7

LT1620/LT1621

supply rails is also imperative in these types of circuits, as

large current transients are the norm. Power supply

decoupling should be placed as close as possible to the

ICs, and each IC should have a dedicated capacitor.

Design Equations

Current limit programming voltage:

Voltage feedback resistors:

End-of-Cycle Flag Application

Figure 3 illustrates additional connections using the

LT1620GN, including the end-of-cycle (EOC) flag feature.

The EOC threshold is used to notify the user when the

required load current has fallen to a programmed value,

usually a given percentage of maximum load.

The end-of-cycle output (MODE) is an open-collector pull-

down; the circuit in Figure 3 uses a 10k pull-up resistor on

The EOC flag threshold is determined through program-

sponds to 20 times the voltage across the sense amplifier

inputs.

As mentioned in the previous circuit discussion, the

charging current level is set to correspond to a sense

voltage of 80mV. The circuit in Figure 3 uses a resistor

0.5V. The MODE flag will therefore trip when the charging

current sense voltage has fallen to 0.5V/20 or 0.025V.

Thus, the end-of-cycle flag will trip when the charging

current has been reduced to about 30% of the maximum

value.

Input Current Sensing Application

system is achieved by placing a second current sense

useful for systems that will overstress the input supply

(wall adapter, etc.) if both battery charging and other

system functions simultaneously require high currents.

This allows use of input supply systems that are capable

of driving full-load battery charging and full-load system

requirements, but not simultaneously. If the input supply

current exceeds a predetermined value due to a combina-

tion of high battery charge current and external system

demand, the input current sense function automatically

Figure 3. End-of-Cycle Flag Implementation with LT1620GN

Figure 4. Input Current Sensing Application

AVG

PROG

PROG2

AVG2

SENSE

MODE

LT1620GN

LT1620/21 • F03

CONNECTED AS IN FIGURE 2

R1

5.5k

R2

50k

C2

3.3

µF

C1, 3.3

µF

R3

10k

END-OF-CYCLE

(ACTIVE LOW)

+

AVG

PROG

SENSE

LT1620MS8

1

2

3

4

8

7

GND

6

5

7

5

81

6

S/S

2

4

GND

GND

TAB

3

C1

1

µF

22

µF

3k

1%

12k

1%

C2

1

µF

R1

0.033

Ω

L1B

10

µH

22

µF

TO

SYSTEM LOAD

4.7

µF

L1A

10

µH

24

Ω

0.22

µF

0.1

µF

X7R

LT1513

RUN

5V

57k

6.4k

22

µF

× 2

MBRS340

1620/21 • F04

0.1

Ω

+

+

+

Li-ION