LTC4012 datasheet(14/28 Pages) LINER

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LTC4012/

LTC4012-1/LTC4012-2

4012f

Adapter Input Current Limit

The LTC4012 can monitor and limit current from the input

DC supply, which is normally an AC adapter. When the

programmed adapter input current is reached, charge

current is reduced to maintain the desired maximum input

current. The ITH and PROG pins will reﬂect the reduced

charge current. This limit function avoids overloading the

DC input source, allowing the product to operate at the

same time the battery is charging without complex load

management algorithms. The battery will automatically be

charged at the maximum possible rate that the adapter will

support, given the application’s operating condition. The

LTC4012 can only limit input current by reducing charge

current, and in this case the charger uses nonsynchronous

PWM operation to prevent boosting if the average

charge current falls below about 25% of the maximum

programmed current. Note that the ICL indicator output

becomes active (low) at an adapter input current level just

slightly less than that required for the internal ampliﬁer to

begin to assert control over the PWM loop.

Charger Status Indicator Outputs

The LTC4012 open-drain indicator outputs provide valuable

information about the IC’s operating state and can be

used for a variety of purposes in applications. Table 1

summarizes the state of the three indicator outputs as a

function of LTC4012 operation.

OPERATION

Table 1. LTC4012 Open-Drain Indicator Outputs

ACP

CHRG

ICL

CHARGER STATE

Off

No DC Input (Shutdown)

Off

Shutdown or Reverse Current

Off

Bulk Charge

25μA

Off

Low Current Charge or Initial

DCIN – BAT <500mV

Input Current Limit During Bulk

Charge

25μA

Input Current Limit During Low

Current Charge

Off

On or

25μA

On or

Off

Indicated Charge with DCIN - BAT

< 300mV. Bulk charge may be less

than programmed value.

PWM Controller

The LTC4012 uses a synchronous step-down architecture

to produce high operating efﬁciency. The nominal operating

frequency of 550kHz allows use of small ﬁlter components.

The following conceptual discussion of basic PWM

operation references Figure 1.

The voltage across the external charge current sense

resistor RSENSE is measured by current ampliﬁer CA. This

instantaneous current (VSENSE/RIN) is fed to the PROG

pin where it is averaged by an external capacitor and

converted to a voltage by the programming resistor RPROG

between PROG and GND. The PROG voltage becomes

the average charge current input signal to error ampliﬁer

EA. EA also receives loop control information from the

battery voltage feedback input VFB and the adapter input

current limit circuit.

Figure 2. PWM Waveforms

OFF

INDUCTOR

CURRENT

TOP FET

BOTTOM FET

tOFF

THRESHOLD

SET BY ITH

VOLTAGE

4012 F02

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