6

FN7377.8

September 18, 2009

brightness and 1.25V represents the maximum. The first

maximum output current coming from the EL7900 under the

application's maximum light exposure. Suppose the current

calculated as shown in Equations 5 and 6:

and

In Figure 11, user adjustable bias control has been added to

allow control over the minimum and maximum output

voltage. This allows the user to adjust the output brightness

to personal preference over a limit range via the 3V PWM

control.

Figure 12 shows how a fully manual override can be quickly

added by using the EN pin.

Short Circuit Current Limit

The EL7900 does not limit the output short circuit current. If

the output is directly shorted to the ground continuously, the

output current could easily increase for a strong input light

such that the device may be damaged. Maximum reliability is

maintained if the output continuous current never exceeds

6mA by adding a load resistor at the output. This limit is set

by the design of the internal metal interconnects.

Suggested PCB Footprint

Footprint pads should be a nominal 1-to-1 correspondence

with package pads. The large, exposed central die-mounting

paddle in the center of the package requires neither thermal

nor electrical connections to PCB, and such connections

should be avoided.

Power Supply Bypassing and Printed Circuit

Board Layout

The EL7900 is relatively insensitive to the printed circuit

board layout due to its low speed operation. Nevertheless,

good printed circuit board layout is necessary for optimum

performance. Ground plane construction is highly

recommended, lead length should be as short as possible

and the power supply pins must be well bypassed to reduce

the risk of oscillation. For normal single supply operation,

where the GND pin is connected to ground, a 0.1µF ceramic

capacitor should be placed from VCC pin to GND pin. A

4.7µF tantalum capacitor should then be connected in

parallel, placed close to the device.

R

1

R

2

3.0V

0.25V

----------------

1

–

⎝⎠

⎛⎞

×

11

R

2

×

==

(EQ. 4)

3V TO 5V

SUPPLY VOLTAGE

3V DC VOLTAGE

EL7900

OUT

EN

GND

VDO

TO INVERTER BRIGHTNESS

CONTROLLER

FIGURE 10. AUTOMATIC DIMMING SOLUTION

10k

25µF

110k

INPUT OR LED DRIVER

R

2

1.25V

125

μA

------------------

⎝⎠

⎛⎞

10k

Ω

==

(EQ. 5)

R

1

11

R

2

×

110k

Ω

==

(EQ. 6)

3V TO 5V

SUPPLY VOLTAGE

3V PWM CONTROL

EL7900

OUT

EN

GND

VDO

FIGURE 11. AUTOMATIC DIMMING SOLUTION WITH

ADJUSTABLE BIAS CONTROL

10k

25µF

110k

TO INVERTER BRIGHTNESS

CONTROLLER

INPUT OR LED DRIVER

3V TO 5V

SUPPLY VOLTAGE

3V PWM CONTROL

EL7900

OUT

EN

GND

VDO

FIGURE 12. AUTOMATIC DIMMING SOLUTION WITH

ADJUSTABLE BIAS CONTROL AND MANUAL

OVERRIDE

ENABLE/

CONTROL

DISABLE

10k

25µF

110k

TO INVERTER

CONTROLLER

BRIGHTNESS

LED DRIVER

INPUT OR

EL7900