LTC3566-2_15 Datasheet, PDF (16/28 Pages) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566/LTC3566-2
OPERATION
The signal at the CHRG pin can be easily recognized as
one of the above four states by either a human or a mi-
croprocessor. An open drain output, the CHRG pin 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 charg-
ing is complete, i.e., the BAT pin reaches 4.200V 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 LTC3566 family is in V_BUS current limit. This prevents
false end of charge indications due to insufficient power
available to the battery charger.
Table 1 illustrates the four possible states of the CHRG
pin when the battery charger is active.
Table 1. CHRG Output Pin
STATUS
MODULATION (BLINK)
FREQUENCY
FREQUENCY
Charging
0Hz
0Hz (Lo-Z)
Not Charging
0Hz
0Hz (Hi-Z)
NTC Fault
35kHz
1.5Hz at 50%
Bad Battery
35kHz
6.1Hz at 50%
DUTY CYCLE
100%
0%
6.25%, 93.75%
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 2.85V, for 1/2 hour), the CHRG
pin gives the battery fault indication. For this fault, a human
would easily recognize the frantic 6.1Hz fast blink 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.
Note that the LTC3566 family is a 3-terminal PowerPath
product where system load is always prioritized 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, R_NTC, be-
tween the NTC pin and ground and a resistor, R_NOM, from
V_BUS to the NTC pin. R_NOM should be a 1% resistor with
a value equal to the value of the chosen NTC thermistor
at 25°C (R25). A 100k thermistor is recommended since
thermistor current is not measured by the LTC3566 family
and will have to be budgeted for USB compliance.
The LTC3566 family 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
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