LTC3576-1_15 Datasheet, PDF(26/48 Page) Linear Technology – Switching Power Manager with USB On-the-Go Triple Step-Down DC/DCs

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LTC3576-1_15 Datasheet, PDF (26/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go Triple Step-Down DC/DCs

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LTC3576/LTC3576-1

OPERATION

Each of the two faults has its own unique âblinkâ 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 VBUS current limit. This

prevents false end of charge indications due to insufï¬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

MODULATION

STATUS FREQUENCY (BLINK) FREQUENCY

Charging

0Hz

0Hz (Low-Z)

Not Charging

0Hz

0Hz (Hi-Z)

NTC Fault

35kHz

1Hz at 50%

Bad Battery

or On-The-Go

Short-Circuit

Fault

35kHz

4Hz at 50%

DUTY CYCLES

100%

6%, 94%

12%, 88%

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 âslowâ 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 âfastâ 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

PowerPath products where system load is always priori-

tized over battery charging. Due to excessive system load,

there may not be sufï¬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.

In addition to charge status, the CHRG pin is also used

to indicate whether there is a short-circuit condition on

VBUS when the bidirectional switching regulator is in on-

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 VBUS. System software

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 coefï¬cient (NTC) thermistor close to the

battery pack.

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

tween the NTC pin and ground and a bias resistor, RNOM,

from NTCBIAS to NTC. RNOM should be a 1% 200ppm

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

thermistor at 25Â°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 40Â°C. If the battery charger is in constant

voltage (ï¬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 0Â°C. The hot and cold comparators each

have approximately 3Â°C of hysteresis to prevent oscilla-

tion about the trip point. Grounding the NTC pin disables

all NTC functionality.

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