ADP2291ACPZ-R7 datasheet(12/20 Pages) AD

12 / 20 page

ADP2291

Rev. 0 | Page 12 of 20

APPLICATION INFORMATION

SETTING THE MAXIMUM CHARGE CURRENT

The maximum charge current is set by choosing the proper

current sense resistor, RS, and the voltage on the ADJ input. The

charger nominally regulates its output current at the point

where the voltage across the current sense resistor VIN–VCS

(defined as VRS) is 150 mV. This setpoint voltage can be adjusted

by pulling down on the ADJ input, which is internally attached

through a 100 kΩ pull-up resistor to 3 V. Each volt of pull-down

from 3 V will reduce VRS by 67 mV during fast charge. A

minimum of 50 mV is reached when a 100 kΩ resistor is

attached between ADJ and ground. During slow charge the

voltage across the current sense resistor is 15 mV with no

connection to ADJ and drops to 10 mV with a 100 kΩ resistor

attached to ground. Therefore the maximum charge rate IMAX

can be calculated as

)

mΩ

(

)

(

MAX

(1)

where 50 mV ≤ VRS ≤ 150 mV

After determining suitable values for VRS and RS, the value of

VADJ and RADJ can be calculated as

ADJ

)

(

(2)

RADJ = 100 kΩ ×

⎟

⎠

⎞

⎜

⎝

⎛

− ADJ

ADJ

(3)

Examples of resistor combinations are shown in Table 4.

Table 4. Examples of RS and RADJ Selection

IMAX

VRS

VADJ

RADJ

1.5 A

100 mΩ

150 mV

3 V

Open

1 A

100 mΩ

100 mV

2.25 V

300 K

750 mA

100 mΩ

75 mV

1.87 V

167 K

500 mA

100 mΩ

50 mV

1.5 V

100 K

750 mA

200 mΩ

150 mV

3 V

Open

500 mA

200 mΩ

100 mV

2.25 V

300 K

375 mA

200 mΩ

75 mV

1.87 V

167 K

250 mA

200 mΩ

50 mV

1.5 V

100 K

500 mA

300 mΩ

150 mV

3 V

Open

333 mA

300 mΩ

100 mV

2.25 V

300 K

250 mA

300 mΩ

75 mV

1.87 V

167 K

167 mA

300 mΩ

50 mV

1.5 V

100 K

SETTING THE MAXIMUM CHARGE TIME

The maximum charge time is intended as a safety mechanism to

prevent the charger from trickle charging the cell indefinitely. It

does not terminate charging under normal charging conditions,

but only when there is a failure to reach end-of-charge. A typical

cell charges at a 1 C rate in about 1.5 hours, depending on the

cell type, temperature, and manufacturer. Generally, a three

hour time limit is sufficient to prevent a normal charge cycle

from being interrupted by the charge timer. It is recommended

that the cell manufacturer be consulted for timing details.

The maximum charge time is set by selecting the value of the

CTIMER capacitor. Calculate the timer capacitance using

CTIMER = tCHG(minutes) ×

minutes

1800

1 F

(4)

The precharge and end-of-charge periods are 1/6 the duration

of the fast charge time limit. The charge timers are completely

disabled by connecting the TIMER pin to ground. If the timers

are disabled, the FAULT and TIMEOUT states are never

reached, so the timers should only be disabled if charging is

monitored and controlled externally.

EXTERNAL CAPACITORS

Use an input supply capacitor (CIN) with a value in the

1 µF to 10 µF range and place it close to the ADP2291. This

should provide adequate input bypassing, but the selected

capacitor should be checked in the actual application circuit.

Check that the input voltage does not droop or overshoot

excessively during the start-up transient.

Use a battery output capacitor (COUT) with a value of at least

10 µF. This capacitance provides compensation when no battery

load is present. In addition, the battery and interconnections

appear inductive at high frequencies and must be accounted for

when the charger is operated with a battery load. Therefore, a

small amount of output capacitance is necessary to compensate

for the inductive nature of the battery and connections. Use a

minimum output capacitance value of 1 µF for applications

where the battery cannot be removed.

REVERSE INPUT PROTECTION

The diode, D1, shown in Figure 22 through Figure 25 is

optional. It is only required if the input adapter voltage can

be applied with a reverse polarity.

If the adapter voltage is high enough, a Schottky diode is recom-

mended to minimize the voltage difference from the adapter to

the charger input and the power dissipation. Choose a diode

with a continuous current rating high enough to handle battery

charging current at the maximum ambient temperature. Use a

diode whose voltage rating is greater than the maximum

adapter voltage.

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