LTC4012 datasheet(24/48 Pages) AD

24 / 48 page

LTC4162-S

Rev B

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OPERATION

The charge current programming resistor, RSNSB, should

always be set to match the capacity of the battery with-

out regard to source or load limitations from any other

control loop. The multiple control-loop architecture of

the LTC4162 will correct for any discrepancies, always

optimizing transfer of power to the battery and the load.

Thermal Regulation

When the switching battery charger is enabled at an el-

evated ambient temperature, LTC4162 self heating may

push its junction temperature to an unacceptable level.

To prevent overheating the LTC4162 monitors its own

die_temp and automatically reduces the icharge_dac to

limit power dissipation. The differential servo voltage at

CSP to CSN can drop to as low as 1mV giving about 3%

(1/32) of the maximum charge current. The thermal regu-

lation algorithm achieves this by enforcing a maximum

icharge_dac setting which drops linearly from 31 to 0 as

die_tempincreasesfromthermal_reg_start_temp(default

120°C) to thermal_reg_end_temp (default 125°C). When

the thermal regulation algorithm is active, charge_status

becomes thermal_reg_active. A thermal_reg_active_alert

can be set with en_thermal_reg_active_alert and cleared

by writing either back to 0. Thermal regulation can be

programmed to any temperature within the LTC4162's

operating range.

Constant-Voltage Charging

Once the BATSENS+ voltage reaches the programmed

charging voltage the switching regulator will reduce

its output power and hold the battery voltage steady at

(6V + 28.571mV • vcharge_setting) • N where N is 1 for

a 6V battery, 2 for a 12V battery, 3 for an 18V battery and

4 for a 24V battery. In constant voltage mode, the charge

currentwilldecreasenaturallytowardzeroprovidinginher-

ently safe operation by preventing the battery from being

over charged. Multiple charge voltage settings are avail-

able for final top-off voltage selection via vcharge_setting.

While charge voltage trade-offs can be made to preserve

battery life or maximize capacity, it is not possible for the

LTC4162 to be set to a charge voltage that is dangerously

high or inconsistent with a lead-acid battery.

Note that charge_current_setting and vcharge_setting do

notdirectlycontroltheicharge_dacandvcharge_dac.They

areonlytargetvalues.Forexample,ifen_sla_temp_compis

true(default),thevcharge_dacwillbecontrolledbythetem-

perature compensation system. Several constant_voltage

levels will be used depending on which phase the charger

is in, absorb_charge, cc_cv_charge or equalize_charge.

Absorb Charge

The LTC4162 begins charging with an absorb_charge

phase. During absorb_charge, the charge voltage is in-

creasedtovcharge_setting+vabsorb_deltawhichdefaults

to N • 7.2V where N is 1,2,3 or 4 for a 6V, 12V, 18V or 24V

battery respectively. At the beginning of the charge cycle

the charger may be in constant_current for some period

beforereachingconstant_voltageattheabsorblevel.Once

the charger reaches constant_voltage the tabsorbtimer

starts and the absorption phase proper commences. It will

continue until either the tabsorbtimer reaches max_ab-

sorb_time or ibat drops below the c_over_x_threshold.

At the end of the absorb_charge phase the charge voltage

drops back to just vcharge_setting for the continuous

cc_cv_charge phase.

Constant-Current/Constant-Voltage (CC-CV) Charge

In the cc_cv_charge phase, the LTC4162 sets the battery

voltage servo loop to the vcharge_setting. If the load is

powered directly from the battery, the switching charger

will provide the load current as long as the load current

is below the charge_current_setting. If the load exceeds

the charge_current_setting the battery will eventually be

depleted. There is no termination in the charging algo-

rithm; the charger will remain in cc_cv_charge as long

as input power is available but can be forced off with

suspend_charger.

Equalization Charge

An optional equalize_charge phase is available via the I2C

port with equalize_req wherein the charge voltage is set to

vcharge_setting+v_equalize_delta,limitedtoamaximum

of N • 7.8V where N is 1, 2, 3 or 4 for a 6V, 12V, 18V or

24V battery respectively. In equalize_charge, the battery

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