
charge current setting. The fact that the current remains at full
charging until the battery is very close to its final voltage ensures
fast charging times.
The transient performance for various turn-on and turn-off con-
ditions is detailed in Figures 25, 26 and 27. Figure 25 shows
the output voltage when power is applied with no battery con-
nected. As shown, the output voltage quickly rises and over-
shoots its set voltage. The internal comparator responds to this
and clamps the voltage giving a quick recovery. Without the in-
ternal comparator, an external zener would be required to clamp
the voltage to the LED anode. Figure 26 shows the battery cur-
rent when connecting and disconnecting a battery. The actual
trace shown is the voltage across R
rent flowing into the battery. There is an overshoot when the
battery is connected, but the loop quickly takes control and lim-
its the average current to the programmed 0.75 A. When the
battery is removed, the current quickly returns to zero. The
solid band on the scope is due to the current rising and falling
with the switching of the PWM. The time scale is too slow to
show the detail of this. Figure 27 shows the output voltage
when a battery stack charged to 6 V is connected and then dis-
connected. As expected, when the battery is connected, the
voltage immediately goes to 6 V. When the battery is discon-
nected, the voltage returns to the programmed float voltage of
10 V. Again, a small overshoot is present that is clamped by the
internal comparator.
Figure 25. Flyback Charger Output Voltage Transient at
Power Turn On, No Battery Attached
Figure 26. Charge Current Transient Response to Battery
Connect/Disconnect
–200mV
0.0V
10V
0V
100
0%
100
90
10
0%
90
10
0.2V/DIV
2V/DIV
T
NO BATTERY
V
A
IN
= +25 C
= 220V
AC
CS
T
V
V
A
CTRL
IN
, which is negative for cur-
= +25 C
= 220V
20msec/DIV
= 0.775V
0.1sec/DIV
AC
REV. 0