ADP3810/ADP3811

–10–

REV. 0

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

above 2.7 V. This is because of the additional energy in the

flyback transformer, which transfers more energy through the

µF bypass capacitor on V

CC

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:

× 1.67 V = 10 V

Max Charge Current:

Max Control Voltage:

Ω

Pick a Value for R1:

R1 = 80.6 k

Ω

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:

= 0.25 W, 1%, 0.5 W

Battery Divider, R2:

× R1/(V

R2 = 2

× 80.6 kΩ/(10 V–2 V) =

20.15 k

Ω, Pick 20.0 kΩ

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.

1.0

0.3

0.0

211

3

456

789

10

0.9

0.4

0.2

0.1

0.8

0.6

0.7

0.5

Figure 24. Charge Current vs. Battery Voltage at Four Set-

tings for the Flyback Charger in Figure 23

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

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.

10

0%

100

90

0.1sec/DIV

2V/DIV

NO BATTERY

10V

0V

Figure 25. Flyback Charger Output Voltage Transient at

Power Turn On, No Battery Attached

10

0%

100

90

20msec/DIV

0.2V/DIV

0.0V

–200mV

Figure 26. Charge Current Transient Response to Battery

Connect/Disconnect