LTC4060_15 Datasheet, PDF (13/20 Pages) Linear Technology – Standalone Linear NiMH/NiCd Fast Battery Charger

LTC4060
APPLICATIO S I FOR ATIO
resistor to the PROG pin can also be used to program
current. Note that this will alter the timer periods unless
alternate TIMER pin capacitors are also programmed
through an analog switch.
The PROG pin provides a reference voltage of 1.5V (V_PROG)
that may be tapped for system use. Current loading on
PROG is multiplied by 930 and appears as increased I_MAX.
This may be compensated by adjustment of R_PROG. Total
PROG pin current must be limited to 2.3mA otherwise
absolute maximum ratings will be exceeded. When the
LTC4060 is in the shutdown mode, the PROG pin is forced
to ground potential to save power.
Programming the Timer
All LTC4060 internal timing is derived from the internal
oscillator that is programmed with an external capacitor at
the TIMER pin. The time periods shown in Table 1 scale
directly with the timer period. The programmable safety
timer is used to put a time limit on the entire charge cycle
for the case when charging has not otherwise terminated.
The time limit is programmed by an external capacitor at
the TIMER pin and is also dependent on the current set by
the programming resistor connected to the PROG pin. The
time limit is determined by the following equation:
t_MAX (Hours) = 1.567 • 10⁶ • R_PROG (Ω) • C_TIMER (F)
C_TIMER
(F)
=
t_MAX (Hours)
1.567 • 10⁶ • R_PROG
(Ω)
Some typical timing values are detailed in Table 1. The
timer begins at the start of a charge cycle. After the time-
out occurs, the charge current stops and the CHRG output
assumes a high impedance state to indicate that the
charging has stopped.
Excessively short time-out periods may not allow enough
time for the battery to receive full charge or may result in
premature –∆V termination due to too short a battery
voltage stabilization hold-off period. Excessively long time-
out periods may indicate too low a charge current which
may not allow voltage-based termination (–∆V) to work
properly. Time-out limits of less than 0.75 hour for faster
2C charge rates, or more than 3.5 hours for slower C/2
charge rates, are generally not recommended. Consult the
battery manufacturer for recommended periods.
An external timing source can also be used to drive the
TIMER pin for precise or programmed control. The high
level must be between 2.5V and V_CC and the low level must
be between 0V and 0.25V. Also, the driving source must be
able to overdrive the internal current source and sink
which is 5% of the current through R_PROG.
Battery Temperature Sensing
Temperature sensing is optional in LTC4060 applications.
To disable temperature qualification of all charging opera-
tions, the NTC pin must be wired to ground. A circuit for
temperature sensing using a thermistor with a negative
temperature coefficient (NTC) is shown in Figure 2. Inter-
nally derived V_CC proportional voltages (V_CLD, V_HTI, V_HTC)
are compared to the voltage on the NTC input pin to test the
temperature thresholds. The battery temperature is mea-
sured by placing the thermistor close to the battery pack.
In Figure 2, a common 10k NTC thermistor such as a
Murata NTH4G series NTH4G39A103F can be used. R_HOT
should be a 1% resistor with a value equal to the value of
the chosen NTC thermistor at 45°C (V_NTC = V_HTI = 0.5 • V_CC
typ). Another temperature may be chosen to suit the
battery requirements. The LTC4060 will not initiate a
charge cycle or continue with a precharge if the value of the
thermistor falls below 4.42k which is a temperature rising
to approximately 45°C. However, once fast charging is in
progress, it will not be stopped until the thermistor drops
below 3k which is a temperature rising to approximately
55°C (V_NTC = V_HTC = 0.4 • V_CC typ). Once reaching this
charge cutoff threshold, charging is suspended until the
value of the thermistor rises above approximately 4.8k
(falling temperature) or approximately 43°C (45°C – 2°C
hysteresis at V_CC = 5V) and then charging is resumed.
Hysteresis avoids possible oscillation about the trip points.
Note that the comparator hysteresis voltages are constant
and when V_CC increases the signal level from the ther-
mistor increases thus making the temperature hysteresis
look smaller.
During suspension the charge current is turned off and the
safety timer is frozen. The LTC4060 is also designed to
suspend when the thermistor rises above 34k (falling
4060f
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