Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 13
Copyright © 2000
Rev. 1.1, 2006-02-27
WWW.Microsemi .COM
LX1689
Third Generation CCFL Controller
TM
®
DESCRIPTION (CONTINUED)
due to varying input signal amplitude. Input impedance is very
high so BRITE_IN can also be driven from a 100K potentiometer
with no offset error.
BRITE_R and BRITE_OUT Resistor values.
The BRITE_OUT pin is the output from the BRITE_IN signal
processor. It is a linear current source that varies from 0 to the
current value established at pin BRITE_R multiplied by the DC
voltage at pin BRITE_C. The optimum value for BRITE_R is
usually 10K ohms. The BRITE_OUT voltage range can be scaled
from 300mV to 2.0V. However, it is recommended that the
scaling of BRITE_OUT (including Analog mode BRITE_IN
range) be within 400mV to 1.2V. Maximum voltage correlates to
full brightness settings. It is the ratio of the two resistors
multiplied by the voltage at BRITE_C:
V
BRITE_OUT
= V
BRITE_C
(R
BRITE_OUT
/ R
BRITE_R_
)
In some applications, a precision 10K resistor is connected from
BRITE_OUT and BRITE_R to ground to develop 1.0V to
represent maximum lamp brightness. In Analog mode the
BRITE_OUT voltage potential is proportional to BRITE_IN. The
minimum brightness setting at BRITE_IN corresponds to the
minimum voltage at BRITE_OUT. In digital mode, BRITE_IN
has no effect on BRITE_OUT.
Because the BRITE_OUT output is a linear current source, you
can place other components, such as a thermistor or photo resistor,
at this pin to generate complex functions for controlling brightness.
For example; use a PWM input at the BRITE_IN pin to control
dimming, and boost analog lamp current amplitude at cold lamp
temperature with a thermistor at the BRITE_OUT pin. This will
help warm the lamp faster at start up so final brightness is reached
sooner.
SETTING THE OUTPUT CURRENT.
Referring to the application examples figures 2 and 3. The
current setting resistor(s) are R7, and R19 and R20 respectively.
The value these resistor(s) are in the range of 200 to 400ohms. The
following formula can be used to determine the current setting
resistor value. Use 1180 for Digital mode and 1260 for analog dim.
R
SNS
= 1180 or 1260 x R
BRITE_OUT
/ I
OUT
(mA
RMS
) x R
BRITE_R
In the 1W burst dimming application example shown in figure 2 the
output current is set for nominally 3.5mA. RSNS is calculated
using the formula above as follows:
R
SNS
= 1180 x 4990 / 3.5 x 6550 = 256.8 ohms
A standard value of 255 ohms was chosen. It is recommended to
keep the value of the sense resistor in the range of 200 to 400 ohms
as stated above. If calculated value exceeds 400 ohms its best to
increase the value of the R
BRITE_R
resistor.
MECHANICAL DRAWING
PW
20-Pin Thin Small Shrink Outline (TSSOP)
c
123
E
D
E1
e
A2
A1
A
L
b
SEATING PLANE
1
MILLIMETERS INCHES
Dim
MIN MAX MIN MAX
A - 1.10 - 0.043
A1 0.05 0.15 0.002 0.006
A2 0.80 1.05 0.031 0.041
b 0.19 0.30 0.007 0.012
c 0.09 0.20 0.004 0.008
D 6.40 6.60 0.252 0.260
E 6.25 6.55 0.246 0.258
E1 4.30 4.50 0.169 0.177
e 0.65 BSC 0.026 BSC
L 0.45 0.75 0.018 0.030
Θ1 0° 8° 0° 8°
*LC - 0.10 - 0.004
* Lead Coplanarity
Note:
1. Dimensions do not include mold flash or protrusions;
these shall not exceed 0.155mm(.006”) on any side.
Lead dimension shall not include solder coverage.
M
M
E
E
C
C
H
H
A
A
N
N
I
I
C
C
A
A
L
L
S
S
