LTC4156IUFD View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4156IUFD

Dual-Input Power Manager/3.5A LiFePO4 Battery Charger with I2C Control and USB OTG

Linear Technology 

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LTC4156

OPERATION

Battery Charger Operation

The LTC4156 contains a fully featured constant-current/

constant-voltage LiFePO4 battery charger with automatic

recharge, bad cell detection, trickle charge, programmable

safety timer, thermistor temperature qualified charging,

programmable end-of-charge indication, programmable

float voltage, programmable charge current, detailed I2C

status reporting, and programmable interrupt generation.

Precharge/Low Battery

When a battery charge cycle begins, the battery charger

first determines if the battery is deeply discharged. If

the battery voltage is below VLOWBAT, typically 2.7V, the

LTC4156 will report the LOWBAT condition via I2C (see

Table 17). If the low battery voltage persists for more than

one-half hour, the battery charger automatically terminates

and indicates via the I2C port that the battery was unre-

sponsive. When the battery voltage is low, charge current

is reduced, both to protect the battery and to prevent ex-

cessive power dissipation in the external PMOS transistor.

Figure 1 shows the relationship between battery voltage

and charge current reduction. When input power (USB

or WALL) is unavailable, the I2C LOWBAT indication will

always be true, independent of the actual state of charge

of the battery and can be disregarded.

Constant-Current

When the battery voltage is above approximately 2.8V, the

charger will attempt to deliver the programmed charge cur-

rent in constant-current mode. Depending on available input

power and external load conditions, the battery charger

may or may not be able to charge at the full programmed

rate. The external load will always be prioritized over the

battery charge current. Likewise, the USB and WALL input

current limit programming will always be observed and

only additional power will be available to charge the bat-

tery. When system loads are light, battery charge current

will be maximized.

The upper limit of charge current is programmed by the

combination of a resistor from PROG to ground and the

PROG servo voltage value set in the I2C port. The maximum

charge current will be given by the following expression:

ICHARGE

=

VPROG

RPROG

• 1000

VPROG can be set by the I2C port and ranges from 150mV

to 1.2V in 75mV steps. The default value of VPROG is 1.2V.

VPROG is controlled by bits ICHARGE[3:0] located at sub

address 0x02. See Table 10.

In either the constant-current or constant-voltage charging

modes, the voltage at the PROG pin will be proportional

to the actual charge current delivered to the battery. The

charge current can be determined at any time by monitoring

the PROG pin voltage and using the following relationship:

IBAT

=

VPROG

RPROG

• 1000

Recall, however, that in some cases the actual battery

charge current, IBAT, will be lower than the programmed

current, ICHARGE, due to limited input power available and

prioritization of the system load drawn from VOUT. RPROG

should be set to match the capacity of the battery without

regard to input power limitations.

Constant-Voltage

Once the battery terminal voltage reaches the preset float

voltage, the battery charger will hold the voltage steady

and the charge current will decrease naturally toward

zero. Four voltage settings are available for final float

voltage selection via the I2C port using bits VFLOAT[1:0]

(Table 11).

4156f

30

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