LTC4060 datasheet(13/20 Pages) LINER

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LTC4060

4060f

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 (VPROG)

that may be tapped for system use. Current loading on

PROG is multiplied by 930 and appears as increased IMAX.

This may be compensated by adjustment of RPROG. 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:

tMAX (Hours) = 1.567 • 106 • RPROG (Ω) • CTIMER (F)

Hours

TIMER

MAX

PROG

()

.•

•

( )

Ω

1 567 106

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

APPLICATIO S I FOR ATIO

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 VCCand 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 RPROG.

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 VCC proportional voltages (VCLD, VHTI, VHTC)

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. RHOT

should be a 1% resistor with a value equal to the value of

the chosen NTC thermistor at 45°C (VNTC = VHTI = 0.5 • VCC

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 (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.

Hysteresis avoids possible oscillation about the trip points.

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.

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

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