LTC4012-1 datasheet(20/28 Pages) LINER

20 / 28 page

LTC4012/

LTC4012-1/LTC4012-2

4012f

APPLICATIONS INFORMATION

Table 5. Digital Read Back State (IN, Figure 10)

LTC4012

CHARGER STATE

OUT STATE

Hi-Z

Off

C/10 Charge

Bulk Charge

Input and Output Capacitors

In addition to typical input supply bypassing (0.1μF) on

DCIN, the relatively high ESR of aluminum electrolytic

capacitors is helpful for reducing ringing when hot pluging

the charger to the AC adapter. Refer to LTC Application

Note 88 for more information.

The input capacitor between system power (drain of top

FET, Figure 1) and GND is required to absorb all input PWM

ripple current, therefore it must have adequate ripple current

rating. Maximum RMS ripple current is typically one-half

of the average battery charge current. Actual capacitance

value is not critical, but using the highest possible voltage

rating on PWM input capacitors will minimize problems.

Consult with the manufacturer before use.

Figure 10. Microprocessor Status Interface

33k

200k

4012 F10

VDD

3.3V

μP

OUT

LTC4012

CHRG 7

C/10 CHRG Indicator

The value chosen for RPROG has a strong inﬂuence

on charge current monitoring and the accuracy of the

C/10 charge indicator output (CHRG). The LTC4012

uses the voltage on the PROG pin to determine when

charge current has dropped to the C/10 threshold.

The nominal threshold of 400mV produces an accu-

rate low charge current indication of C/10 as long as

RPROG = 26.7k, independent of all other current pro-

gramming considerations. However, it may sometimes

be necessary to deviate from this value to satisfy other

application design goals.

If RPROG is greater than 26.7k, the actual level at which

low charge current is detected will be less than C/10. The

highest value of RPROG that can be used while reliably

indicating low charge current before reaching ﬁnal VBAT

is 30.1k. RPROG can safely be set to values higher than

this, but low current indication will be lost.

If RPROG is less than 26.7k, low charge current detection

occurs at a level higher than C/10. More importantly, the

LTC4012 becomes increasingly sensitive to reverse cur-

rent. The lowest value of RPROG that can be used without

the risk of erroneous boost operation detection at end of

charge is 26.1k. Values of RPROG less than this should not

be used. See the Operation section for more information

about reverse current.

The nominal fractional value of IMAX at which C/10 indica-

tion occurs is given by:

μA

MAX

PROG

400

11 67

12085

()

–•

.–

••.

11 67μA

()

Direct digital monitoring of C/10 indication is possible

with an external application circuit like the one shown in

Figure 9.

By using two different value pull-up resistors, a micropro-

cessor can detect three states from this pin (charging, C/10

and not charging). See Figure 10. When a digital output port

(OUT) from the microprocessor drives one of the resistors

and a second digital input port polls the network, the charge

state can be determined as shown in Table 5.

Figure 9. Digital C/10 Indicator

INTVDD

CHRG

TP0610T

2N7002

100k

LTC4012

4012 F09

100k

VLOGIC

100k

C/10

CHRG

100k

2N7002

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