Analog Devices ADP2291 Evaluation Board ADP2291RM-EVALZ ADP2291RM-EVALZ 数据表

ADP2291 

 

 

Rev. A | 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 ensure safety and long battery lifetime, the ADP2291 charges 
the battery using a simple step-by-step cycle, as shown in Figure 26. 
The normal charge cycle begins by measuring the battery 
voltage to determine charge level. If the battery is deeply dis-
charged, then low current precharge is initiated. Once precharge 
is complete, normal fast charge at the maximum current (denoted 
as I

MAX

) begins. This maximum 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 is discussed in detail. 

For deeply discharged cells, the ADP2291 charges at a reduced 
rate when the battery voltage V

BAT

 < 2.8 V. This reduced rate is 

I

MAX

/10 when the ADJ pin voltage is 3 V and I

MAX

/5 when the 

ADJ pin voltage is 1.5 V. For ADJ pin voltages in between 3 V 
and 1.5 V, 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. 

Once the voltage loop reduces the charge current to 1/10 of its 
nominal value, I

MAX

 (irrespective of the ADJ voltage), the 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). 

When the ADJ input is pulled below 0.4 V, the ADP2291 is put 
into shutdown mode. When in this mode, the charger is dis-
abled; 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. 

Once the charge is complete in the end-of-charge or timeout 
mode, 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 
Figure 26. 

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 timeout 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. 

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 
can be used to generate a logic-level charge status signal by 
connecting a resistor between CHG and logic high. 

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. 

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 
V

BAT

 rises to 5 V and sinks up to 1.5 A to protect the external 

components. 

To ensure 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 
1 µA from the battery. The V

IN

 good comparator halts operation 

if the supply voltage is less than 165 mV above the battery 
voltage, ensuring that charging occurs only if the supply voltage 
is sufficient.