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LTM8061

8061f

APPLICATIONS INFORMATION

RNG/SS: Soft-Start

Soft-start functionality is also supported by the RNG/SS

pin. 50μA is sourced from the RNG/SS pin, so connect-

ing a capacitor from the RNG/SS pin to ground (CRNG/SS

in Figure 7) creates a linear voltage ramp. The maximum

charge current follows this voltage. Thus, the charge current

increases from zero to the fully programmed value as the

capacitor charges from 0V to 1V. The value of CRNG/SS is

calculated based on the desired time to full current (tSS)

following the equation:

CRNG/SS = 50μA • tSS

The RNG/SS pin is pulled to ground internally when charg-

ing terminates so each new charge cycle begins with a

soft-start cycle. RNG/SS is also pulled to ground during

bad-battery and NTC fault conditions, producing a graceful

recovery from a fault.

If the battery is removed from an LTM8061 charger that is

configured for C/10 termination, a low amplitude sawtooth

waveform appears at the charger output, due to cycling

between termination and recharge events. This cycling

results in pulsing at the CHRG output. An LED connected

to this pin will exhibit a blinking pattern, indicating to the

user that a battery is not present. The frequency of this

blinking pattern is dependent on the output capacitance.

C/10 Termination

The LTM8061 supports a low current based termination

scheme, where a battery charge cycle terminates when

the charge current falls below one-tenth the programmed

charge current, or about 200mA. This termination mode is

engaged by shorting the TMR pin to ground. When C/10

termination is used, an LTM8061 charger sources battery

charge current as long as the average current level remains

above the C/10 threshold. As the full-charge float voltage is

achieved, the charge current falls until the C/10 threshold

is reached, at which time the charger terminates and the

LTM8061 enters standby mode. The CHRG status is high

impedance when the charger is sourcing less than C/10.

There is no provision for bad-battery detection if C/10

termination is used.

Timer Termination

The LTM8061 supports a timer-based termination scheme,

where a battery charge cycle terminates after a specific

amount of time elapses. Timer termination is enabled

by connecting a capacitor (CTIMER) from the TMR pin to

GND. The timer cycle time span (tEOC) is determined by

CTIMER in the equation:

CTIMER = tEOC • 2.27 • 10–7 (Hours)

When charging at a 1C rate, tEOC is commonly set to three

hours, which requires a 0.68μF capacitor.

The CHRG status pin continues to signal charging, regard-

less of which termination scheme is used. When timer

termination is used, the CHRG status pin is pulled low

during a charge cycle until the charge current falls below

the C/10 threshold. The charger continues to top off the

battery until timer EOC, when the LTM8061 terminates the

charge cycle and enters standby mode.

RNG/SS

8061 F07

LTM8061

CRNG/SS

Figure 7. Using the RNG/SS Pin for Soft-Start

Status Pins

The LTM8061 reports charger status through two open-col-

lector outputs, the CHRG and FAULT pins. These pins can

accept voltages as high as VIN, and can sink up to 10mA

when enabled. The CHRG pin indicates that the charger is

delivering current at greater than a C/10 rate, or one-tenth

of the programmed charge current. The FAULT pin signals

bad-battery and NTC faults. These pins are binary coded

as shown in Table 2:

Table 2. Status Pin State

CHRG

FAULT

STATUS

High

Standby, Shutdown Mode, or Charging at

Less than C/10

High

Low

Bad-Battery Fault (Precondition Timeout/

EOC Failure)

Low

High

Normal Charging at C/10 or Greater

Low

NTC Fault (Pause)

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