LTC4069 Просмотр технического описания (PDF) - Linear Technology

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LTC4069

Standalone 750mA Li-Ion Battery Charger in 2 × 2 DFN with NTC Thermistor Input

Linear Technology 

LTC4069

Operation

below 4.1V (which corresponds to 80% to 90% battery

capacity), a new charge cycle is initiated and a 2.25 hour

timer begins. This ensures that the battery is kept at, or

near, a fully charged condition and eliminates the need for

periodic charge cycle initiations. Also, if the battery voltage

does not exceed the recharge threshold voltage when the

timer ends, the timer resets and a 2.25 hour recharge cycle

begins. The CHRG output assumes a strong pull-down

state during recharge cycles until C/10 is reached when

it transitions to a high impendance state.

Trickle Charge and Defective Battery Detection

At the beginning of a charge cycle, if the battery voltage

is low (below 2.9V), the charger goes into trickle charge,

reducing the charge current to 10% of the full-scale current.

If the low-battery voltage persists for one quarter of the total

time (1.125 hour), the battery is assumed to be defective,

the charge cycle is terminated and the CHRG pin output

pulses at a frequency of 2Hz with a 75% duty cycle. If for

any reason the battery voltage rises above 2.9V, the charge

cycle will be restarted. To restart the charge cycle (i.e.,

when the defective battery is replaced with a discharged

battery), simply remove the input voltage and reapply it

or momentarily float the PROG pin and reconnect it.

CHRG Status Output Pin

The charge status indicator pin has three states: pull-down,

pulse at 2Hz (see Trickle Charge and Defective Battery

Detection and Battery Temperature Monitoring) and high

impedance. The pull-down state indicates that the LTC4069

is in a charge cycle. A high impedance state indicates that

the charge current has dropped below 10% of the full-scale

current or the LTC4069 is disabled. Figure 2 shows the

CHRG status under various conditions.

Charge Current Soft-Start and Soft-Stop

The LTC4069 includes a soft-start circuit to minimize

the inrush current at the start of a charge cycle. When a

charge cycle is initiated, the charge current ramps from

zero to the full-scale current over a period of approximately

170µs. Likewise, internal circuitry slowly ramps the charge

current from full-scale to zero when the charger is shut off

or self terminates. This has the effect of minimizing the

transient current load on the power supply during start-up

and charge termination.

Constant-Current/Constant-Voltage/

Constant-Temperature

The LTC4069 uses a unique architecture to charge a

battery in a constant-current, constant-voltage and con-

stant-temperature fashion. Figure 1 shows a Simplified

Block Diagram of the LTC4069. Three of the amplifier

feedback loops shown control the constant-current (CA),

constant-voltage (VA), and constant-temperature (TA)

modes. A fourth amplifier feedback loop (MA) is used to

increase the output impedance of the current source pair,

M1 and M2 (note that M1 is the internal P-channel power

MOSFET). It ensures that the drain current of M1 is exactly

1000 times greater than the drain current of M2.

Amplifiers CA and VA are used in separate feedback loops

to force the charger into constant-current or constant-volt-

age mode, respectively. Diodes D1 and D2 provide priority

to either the constant-current or constant-voltage loop,

whichever is trying to reduce the charge current the most.

The output of the other amplifier saturates low which

effectively removes its loop from the system. When in

constant-current mode, CA servos the voltage at the PROG

pin to be precisely 1V. VA servos its inverting input to an

internal reference voltage when in constant-voltage mode

and the internal resistor divider, made up of R1 and R2,

ensures that the battery voltage is maintained at 4.2V.

The PROG pin voltage gives an indication of the charge

current during constant-voltage mode as discussed in

“Programming Charge Current”.

Transconductance amplifier, TA, limits the die temperature

to approximately 115°C when in constant-temperature

mode. Diode D3 ensures that TA does not affect the charge

current when the die temperature is below approximately

115°C. The PROG pin voltage continues to give an indication

of the charge current.

For more information www.linear.com/LTC4069

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