ADP3801 datasheet(17/20 Pages) AD

17 / 20 page

ADP3801/ADP3802

–17–

REV. 0

The 60 second wait period allows the output capacitor to dis-

charge before switching from one battery to the next. Without

this wait period, the capacitor would be fully charged when

switched to an uncharged battery. The current under this condi-

tion is only limited by the ESR of the capacitor, the ON resis-

tance of the FET and diode, and the series resistance of the

battery. These values are typically very small, so current in

excess of 5 amps can flow for a short time period. In most

practical circuits the wait period is not required, but it is good

practice to have it in

µC controlled systems. The duration of the

wait period is determined by the RC time constant of CO and RB

and can be adjusted by changing these components.

The two Si4463 switches are turned on by connecting their

gates to ground. In a short circuit or overdischarged battery

condition, the switches could be operated in their linear region.

This may result in high power dissipation and excessive die

temperature rise. In

µC controlled chargers a simple monitor

routine can reduce the charge current if the battery voltage is

lower than about 2 V. This should not happen under normal

circumstances as the Li-Ion cells are not discharged below

2.5 V/cell.

Low Overhead Charging

For applications where the input supply is less than 2 V higher

than the final battery voltage, the circuit of Figure 32 can be

used. This circuit adds a resistor divider to the input of the

current sense amplifier to increase its common-mode input

voltage range. The value of this resistor divider should divide

down the battery voltage such that the common-mode voltage at

CS+ and CS– is at least 2 V less than the chip’s VCC. The

formula for the ratio is:

VCC

VBAT

MIN

MAX

≤

−

For example, if VCCMIN = 9 V and VBATMAX = 8.4 V, then the

ratio would be 0.833. To provide some headroom for resistor

tolerances and line drops, the actual ratio should be lowered to

0.8. The resistors should be reasonably large to keep the current

drain low. Values of R1 = 20 k

Ω (0.1%) and R2 = 80 kΩ (0.1%)

work well. A diode is added between the current sense resistor

and the battery to prevent discharging the battery through these

resistors.

ADP3801/

ADP3802

CS+

CS–

20k

80k

430pF

RCS

0.1

430pF

Figure 32. Low Overhead Charging

Because the current sense voltage is divided down by these

input resistors, the current sense programming function also

changes. Remember to adjust the programming function by the

same ratio. In this example, the programming function would

become 0.125 V/V. Also, the EOC current detection point changes

by the same factor.

An alternative to adding the resistor divider is to use a low side

current sense. The CS+ and CS– inputs have a common-mode

range that extends approximately 300 mV below ground. The

circuit in Figure 33 shows how a low side current sense would

be configured. The current programming function, EOC detec-

tion point, and ac performance do not change from the normal

configuration.

Si4463

VCC

DRV

EOC CS–

CS+

ISET

BATA

BATB

A/B

PROG

ADJ

COMP

GND

RESET

VISET

RCS

40m

4.3k

2.2nF

GND

VIN

0.1 F

VBATPROG

210 F

ADP3801/ADP3802

2.2nF

Figure 33. Low Side Current Sensing For Low Overhead

Charging

ADP3801 Datasheet(PDF) 17 Page - Analog Devices

ADP3801 Datasheet(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL