LTC4068EDD-4.2#PBF Datasheet LTC4068EDD-4.2#PBF, LTC4068XEDD-4.2#PBF, LTC4068EDD-4.2#TRPBF | P7-P9

LTC4068-4.2/LTC4068X-4.2

406842fa

OPERATIO

The LTC4068 is a single cell lithium-ion battery charger

using a constant-current/constant-voltage algorithm. It

can deliver up to 950mA of charge current (using a good

thermal PCB layout) with a final float voltage accuracy of

±1%. The LTC4068 includes an internal P-channel power

MOSFET and thermal regulation circuitry. No blocking

diode or external current sense resistor is required; thus,

the basic charger circuit requires only two external com-

ponents. Furthermore, the LTC4068 is capable of operat-

ing from a USB power source.

Normal Charge Cycle

A charge cycle begins when the voltage at the V

pin rises

above the UVLO threshold level and a 1% program resistor

is connected from the PROG pin to ground. If the BAT pin

is less than 2.9V, the charger enters trickle charge mode.

In this mode, the LTC4068 supplies approximately 1/10th

the programmed charge current to bring the battery volt-

age up to a safe level for full current charging. (Note: The

LTC4068X does not include this trickle charge feature.)

When the BAT pin voltage rises above 2.9V, the charger

enters constant-current mode where the programmed

charge current is supplied to the battery. When the BAT pin

approaches the final float voltage (4.2V), the LTC4068

enters constant-voltage mode and the charge current

begins to decrease. When the charge current drops to the

programmed termination threshold (set by the external

resistor R

TERM

), the charge cycle ends.

Programming Charge Current

The charge current is programmed using a single resistor

from the PROG pin to ground. The charge current out of

the BAT pin is 1000 times the current out of the PROG pin.

The program resistor and the charge current are calcu-

lated using the following equations:

PROG

CHG

PROG

1000 1000

Charge current out of the BAT pin can be determined at any

time by monitoring the PROG pin voltage and using the

following equation:

BAT

PROG

= • 1000

Programming Charge Termination

The charge cycle terminates when the charge current

falls below the programmed termination threshold. This

threshold is set by connecting an external resistor, R

TERM

from the ITERM pin to ground. The charge termination

current threshold (I

TERM)

is set by the following equation:

TERM

CHG PROG

TERM

== =

100

•,

The termination condition is detected by using an internal

filtered comparator to monitor the ITERM pin. When the

ITERM pin voltage drops below 100mV

for longer than

TERM

(typically 1ms), charging is terminated. The charge

current is latched off and the LTC4068 enters standby

mode where the input supply current drops to 200µA.

(Note: Termination is disabled in trickle charging and

thermal limiting modes.)

TERM

can be set to be 1/10th of I

CHG

by shorting the ITERM

pin to the PROG pin, thus eliminating the need for external

resistor R

TERM

. When configured in this way, I

TERM

always set to I

CHG

/10, and the programmed charge current

is set by the equation:

CHG

PROG

CHG

500 500

When charging, transient loads on the BAT pin can cause

the ITERM pin to fall below 100mV for short periods of

time before the DC charge current has dropped to 10% of

the programmed value. The 1ms filter time (t

TERM

) on the

termination comparator ensures that transient loads of

this nature do not result in premature charge cycle termi-

nation. Once the

average

charge current drops below the

programmed termination threshold, the LTC4068 termi-

nates the charge cycle and ceases to provide any current

out of the BAT pin. In this state, any load on the BAT pin

must be supplied by the battery.

The LTC4068 constantly monitors the BAT pin voltage in

standby mode. If this voltage drops below the 4.1V recharge

Any external sources that hold the ITERM pin above 100mV will prevent the LTC4068 from

terminating a charge cycle.

These equations apply only when the ITERM pin is shorted to the PROG pin.

LTC4068-4.2/LTC4068X-4.2

406842fa

OPERATIO

threshold (V

RECHRG

), another charge cycle begins and

charge current is once again supplied to the battery. To

manually restart a charge cycle when in standby mode, the

input voltage must be removed and reapplied or the charger

must be shut down and restarted using the EN pin. Figure 1

shows the state diagram of a typical charge cycle.

Thermal Limiting

An internal thermal feedback loop reduces the programmed

charge current if the die temperature attempts to rise above

a preset value of approximately 120°C. This feature protects

the LTC4068 from excessive temperature and allows the

user to push the limits of the power handling capability of

a given circuit board without risk of damaging the LTC4068.

The charge current can be set according to typical (not worst

case) ambient temperature with the assurance that the

charger will automatically reduce the current in worst-case

conditions. DFN power considerations are discussed fur-

ther in the Applications Information section.

Undervoltage Lockout (UVLO)

An internal undervoltage lockout circuit monitors the input

voltage and keeps the charger in shutdown mode until V

rises above the undervoltage lockout threshold. The UVLO

circuit has a built-in hysteresis of 200mV. Furthermore, to

protect against reverse current in the power MOSFET, the

UVLO circuit keeps the charger in shutdown mode if V

falls to within 30mV of the BAT voltage. If the UVLO com-

parator is tripped, the charger will not come out of shut-

down mode until V

rises 100mV above the BAT voltage.

Manual Shutdown

At any point in the charge cycle, the LTC4068 can be put

into shutdown mode by driving the EN pin high. This

reduces the battery drain current to less than 2µA and the

supply current to less than 50µA. When in shutdown

mode, the CHRG pin is in the high impedance state. A new

charge cycle can be initiated by driving the EN pin low. An

internal resistor pull-down on this pin forces the LTC4068

to be enabled if the pin is allowed to float.

Automatic Recharge

Once the charge cycle is terminated, the LTC4068 continu-

ously monitors the voltage on the BAT pin using a com-

parator with a 2ms filter time (t

RECHARGE

). A charge cycle

restarts when the battery voltage falls below 4.10V (which

corresponds to approximately 80% to 90% battery capac-

ity). This ensures that the battery is kept at, or near, a fully

Figure 1. State Diagram of a Typical Charge Cycle

TRICKLE CHARGE

MODE

1/10TH FULL CURRENT

BAT > 2.9V

BAT < 2.9V

BAT > 2.9V

LTC4068

ONLY

CHRG: STRONG

PULL-DOWN

CHARGE MODE

FULL CURRENT

CHRG: STRONG

PULL-DOWN

SHUTDOWN MODE

CHRG: Hi-Z

EN DRIVEN LOW

UVLO CONDITION

STOPS

EN DRIVEN HIGH

UVLO CONDITION

DROPS TO <25µA

POWER ON

ITERM < 100mV

STANDBY MODE

NO CHARGE CURRENT

CHRG: Hi-Z

2.9V < BAT < 4.1V

406842 F01

Charge Status Indicator (CHRG)

The charge status output has two states: pull-down and

high impedance. The pull-down state indicates that the

LTC4068 is in a charge cycle. Once the charge cycle has

terminated or the LTC4068 is disabled, the pin state

becomes high impedance.

Power Supply Status Indicator (ACPR)

The power supply status output has two states: pull-down

and high impedance. The pull-down state indicates that

is above the UVLO threshold (3.8V) and is also 100mV

above the battery voltage. If these conditions are not met,

the ACPR pin is high impedance indicating that the LTC4068

is unable to charge the battery.

LTC4068-4.2/LTC4068X-4.2

406842fa

APPLICATIO S I FOR ATIO

WUUU

Stability Considerations

The constant-voltage mode feedback loop is stable with-

out an output capacitor, provided a battery is connected to

the charger output. With no battery present, an output

capacitor on the BAT pin is recommended to reduce ripple

voltage. When using high value, low ESR ceramic capaci-

tors, it is recommended to add a 1Ω resistor in series with

the capacitor. No series resistor is needed if tantalum

capacitors are used.

In constant-current mode, the PROG pin is in the feedback

loop, not the battery. The constant-current mode stability

is affected by the impedance at the PROG pin. With no

additional capacitance on the PROG pin, the charger is

stable with program resistor values as high as 20k; how-

ever, additional capacitance on this node reduces the

maximum allowed program resistor. The pole frequency

at the PROG pin should be kept above 100kHz. Therefore,

if the PROG pin is loaded with a capacitance, C

PROG

, the

following equation can be used to calculate the maximum

resistance value for R

PROG

≤

210

••

Average, rather than instantaneous charge current may be

of interest to the user. For example, if a switching power

supply operating in low current mode is connected in

parallel with the battery, the average current being pulled

out of the BAT pin is typically of more interest than the

instantaneous current pulses. In such a case, a simple RC

LTC4068-4.2

GND

PROG

10k

FILTER

406842 F02

CHARGE

CURRENT

MONITOR

CIRCUITRY

Figure 2. Isolating Capacitive Load on PROG Pin and Filtering

Power Dissipation

It is not necessary to design for worst-case power dissi-

pation scenarios because the LTC4068 automatically

reduces the charge current during high power conditions.

The conditions that cause the LTC4068 to reduce charge

current through thermal feedback can be approximated

by considering the power dissipated in the IC. Nearly all

of this power dissipation is generated by the internal

MOSFET—this is calculated to be approximately:

= (V

– V

BAT

) • I

BAT

where P

is the power dissipated, V

is the input supply

voltage, V

BAT

is the battery voltage and I

BAT

is the charge

current. The approximate ambient temperature at which

the thermal feedback begins to protect the IC is:

= 120°C – P

= 120°C – (V

– V

BAT

) • I

BAT

• θ

Example: An LTC4068 operating from a 5V supply is

programmed to supply 800mA full-scale current to a

discharged Li-Ion battery with a voltage of 3.3V. Assuming

is 50°C/W (see Thermal Considerations), the ambient

temperature at which the LTC4068 will begin to reduce the

charge current is approximately:

= 120°C – (5V – 3.3V) • (800mA) • 50°C/W

= 120°C – 1.36W • 50°C/W = 120°C – 68°C

= 52°C

charged condition and eliminates the need for periodic

charge cycle initiations. The CHRG output enters a pull-

down state during recharge cycles.

If the battery is removed from the charger, a sawtooth

waveform of approximately 100mV appears at the charger

output. This is caused by the repeated 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 the sawtooth is

dependent on the amount of output capacitance.

filter can be used on the PROG pin to measure the average

battery current, as shown in Figure 2. A 10k resistor has

been added between the PROG pin and the filter capacitor

to ensure stability.

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LTC4068EDD-4.2#PBF

Mfr. #:

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Manufacturer:

Analog Devices / Linear Technology

Description:

Battery Management Standalone Li-Ion Battery Charger in DFN

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New from this manufacturer.

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