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LTC3559/LTC3559-1

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APPLICATIONS INFORMATION

In any mode, the actual battery current can be determined

by monitoring the PROG pin voltage and using the follow-

ing equation:

PROG

BAT

PROG

•800

Thermal Regulation

To prevent thermal damage to the IC or surrounding

components, an internal thermal feedback loop will auto-

matically decrease the programmed charge current if the

die temperature rises to approximately 115°C. Thermal

regulation protects the battery charger from excessive

temperature due to high power operation or high ambient

thermal conditions and allows the user to push the limits

of the power handling capability with a given circuit board

design without risk of damaging the LTC3559/LTC3559-1

or external components. The beneﬁt of the LTC3559/

LTC3559-1 battery charger thermal regulation loop is that

charge current can be set according to actual conditions

rather than worst-case conditions with the assurance that

the battery charger will automatically reduce the current

in worst-case conditions.

Charge Status Indication

The CHRG pin indicates the status of the battery charger.

Four possible states are represented by CHRG: charging,

not charging, unresponsive battery and battery temperature

out of range.

The signal at the CHRG pin can be easily recognized as one

of the above four states by either a human or a micropro-

cessor. The CHRG pin, which is an open-drain output, can

drive an indicator LED through a current limiting resistor

for human interfacing, or simply a pull-up resistor for

microprocessor interfacing.

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 the

charge current has dropped to below 10% of the full-scale

current, the CHRG pin is released (high impedance). If a

fault occurs after the CHRG pin is released, the pin re-

mains high impedance. However, if a fault occurs before

the CHRG pin is released, 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

microprocessor recognition.

Table 1 illustrates the four possible states of the CHRG

pin when the battery charger is active.

Table 1. CHRG Output Pin

STATUS

FREQUENCY

MODULATION

(BLINK)

FREQUENCY

DUTY CYCLE

Charging

0Hz

0 Hz (Lo-Z)

100%

IBAT < C/10

0Hz

0 Hz (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 VTRKL for over 1/2 hour), the

CHRG pin gives the battery fault indication. For this fault,

a human would easily recognize the frantic 6.1Hz “fast”

blinking of the LED while a microprocessor would be able

to decode either the 12.5% or 87.5% duty cycles as a bad

battery fault.

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.

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