27

LTC1759

Figure 14. Kelvin Sensing of Charging Current

1759 F14

DIRECTION OF CHARGING CURRENT

Charging Indication

When a CHARGECURRENT() command with a value of

zero is sent to the LTC1759, current stops flowing into the

battery. However, the command does not cause the CHGEN

pin to pull low. This prevents use of the CHGEN pin as a

charge termination indication. Figure 16 shows a circuit

that reliably indicates when the battery is receiving a

charge current.

The circuit shown, except for the optional 555 timer IC,

Q1 and R5 partially isolate the capacitance of C1 from

1759 F15

L1

HIGH

FREQUENCY

CIRCULATING

PATH

BAT

SWITCH NODE

D1

Figure 15. High Speed Switching Path

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

a crude voltage level detector looking for voltage going

below approximately one volt on the emitter. The RC

circuit consisting of R4 and C1 filters low pulses from the

logic level type interface. The circuit is powered from the

logic output driver of the CHGEN pin of the LTC1759. The

circuit does not need any capacitive supply bypassing to

function and draws little current. When the CHGEN pin

goes low, the current consumption of the circuit is elimi-

nated. Note that some resistor values must change de-

the LTC1759. An optional low power 555 timer can be

added to to give a blinking LED charge indication.

UV

SYNC

SDB

CHGEN

DGND

PROG

COMP1

AGND

RNR

THERM

SDA

SCL

INTB

22

21

8

32

33

9

10

2

3

35

1

34

30

29

31

23

26

36

U2

LTC1759

7

16

4

5

12

25

24

18

17

28

27

11

6

20

19

14

15

13

DCIN

DCDIV

INFET

BOOSTC

CLP

CLN

TGATE

SW

BGATE

BOOST

GBIAS

SPIN

SENSE

BAT1

BAT2

PGND

GROUND

TRIGGER

OUTPUT

RESET

8

7

6

5

LMC555

(OPTIONAL, SEE TEXT)

R1

R1

3.3V = 240

Ω

5V = 510

Ω

CHARGE

1

2

3

4

DISCHARGE

THRESHOLD

CONTROL

VOLTAGE

1759 F16

R2

4.7k

R3

2.7M

C3

0.1

µF

C1

0.1

µF

C2

0.1

µF

R4

4.7M

R5

10k

R6

R6

3.3V = 330k

5V = 680k

Q1

MMBT3904

D

Q2, 2N7002

G

S

R7

330k

Figure 16. Battery Charge Indicator Circuit