ltc3576 Linear Technology Corporation, ltc3576 Datasheet - Page 26

• Manufacturer Part Number: ltc3576
• Description: Switching Power Manager With Usb On-the-go + Triple Step-down Dc/dcs
• Manufacturer: Linear Technology Corporation

LTC3576/LTC3576-1
OPERATION
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 LTC3576/LTC3576-1 is in V
prevents false end of charge indications due to insuffi cient
power available to the battery charger.
Table 2 illustrates the four possible states of the CHRG
pin when the battery charger is active.
Table 2. CHRG Signal
An NTC fault is represented by a 35kHz pulse train whose
duty cycle alternates between 6% and 94% at a 1Hz rate. A
human will easily recognize the 1Hz rate as a “slow” blink-
ing which indicates the out-of-range battery temperature
while a microprocessor will be able to decode either the
6% or 94% 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
pin gives the bad battery fault indication. For this fault, a
human would easily recognize the 4Hz “fast” blink of the
LED while a microprocessor would be able to decode either
the 12% or 88% duty cycles as a bad battery fault.
Note that the LTC3576/LTC3576-1 are 3-terminal Pow-
erPath products where system load is always prioritized
over battery charging. Due to excessive system load, there
may not be suffi cient 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.
26
or On-The-Go
Not Charging
Short-Circuit
Bad Battery
NTC Fault
Charging
STATUS
Fault
FREQUENCY
35kHz
35kHz
0Hz
0Hz
(BLINK) FREQUENCY
MODULATION
0Hz (Low-Z)
1Hz at 50%
4Hz at 50%
0Hz (Hi-Z)
BUS
current limit. This
DUTY CYCLES
12%, 88%
6%, 94%
100%
0%
In addition to charge status, the CHRG pin is also used
to indicate whether there is a short-circuit condition on
V
the-go mode. When a short-circuit condition is detected,
CHRG will blink with the same modulation frequency and
duty cycle as a bad battery fault. If the charger is on at the
same time that on-the-go is enabled, a 4Hz modulation of
12% and 88% duty cycles on CHRG could indicate a bad
battery or a short-circuit fault on V
should turn off the charger or on-the-go to determine
which fault has occurred.
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 coeffi cient (NTC) thermistor close to the
battery pack.
To use this feature connect the NTC thermistor, R
tween the NTC pin and ground and a bias resistor, R
from NTCBIAS to NTC. R
resistor with a value equal to the value of the chosen NTC
thermistor at 25°C (R25).
The LTC3576/LTC3576-1 pauses charging when the re-
sistance of the NTC thermistor drops to 0.54 times the
value of R25 or approximately 54k for a 100k thermistor.
For a Vishay Curve 1 thermistor, this corresponds to ap-
proximately 40°C. If the battery charger is in constant
voltage (fl oat) 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 LTC3576/LTC3576-1 are also designed to pause
charging when the value of the NTC thermistor increases
to 3.25 times the value of R25. For a Vishay Curve 1
100k thermistor, this resistance, 325k, corresponds to
approximately 0°C. The hot and cold comparators each
have approximately 3°C of hysteresis to prevent oscilla-
tion about the trip point. Grounding the NTC pin disables
all NTC functionality.
BUS
when the bidirectional switching regulator is in on-
NOM
should be a 1% 200ppm
BUS
. System software
NTC
NOM
, be-
3576f
,