ADP3801/ADP3802

–10–

REV. 0

Current Sense Amplifier

A differential, high side current sense amplifier (GM1 in Figure

20) amplifies the voltage drop across a current sense resistor

ground to VCC – 2 V. Sensing to ground ensures current regu-

lation even in short circuit conditions. To stay within the com-

mon-mode range of GM1, VCC must be at least 2 V greater

than the final battery voltage or a circuit such as shown in Fig-

ure 32 must be used. RC filters are included to filter out high

frequency transients, which could saturate the internal circuitry.

The filter’s cutoff is typically set at half the switching frequency

of the oscillator.

The charge current is controlled by the voltage on the ISET pin

according to the following formula:

I

V

R

CHARGE

ISET

CS

=

×

10

The factor of 10 is due the GM1’s gain of 10 V/V. To set a charge

22 shows the linearity of the charge current control as the volt-

age is increased from 0 V to the programmed final battery volt-

age (12.6 V in this case). It is important to state that this curve

is taken with an ideal, zero impedance load. An actual Li-Ion

battery will exhibit a more gradual drop in charge current due to

the internal impedance of the battery as shown in Figure 25.

0

23

45

67

89

10

0.5

11

12

13

1.0

1.5

2.0

2.5

3.0

3.5

Figure 22. CCCV Characteristic with Ideal Load

Overcurrent Comparator

Similar to the voltage loop, the current loop includes a com-

parator to protect the external circuitry from an overcurrent

event. This comparator trips when GM1’s differential input

voltage exceeds 185 mV. Like the overvoltage comparator, it has

two outputs to quickly reduce the duty cycle to 0% and to pro-

vide a soft-start recovery. The response time of the internal

comparator is approximately 1

µs; however, the filter on the

input of GM1 may slow down the total response time of the

loop.

End-of-Charge Output

The ADP3801/ADP3802 provides an active low, end-of-charge

(EOC) logic output to signal when the battery has completed

charging. The typical Li-Ion charging characteristic in Figure 25

shows that when the battery reaches its final voltage the current

decreases. To determine EOC, an internal comparator senses

when the current falls below 6% of full scale, ensuring that the

battery has been fully charged. The comparator has hysteresis to

prevent oscillation around the trip point.

To prevent false triggering (such as during soft-start), the com-

parator is only enabled when the battery voltage is within 5% of

its final voltage. As the battery is charging up, the comparator

will not go low even if the current falls below 6% as long as the

battery voltage is below 95% of full scale. Once the battery has

risen above 95%, the comparator is enabled.

There are two important reasons for this functionality. First,

when the circuit is initially powered on, the charge current is

zero because of the soft start. If the comparator is not gated by

the battery voltage, then EOC would go low erroneously. Sec-

ond, a provision must be made for battery discharge. Assume

that a battery has been fully charged. EOC goes low, and the

charger is gated off. When the battery voltage falls to 95%, due

to self-discharge for example, EOC will return high. Then the

charger can start up and top off the battery, preventing the

battery from “floating” at the end-of-charge voltage.

The EOC output has many possible uses as shown in Figure 23.

One simple function is to terminate the charging to prevent

floating (Figure 23a). It can be used as a logic signal to a

microcontroller to indicate that the battery has finished charg-

ing. The microcontroller can then switch to the next battery if

appropriate or shutdown the ADP3801/ADP3802. It can also be

used to turn on an LED to signal charge completion (Figure

23b). Using a flip-flop, EOC can control the switching from

BATA to BATB (Figure 23d). The RC filter delays switching

between the two batteries to ensure that the output capacitor is

discharged.