LTC4060_15 Datasheet, PDF (12/20 Pages) Linear Technology – Standalone Linear NiMH/NiCd Fast Battery Charger

LTC4060
U
OPERATIO
Battery Chemistry Selection
Insertion and Removal of Batteries
The desired battery chemistry is selected by programming
the CHEM pin to the proper voltage. When wired to GND,
a set of parameters specific to charging NiMH cells is
selected. When CHEM is connected to V_CC, charging is
optimized for NiCd cells. The various charging parameters
are detailed in Table 2.
Cell Selection
The number of series cells is selected using the SEL0 and
SEL1 pins. For one cell, both pins connect to GND. For two
cells, SEL0 connects to V_CC and SEL1 to GND. For three
cells, SEL0 connects to GND and SEL1 to V_CC. For four
cells, both connect to V_CC.
The LTC4060 automatically senses the insertion or re-
moval of a battery by monitoring the V_CELL pin voltage.
Either the charge current, or if not charging then an
internal pull-up current (I_BRD), will pull V_CELL up when the
battery is removed. When this voltage rises above 2.05V
(V_BR) for a time greater than the battery removal event
delay shown in Table 1, the LTC4060 considers the battery
to be absent. Inserting a battery, causing V_CELL to fall
below both V_BR and 1.95V (V_BOV) for a period longer than
the battery insertion event delay shown in Table 1, results
in the LTC4060 recognizing a battery present and initiates
a completely new charge cycle beginning with charge
qualification. All battery currents are inhibited while in
shutdown.
Table 2. LTC4060 Charging Parameters
STATE
CHEM CHARGE TIME LIMIT
Precharge
Both
t_MAX
Fast Charge NiCd
t_MAX
NiMH
t_MAX
T_MIN
T_MAX
I_CHRG TYPICAL TERMINATION CONDITION
5°C
45°C
I_MAX/5 V_CELL ≥ 0.9V
5°C
55°C
I_MAX –16mV Per Cell After Initial t_MAX/12 Delay
5°C
55°C
I_MAX –8mV Per Cell After Initial t_MAX/12 Delay
APPLICATIO S I FOR ATIO
Programming Charge Current
The battery charge current is set with an external program
resistor connected from the PROG pin to GND. The for-
mula for the battery fast charge current or I_MAX is:
I_MAX
=
(^I_PROG)
•
930
=
⎛ 1.5V ⎞
⎝⎜ R_PROG ⎠⎟
•
930
or
R_PROG
=
1395
I_MAX
where R_PROG is the total resistance from the PROG pin to
ground. For example, if 1A of fast charge current is
required:
R_PROG
=
1395
1A
=
1.4k
1%
Resistor
Under precharge conditions, the current is reduced to
20% of the fast charge value (I_MAX).The LTC4060 is
designed for a maximum current of 2A. This translates to
a maximum PROG pin current of 2.15mA and a minimum
program resistor of 698Ω. Reduced accuracy at low
current limits the useful fast charge current to a minimum
of approximately 200mA. Errors in the charge current can
be statistically approximated as follows:
One Sigma Error ≅ 7mA
For best stability over temperature and time, 1% metal-
film resistors are recommended. Capacitance on the PROG
pin should be limited to about 75pF to insure adequate AC
phase margin for its amplifier.
Different charge currents can be programmed by various
means such as by switching in different program resis-
tors. A voltage DAC connected through a resistor to the
PROG pin or a current DAC connected in parallel with a
4060f
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