LX1745CPW Microsemi Analog Mixed Signal Group, LX1745CPW Datasheet - Page 8

• Manufacturer Part Number: LX1745CPW
• Description: IC LED DRVR WT/CLR BCKLT 20TSSOP
• Manufacturer: Microsemi Analog Mixed Signal Group
• Type: Backlight, White LED, Color LEDr

Specifications of LX1745CPW

• Constant Current: Yes
• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 3
• Internal Driver: Both
• Type - Primary: Backlight
• Type - Secondary: Color, White LED
• Voltage - Supply: 1.6 V ~ 6 V
• Voltage - Output: 25V
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 100mA
• Internal Switch(s): Yes
• Efficiency: 90%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Frequency: -

Copyright © 2000
Rev. 1.2, 2006-02-27
less than 100kHz, an external filter capacitor will be
needed (Figure 5). The value of C
based on the PWM frequency and R
following equation.
where
LED D
choosing the appropriate value for R
recommended that a minimum value of 15Ω be used for
this resistor in order to prevent noise coupling issues on the
feedback line. Although, alternate values can be calculated
using the following equation:
LED D
LED current regulation is accomplished by using the
applied BRT pin voltage as the LED current reference.
This reference voltage, in conjunction with the LED current
setting resistor (R
applying a variable DC voltage, or by varying the duty
cycle (DC) of a PWM control signal, directly to the BRT
pin.
Maximum LED current is easily programmed by
The LX1745 features a full range dimming LED driver.
Dimming can be accomplished in one of two ways: by
It is recommended that a maximum signal voltage of
V
BAT
TM
= 1.6V to 6.0V
RIVER
RIVER
Figure 6 – LED Current Programming
– O
– LED B
LED
UTPUT
C
LX1745
PWM
), sets the LED output current.
R
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
R
PWM
LED
RIGHTNESS
=
DRV
SRC
OVP
C
LFB
BRT
CS
L
⋅ π
_
=
1
1
URRENT
<<
f
V
PWM
I
LED
BRTx
R
CS
2
50
C
(
5 .
BRT
⋅
MAX
(
R
MAX
M
PWM
A
)
P
PWM
Ω
R
)
R
D
BRT_2
DJUSTMENT
BRT_1
ROGRAMMING
1
LED
R
R
_
OVP_1
OVP_2
1
is easily calculated
PWM_1
(Figure 6). It is
A P P L I C A T I O N N O T E
4.7μF
Integrated Products Division
C
1
using the
®
Microsemi
R
15Ω
LED
eq. 3
eq. 4
eq. 5
Triple Output Boost – LED Driver / LCD Bias
300mV (V
losses in the LED current sense resistor (R
Like the LCD bias adjustment (ADJx) pins, the BRT pin is
connected to an internal 50pF capacitor to ground that
works with an external resistor to create a low-pass filter,
allowing the BRT pin to driven directly by a PWM signal
whose frequency is greater than 100kHz. When this pin is
driven by a PWM signal whose frequency is less than
100kHz, an external filter capacitor is needed.
capacitor is selected such that the ripple component of the
resultant voltage on the BRT pin is less than 10% of the
nominal input voltage.
BRT input resistor is calculated using the following
equation.
where V
feedback threshold.
BRT input resistors and filter capacitor (Figure 4) are
calculated using the following equations.
where R
maximum LED current sense feedback threshold.
where V
the PWM signal frequency
D
(e.g. Microsemi UPS5817). The low forward voltage drop
and fast recovery time associated with this device supports
the switching demands associated with this circuit
topology. The designer is encouraged to consider the
diode’s average and peak current ratings with respect to the
application’s
requirements.
voltage characteristic must be capable of withstanding a
IODE
For PWM frequencies greater than 100kHz, the external
For PWM frequencies less than 100kHz, the external
A Schottky diode is recommended for most applications
S
R
R
RIPPLE
ELECTION
BRT
BRT_2
BRT
BRT
BRT
P
_
_
is the selected maximum LED current sense
C
1
1
RODUCTION
) be used in order to minimize dissipative
BRT
=
is selected to be 10% of V
=
is selected and V
output
Further, the diode’s reverse breakdown
2
R
5 .
=
BRT
M
⋅ π
_
Ω
2
5
f
PWM
⋅
⋅
⎛
⎜
⎜
⎝
⎛
⎜
⎜
⎝
V
and
V
.
PWM
⋅
PWM
⎛
⎜
⎜
⎝
D
R
R
ATASHEET
(
(
BRT
DC
DC
BRT
peak
V
V
_
_
MAX
BRT
BRT(MAX)
BRT
1
MAX
1
+
⋅
R
(
(
R
)
MAX
)
MAX
BRT
−
−
BRT
V
)
V
)
inductor
_
BRT
BRT
_
LED
2
2
BRT
is the selected
⎞
⎟
⎟
⎠
(
(
MAX
MAX
).
, and f
)
)
⎞
⎟
⎟
⎠
⎞
⎟
⎟
⎠
LX1745
current
PWM
eq. 6
eq. 7
eq. 8
This
is
Page 8