LTC3558
15
3558f
APPLICATIONS INFORMATION
In any mode, the actual battery current can be determined
by monitoring the PROG pin voltage and using the follow-
ing equation:
I
PROG
R
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 LTC3558 or external
components. The benefi t of the LTC3558 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 con-
ditions.
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 a low for charging,
a 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)
0%
NTC Fault
35kHz
1.5Hz at 50%
6.25%, 93.75%
Bad Battery
35kHz
6.1Hz at 50%
12.5%, 87.5%
An NTC fault is represented by a 35kHz pulse train whose
duty cycle alternates 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 V
TRKL
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.
