LTC4060
W U U
APPLICATIO S I FOR ATIO
U
resistor to the PROG pin can also be used to program charge rates, are generally not recommended. Consult the
current. Note that this will alter the timer periods unless battery manufacturer for recommended periods.
alternate TIMER pin capacitors are also programmed
An external timing source can also be used to drive the
through an analog switch.
TIMER pin for precise or programmed control. The high
levelmustbebetween2.5VandVCCandthelowlevelmust
ThePROGpinprovidesareferencevoltageof1.5V(VPROG
)
that may be tapped for system use. Current loading on bebetween0Vand0.25V.Also,thedrivingsourcemustbe
PROG is multiplied by 930 and appears as increased IMAX. able to overdrive the internal current source and sink
This may be compensated by adjustment of RPROG. Total which is 5% of the current through RPROG
PROG pin current must be limited to 2.3mA otherwise
.
Battery Temperature Sensing
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.
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
Programming the Timer
All LTC4060 internal timing is derived from the internal temperature coefficient (NTC) is shown in Figure 2. Inter-
oscillatorthatisprogrammedwithanexternalcapacitorat nally derived VCC proportional voltages (VCLD, VHTI, VHTC
)
the TIMER pin. The time periods shown in Table 1 scale arecomparedtothevoltageontheNTCinputpintotestthe
directly with the timer period. The programmable safety temperature thresholds. The battery temperature is mea-
timer is used to put a time limit on the entire charge cycle sured by placing the thermistor close to the battery pack.
for the case when charging has not otherwise terminated. In Figure 2, a common 10k NTC thermistor such as a
Murata NTH4G series NTH4G39A103F can be used. RHOT
The time limit is programmed by an external capacitor at
should be a 1% resistor with a value equal to the value of
the TIMER pin and is also dependent on the current set by
thechosenNTCthermistorat45°C(VNTC =VHTI =0.5•VCC
theprogrammingresistorconnectedtothePROGpin. The
time limit is determined by the following equation:
typ). Another temperature may be chosen to suit the
battery requirements. The LTC4060 will not initiate a
tMAX (Hours) = 1.567 • 106 • RPROG (Ω) • CTIMER (F)
chargecycleorcontinuewithaprechargeifthevalueofthe
thermistor falls below 4.42k which is a temperature rising
tMAX (Hours)
1.567 •106 •RPROG (Ω)
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 (VNTC = VHTC = 0.4 • VCC 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 VCC = 5V) and then charging is resumed.
Hysteresisavoidspossibleoscillationaboutthetrippoints.
Note that the comparator hysteresis voltages are constant
and when VCC increases the signal level from the ther-
mistor increases thus making the temperature hysteresis
look smaller.
CTIMER (F) =
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
voltagestabilizationhold-offperiod.Excessivelylongtime-
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
Duringsuspensionthechargecurrentisturnedoffandthe
safety timer is frozen. The LTC4060 is also designed to
suspend when the thermistor rises above 34k (falling
4060f
13
Linear Systems [ Linear Systems ]
