ADP2291
Rev. A | Page 12 of 20
APPLICATION INFORMATION
SETTING THE MAXIMUM CHARGE CURRENT
The maximum charge current is set by choosing the proper
current sense resistor, R
S
, 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 V
IN
– V
CS
(defined as V
RS
) is 150 mV. This setpoint voltage can be adjusted
by pulling down on the ADJ input, which is internally attached
through a 100 k
Ω pull-up resistor to 3 V. Each volt of pull-down
from 3 V reduces V
RS
by 67 mV during fast charge. A minimum
of 50 mV is reached when a 100 k
Ω 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
it drops to 10 mV with a 100 k
Ω resistor attached to ground.
Therefore, the maximum charge rate I
MAX
is calculated as
)mΩ(
)mV(
S
RS
MAX
R
V
I =
(1)
where 50 mV ≤ V
RS
≤ 150 mV.
After determining suitable values for V
RS
and R
S
, the value of
V
ADJ
and R
ADJ
are calculated as
V
mV
mVmV
7.66
50)( +
=
RS
ADJ
V
V
(2)
R
ADJ
= 100 kΩ ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
ADJ
ADJ
VV
V
3
(3)
Examples of resistor combinations are shown in Table 4.
Table 4. Examples of RS and R
ADJ
Selection
I
MAX
RS mΩ V
RS
mV V
ADJ
V R
ADJ
1.5 A 100 150 3 Open
1 A 100 100 2.25 300 K
750 mA 100 75 1.87 167 K
500 mA 100 50 1.5 100 K
750 mA 200 150 3 Open
500 mA 200 100 2.25 300 K
375 mA 200 75 1.87 167 K
250 mA 200 50 1.5 100 K
500 mA 300 150 3 Open
333 mA 300 100 2.25 300 K
250 mA 300 75 1.87 167 K
167 mA 300 50 1.5 100 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 condi-
tions, 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 recom-
mended 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 = t
CHG
(minutes) ×
minutes1800
µF1
(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 required only 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 with a
voltage rating greater than the maximum adapter voltage.
