LT1620 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LT1620

Rail-to-Rail Current Sense Amplifier

Linear Technology 

LT1620/LT1621

APPLICATIONS INFORMATION

between the IN+ and IN– inputs. Effective decoupling of

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

Sense resistor: RSENSE = VID /IMAX

Current limit programming voltage:

VPROG = VCC – [(10)(VID)]

Voltage feedback resistors:

RF1/RF2 = (VBATT(FLOAT) – 1.19)/1.19

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 MODE pin, connected to VCC.

The EOC flag threshold is determined through program-

ming VPROG2. The magnitude of this threshold corre-

sponds to 20 times the voltage across the sense amplifier

inputs.

CONNECTED AS IN FIGURE 2

LT1620GN

SENSE

AVG

IOUT

VEE

PROG

PROG2

MODE

IN–

AVG2

VCC

IN+

C1, 3.3µF

+ C2

R1

3.3µF 5.5k

R3

10k

R2

50k

END-OF-CYCLE

(ACTIVE LOW) LT1620/21 • F03

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

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

divider to create a programming voltage (VCC –VPROG2)of

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

Monitoring the load placed on the VIN supply of a charging

system is achieved by placing a second current sense

resistor in front of the charger VIN input. This function is

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

5V +

22µF

C1

1µF

1

SENSE

8

AVG

2

IOUT

PROG 7

3

LT1620MS8

GND

VCC

6

4 IN–

IN+ 5

+

22µF

7

VIN

5

VSW

LT1513

6

RUN S/S

4

GND

GND

TAB

8

2

VFB

3

IFB

VC

1

0.1µF

X7R

RP1

3k

1%

C2

1µF

RP2

12k

1%

R1

0.033Ω

TO

SYSTEM LOAD

L1B

10µH

MBRS340

VBATT = 12.3V

4.7µF L1A

10µH

24Ω

57k+ 22µF

×2

6.4k

Li-ION

0.22µF RSENSE

0.1Ω

1620/21 • F04

Figure 4. Input Current Sensing Application

7

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