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EL7900
brightness and 1.25V represents the maximum. The first
step would be to determine the ratio of R 1 and R 2 :
Figure 12 shows how a fully manual override can be quickly
added by using the EN pin.
R 1 = R 2 × ? ---------------- – 1 ? = 11 × R 2
3.0V
? 0.25V ?
(EQ. 4)
3V TO 5V
SUPPLY VOLTAGE
3V PWM CONTROL
3V TO 5V
SUPPLY VOLTAGE
3V DC VOLTAGE
VDO
R 1
110k
VDO
R 1
110k
ENABLE/
DISABLE
CONTROL
EN
EL7900
OUT
TO INVERTER
BRIGHTNESS
INPUT OR
CONTROLLER
EN
EL7900
OUT
TO INVERTER BRIGHTNESS
GND
R 2
10k
C 1
25μF
INPUT OR LED DRIVER
CONTROLLER
GND
R 2
10k
C 1
25μF
LED DRIVER
FIGURE 12. AUTOMATIC DIMMING SOLUTION WITH
ADJUSTABLE BIAS CONTROL AND MANUAL
OVERRIDE
FIGURE 10. AUTOMATIC DIMMING SOLUTION
Short Circuit Current Limit
Next, the value of R 2 can be calculated based on the
maximum output current coming from the EL7900 under the
application's maximum light exposure. Suppose the current
has been determined to be about 125μA. Thus, R 2 can be
calculated as shown in Equations 5 and 6:
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
R 2 = ? ------------------ ? = 10k Ω
1.25V
? 125 μ A ?
and
R 1 = 11 × R 2 = 110k Ω
(EQ. 5)
(EQ. 6)
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
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.
3V TO 5V
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
SUPPLY VOLTAGE
3V PWM CONTROL
board layout due to its low speed operation. Nevertheless,
good printed circuit board layout is necessary for optimum
VDO
R 1
110k
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
EN
EL7900
GND
OUT
R 2
10k
TO INVERTER BRIGHTNESS
INPUT OR LED DRIVER
CONTROLLER
C 1
25μF
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.
FIGURE 11. AUTOMATIC DIMMING SOLUTION WITH
ADJUSTABLE BIAS CONTROL
6
FN7377.8
September 18, 2009
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