ADP2291

Rev. 0 | Page 10 of 20

THEORY OF OPERATION

The ADP2291 is intended to charge a single cell, lithium battery

from a supply voltage or wall adapter providing 4.5 V to 12 V.

The charge controller adjusts the base current of an external

PNP transistor to optimize current and voltage applied to the

battery during charging. A low value resistor placed in series

with the battery charging current provides current measure-

ment for the ADP2291.

To assure safety and long battery lifetime, the ADP2291 charges

the battery using a simple step-by-step cycle, as shown in the

state diagram of Figure 26. The normal charge cycle begins by

measuring the battery voltage to determine charge level. If the

battery is deeply discharged, then low current precharge is

initiated. Once precharge is complete, normal fast charge at the

current can be adjusted by varying the sense resistor value or by

varying the voltage at the adjust pin. As the battery approaches

full capacity, the charging current is reduced until the end-of-

charge condition is reached. Batteries that are not deeply

discharged skip the precharge mode and immediately begin fast

charging. Each of these modes and associated fault conditions

are discussed in detail.

PRECHARGE MODE

For deeply-discharged cells, the ADP2291 charges at a reduced

ADJ pin voltage is 1.5 V. For ADJ pin voltages in between, the

charge current can be interpolated. If the battery voltage does

not increase past 2.8 V before the precharge timer elapses

(typically 30 minutes), a battery fault is assumed and the

ADP2291 shuts down and does not restart until the input

voltage is cycled OFF and then back ON. Note that in this mode

shutdown commands are ignored.

END-OF-CHARGE MODE

Once the voltage loop reduces the charge current to 1/10 of its

ADP2291 detects the end-of-charge (EOC) state and the charge

status indicator becomes high-impedance.

Low level charging continues until the timer terminates the

charge (nominally 30 minutes).

SHUTDOWN MODE

When the ADJ input is pulled below 0.4 V, the ADP2291 is put

into shutdown mode. When in this mode, the charger is

disabled, the current drawn from the battery falls to less than

1 µA and the current drawn from IN falls to 0.7 mA.

When the charger is re-enabled, the charger returns to the

START state but quickly sequences through the states until the

proper charge mode is reached.

CHARGE RESTART

Once charge is complete in end-of-charge or timeout modes,

the ADP2291 continually monitors the cell voltage and charge

current. When the cell voltage falls by 100 mV or the charge

current increases beyond the EOC hysteresis, the ADP2291

initiates another charge cycle to keep the cell fully charged. See

the state diagram in Figure 26.

PROGRAMMABLE TIMER

The on-chip timer, controlled by an external capacitor

CTIMER, determines the timeout intervals of the various

charger modes. For example, a CTIMER value of 0.1 µF results

in a precharge timeout interval of 30 minutes, a fast charge

timeout of 3 hours, and an end-of-charge timeout of 30

minutes. The ratio between precharge and end-of-charge to fast

charge time-out is always 1/6. All these time intervals are

proportional to the CTIMER capacitor value, allowing them to

be adjusted over a wide range. Connecting the TIMER pin to

ground disables the timer.

CHARGE STATUS INDICATOR

The ADP2291 contains a charge status output, CHG, that sinks

current when the ADP2291 is charging the battery. This output

can be used as a visual signal by connecting it to an LED, or it

may be used to generate a logic-level charge status signal by

connecting a resistor between CHG and logic high.

AUTOMATIC REVERSE ISOLATION

When the voltage on the BAT pin is higher than the voltage on

IN, the ADP2291 automatically connects the base of the pass

device to BAT. This removes the necessity of having an external

diode between the pass device and battery, further reducing the

charger’s footprint and component count.

OVERSHOOT PROTECTION

In the event of a battery disconnect during charging, a voltage

overshoot condition on BAT could occur. The ADP2291

includes an overshoot protection circuit that activates when

components.

POWER SUPPLY CHECKS

To assure proper operation, the ADP2291 checks the absolute

voltage level of the input supply and the supply voltage relative

to the battery. When the supply IN is below 3.8 V, the chip is

internally powered down and does not respond to external

control. In this power-down mode, the device draws less than

if the supply voltage is less than 165 mV above the battery

voltage, insuring that charging only occurs if the supply voltage

is sufficient.