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LTC3586/LTC3586-1

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To make the CHRG pin easily recognized by both humans

and microprocessors, the pin is either LOW for charging,

HIGH for not charging, or it is switched at high frequency

(35kHz) to indicate the two possible faults, unresponsive

battery and battery temperature out of range.

When charging begins, CHRG is pulled low and remains

low for the duration of a normal charge cycle. When

charging is complete, i.e., the BAT pin reaches the float

voltage and the charge current has dropped to one tenth

of the programmed value, the CHRG pin is released (Hi-Z).

If a fault occurs, the pin is switched at 35kHz. While

switching, its duty cycle is modulated between a high

and low value at a very low frequency. The low and high

duty cycles are disparate enough to make an LED appear

to be on or off thus giving the appearance of “blinking”.

Each of the two faults has its own unique “blink” rate for

human recognition as well as two unique duty cycles for

machine recognition.

The CHRG pin does not respond to the C/10 threshold if

the LTC3586/LTC3586-1 are in VBUS current limit. This

preventsfalseend-of-chargeindicationsduetoinsufﬁcient

power available to the battery charger.

Table 3 illustrates the four possible states of the CHRG

pin when the battery charger is active.

Table 3. CHRG Signal

STATUS

FREQUENCY

MODULATION

(BLINK) FREQUENCY

DUTY CYCLES

Charging

0Hz

0Hz (Lo-Z)

100%

Not Charging

0Hz

0Hz (Hi-Z)

NTC Fault

35kHz

1.5Hz at 50%

6.25% to 93.75%

Bad Battery

35kHz

6.1Hz at 50%

12.5% to 87.5%

An NTC fault is represented by a 35kHz pulse train whose

duty cycle varies between 6.25% and 93.75% at a 1.5Hz

rate. A human will easily recognize the 1.5Hz rate as a

“slow” blinking which indicates the out-of-range battery

temperature while a microprocessor will be able to decode

either the 6.25% or 93.75% duty cycles as an NTC fault.

If a battery is found to be unresponsive to charging (i.e.,

its voltage remains below 2.85V for 1/2 hour), the CHRG

pingivesthebatteryfaultindication.Forthisfault,ahuman

would easily recognize the frantic 6.1Hz “fast” blink of the

LEDwhileamicroprocessorwouldbeabletodecodeeither

the 12.5% or 87.5% duty cycles as a bad battery fault.

Note that the LTC3586/LTC3586-1 are 3-terminal

PowerPath products where system load is always pri-

oritized over battery charging. Due to excessive system

load, there may not be sufﬁcient power to charge the

battery beyond the trickle charge threshold voltage

within the bad battery timeout period. In this case, the

battery charger will falsely indicate a bad battery. System

software may then reduce the load and reset the battery

charger to try again.

Although very improbable, it is possible that a duty cycle

reading could be taken at the bright-dim transition (low

duty cycle to high duty cycle). When this happens the

duty cycle reading will be precisely 50%. If the duty cycle

reading is 50%, system software should disqualify it and

take a new duty cycle reading.

NTC Thermistor

The battery temperature is measured by placing a nega-

tive temperature coefﬁcient (NTC) thermistor close to the

battery pack.

To use this feature, connect the NTC thermistor, RNTC,

between the NTC pin and ground and a resistor, RNOM,

from VBUS to the NTC pin. RNOM should be a 1% resis-

tor with a value equal to the value of the chosen NTC

thermistor at 25°C (R25). A 100k thermistor is recom-

mended since thermistor current is not measured by the

LTC3586/LTC3586-1 and will have to be budgeted for USB

compliance.

The LTC3586/LTC3586-1 will pause charging when the

resistance of the NTC thermistor drops to 0.54 times the

value of R25 or approximately 54k. For Vishay “Curve 1”

thermistor, this corresponds to approximately 40°C. If the

battery charger is in constant voltage (float) mode, the

safety timer also pauses until the thermistor indicates a

return to a valid temperature. As the temperature drops,

the resistance of the NTC thermistor rises. The LTC3586/

LTC3586-1 are also designed to pause charging when the

value of the NTC thermistor increases to 3.25 times the

value of R25. For Vishay “Curve 1” this resistance, 325k,

corresponds to approximately 0°C. The hot and cold

comparators each have approximately 3°C of hysteresis

to prevent oscillation about the trip point. Grounding the

NTC pin disables the NTC charge pausing function.

operaTion

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