Datasheet

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LTC4020

4020fd

For more information www.linear.com/LTC4020

TIMER: C/10 Termination

The LTC4020 supports a low current based termination

scheme. This termination mode is engaged by shorting

the TIMER pin to ground.

When in CC/CV charge mode, a battery charge cycle ter

minates when the current output from the charger falls to

below

one-tenth the maximum

charge current, or I

CSMAX

as programmed with R

. The C/10 threshold current

corresponds to 5mV across R

During lead-acid charging, the LTC4020 initiates float

charging when the absorption stage charge current is re

duced to one-tenth of the programmed maximum current.

When charging in CC mode, the current source function

remains active indefinitely.

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

termination is used.

TIMER: Timed Functions

The LTC4020 supports timer based functions, where bat

tery charge cycle control occurs after a specific amount

of time elapses.

Timer termination is engaged when a

capacitor (C

TIMER

) is connected from the TIMER pin to

ground. C

TIMER

for a desired end-of-cycle time (T

EOC

)

follows the relation:

TIMER

= T

EOC

• 6.87 x 10

–2

(µF)

where T

EOC

is hours.

A typical timer T

EOC

for Li-Ion charge cycle termination

is three hours, which requires a 0.2µF timer capacitor.

The timer cycle starts when the charger transitions from

constant-current to constant-voltage charging, thus, ter

mination at the end of the timer cycle only occurs if the

charging cycle was successful.

When timer termination is

used, the STAT1 status pin is pulled low during a charging

cycle until the battery charge current falls below the C/10

threshold. The STAT1 pin stays high impedance with charge

currents below C/10, but the charger continues to top off

the battery until timer T

EOC

, when the LTC4020 terminates

the charging cycle and the PowerPath FET disconnects the

battery from the DC/DC converter output.

During lead-acid charging, the timer acts as an absorp

tion mode safety timer. Normally, the LTC4020

initiates

float charging when the absorption stage charge current

is reduced to one-tenth of the programmed maximum

current, however, the maximum duration of absorption

charging is limited by the timer. If the charge current does

not fall to one-tenth of the programmed maximum current

by T

EOC

, the LTC4020 forces the battery charger to begin

float mode charging. A typical timer T

EOC

for lead-acid

charging is six to eight hours, which is accommodated

by a 0.47µF timer capacitor.

When charging in CC mode, after charge termination, once

the timer reaches T

EOC

and the charge cycle terminates,

input power or SHDN must be cycled to initiate another

battery charge cycle.

A bad battery detection function is available during

CC/CV or lead-acid charging. This fault condition is achieved

if the battery does not respond to preconditioning (V

1.75V), such that the charger remains in (or enters) pre-

condition mode after one-eighth of the programmed T

EOC

time. A bad battery fault halts the charging cycle, and the

fault condition is reported on the status pins. The bad battery

fault remains active until the battery voltage rises above the

precondition threshold, or until power or SHDN is cycled.

Battery Temperature Qualified Charging: NTC

The LTC4020 can accommodate battery temperature moni

toring by using an NTC (negative temperature co-efficient)

thermistor close to the battery pack

. The temperature

monitoring function is enabled by connecting a 10kΩ,

β = 3380 NTC thermistor from the NTC pin to ground. If

the NTC function is not desired, leave the pin unconnected.

The NTC pin sources 50µA, and monitors the voltage

dropped across the 10kΩ thermistor. When the voltage

on this pin is above 1.35V (0°C) or below 0.3V (40°C),

the battery temperature is out of range, and the LTC4020

triggers an NTC fault. The NTC fault condition remains until

the voltage on the NTC pin corresponds to a temperature

within the 0°C to 40°C range. Both hot and cold thresholds

incorporate hysteresis that corresponds to 5°C.

If higher operational charging temperatures are desired,

the temperature range can be expanded by adding series

resistance to the 10k NTC resistor. Adding a 910Ω resistor

will increase the effective HOT temperature threshold to

45°C. The effect of this additional resistance on the COLD

threshold is negligible.

applicaTions inForMaTion

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