FAN5640 datasheet(8/11 Pages) FAIRCHILD

Numer części	FAN5640

Szczegółowy opis	FAN5640 Dual High-Side Constant Current Source for High-Voltage Keypad LED Illumination

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Producent	FAIRCHILD [Fairchild Semiconductor]

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FAN5640 • Rev. 1.0.2

8

Application Information

Setting the Output Current Level

An internally generated reference current is mirrored on

the MOSFETs connected to the outputs IOUT1 and

IOUT2 (pins 1 and 6, respectively). The current mirror

ratio is 275 (typical). The voltage on pin 3 (RSET) is

0.475V in steady state; therefore, the programmed

current through each of the outputs is:

EXT

OUT

R

475

.

0

275

I





EQ. 1

where REXT is the external resistor connected from pin 3

to ground. Increasing this external programming resistor

reduces the output current. For the maximum rated

25mA rating of each output, the minimum value of the

external resistor is:













k

225

.

5

025

.

0

475

.

0

275

I

475

.

0

275

R

OUT

EXT

EQ. 2

The LED output current accuracy is ±10% for 25mA

current (see the Electrical Characteristics table). In the

worst-case scenario, the calculated value of IOUT can

lead to an error of ±10% in the LED current. Since the

tolerance of REXT also affects the LED current accuracy,

a precision resistor should be chosen to have the least

effect on the overall accuracy of the LED current

(see Figure 12).

Floating vs. Tied Outputs

Unused outputs can be left floating. The current through is

zero, regardless of the current programmed at pin 3.

However, ESD protection is enhanced if the unused

output pin is tied to VIN (pin 5).

If the two output pins are tied together, they can deliver

a combined 50mA for the same programming resistor of

5.225kΩ.

External Capacitors

Because the FAN5640 is stable without capacitors on

the outputs, no capacitors are recommended. Typical

input decoupling usually present on incoming supply

rails should suffice in most applications. If necessary, a

small input capacitance may be placed between the

input pin and ground without adverse effects.

Dropout Limitations

As for any LDO regulator, there are limitations on how

close the input and output rails can be to maintain

regulation. The minimum difference is referred to as the

dropout. The relevant information is provided in the

Typical Performance curve Dropout Voltage vs. LED

Current (see Figure 8). The equation for the data is:









64

I

V

35

.

0

V

OUT

DO

EQ. 3

This is equivalent to an RDS of 64Ω with an additional

offset of 350mV. This equation is helpful in determining

the minimum dissipation in the device and the lowest

input voltage for a given application.

Multiple LED Displays

For portable applications, the FAN5640 can be powered

from the output of any typical boost regulator. Multiple

LED displays can be created with the FAN5640 powered

from the output of the FAN5333, as shown in Figure 20.

Note that the output voltage of the FAN5333 depends

upon the number of LEDs in its output string. Being

conscious of the minimum dropout requirements of the

FAN5640; if three series LEDs are required to be

present at its output, then the FAN5333 should have

four series LEDs in its output string.

IOUT1

GND

RSET 3

2

1

4

6

5

R

EXT

GND

SHDN

C

IN

C

OUT

FB

VIN

SW

GND

SHDN

L

6.8µH to

10µH

IOUT2

VIN

EN

ON

OFF

4.7µF

to

10µF

V

IN

0.1µF

to

2.2µF

FAN5333

FAN5640

Figure 20. LED Display Example

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