LTC4012-1 datasheet(13/28 Pages) LINER

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4012f

LTC4012/

LTC4012-1/LTC4012-2

the end of soft-start will depend on which loop (current,

voltage or adapter limit) is in control of the PWM. If this

current is below that required by the ITH start-up threshold,

the resulting charge current transient duration depends on

loop compensation but is typically less than 100μs.

Bulk Charge

When soft-start is complete, the LTC4012 begins sourcing

the current programmed by the external components

connected to CSP, CSN and PROG. Some batteries may

require a small conditioning trickle current if they are heavily

discharged. As shown in the Applications Information

section, the LT4012 can address this need through a variety

of low current circuit techniques on the PROG pin. Once

a suitable cell voltage has been reached, charge current

can be switched to a higher, bulk charge value.

End of Charge and CHRG Output

As the battery approaches the programmed output voltage,

charge current will begin to decrease. The open-drain

CHRG output can indicate when the current drops to

10% of its programmed full-scale value by turning off

the strong pull-down (open-drain FET) and turning on a

weak 25μA pull-down current. This weak pull-down state

is latched until the part enters shutdown or the sensed

current rises to roughly C/6. C/10 indication will not be set

if charge current has been reduced due to adapter input

current limiting. As the charge current approaches 0A, the

PWM continues to operate in full continuous mode. This

avoids generation of audible noise, allowing bulk ceramic

capacitors to be used in the application.

Charge Current Monitoring

When the LTC4012 is charging, the voltage on the PROG pin

varies in direct proportion to the charge current. Referring

to Figure 1, the nominal PROG voltage is given by

μA R

PROG

CHRG

SENSE

PROG

••

.•

11 67

Voltage tolerance on PROG is limited by the charge

current accuracy speciﬁed in the Electrical Characteristics

table. Refer to the Applications Information section on

programming charge current for additional details.

OPERATION

Figure 1. PWM Circuit Diagram

–

PWM

LOGIC

OSCILLATOR

WATCHDOG

TIMER

LOOP

COMPENSATION

TGATE

SYSTEM

POWER

RIN

–

BGATE

CSP

CSN

PROG

VFB

ITH

RPROG

FROM ICL

1.2085V

RSENSE

VSENSE

ICHRG

4012 F01

CPROG

CLOCK

LTC4012

CLP

BAT

RIN

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