LX1745CPW Microsemi Analog Mixed Signal Group, LX1745CPW Datasheet - Page 7

• 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
F
Modulated (PFM) boost converter that is optimized for
large step-up voltage applications like LCD biasing and
LED drive.
switch “off time” of 300ns, converter switching is enabled
when the feedback voltage (V
reference voltage or the ADJ pin voltage managed by the
reference logic block (see Block Diagram).
occurs, the feedback comparator activates the switching
logic, pulling the gate of the power MOSFET high. This in
turn connects the boost inductor to ground causing current
to flow building up the energy stored in the inductor. The
output remains “on”, until the inductor current ramps up to
the peak current level set either by the CS pin programming
resistor (R
reference threshold for the LCD bias outputs. During this
switch cycle, the load is powered from energy stored in the
output capacitor. Once the peak inductor current value is
achieved, the driver output is turned off, for the fixed off-
time period of 300ns, allowing a portion of the energy
stored in the inductor to be delivered to the load causing
output voltage to rise at the input to the feedback circuit. If
the voltage at the feedback pin is less than the internal
reference at the end of the off-time period, the output
switches the power MOSFET “on” and the inductor
charging cycle repeats until the feedback pin voltage is
greater than the internal reference.
switching behavior is shown in Figure 14.
LCD B
voltage divider (Figure 3), connected to the feedback pin,
programs the output voltage.
UNCTIONAL
The LX1745 is a triple output Pulse Frequency
Operating in a pseudo-hysteretic mode with a fixed
Selecting the appropriate values for LCD Bias output
V
TM
OUT
IAS
V
CS
– O
BAT
) in the case of the LED driver or by an internal
Figure 3 – LCD Bias Output Voltage
D
= 1.6V to 6.0V
UTPUT
ESCRIPTION
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
V
R
R
1
2
OLTAGE
FB
P
) falls below the bandgap
ROGRAMMING
Typical converter
SWx
LX1745
A P P L I C A T I O N N O T E
Integrated Products Division
FBx
When this
®
Microsemi
Triple Output Boost – LED Driver / LCD Bias
Using a value between 40kΩ and 75kΩ for R
in most applications.
following equation (where V
LCD B
of the voltage outputs via an adjustment pin (ADJx). Any
voltage applied to the adjustment pin(s) works in
conjunction with the internal reference logic. The LX1745
will automatically utilize the internal reference when no
signal is detected or when the adjustment signal voltage is
below approximately 0.6V.
ground (Figure 4) that works with an external resistor to
create a low-pass filter. This allows a direct PWM (f
100KHz) signal input to be used for the voltage adjustment
signal. (Consequently a DC bias signal can also be used).
selecting a value for R
value is equal to the reference voltage (eq. 2)
approximately 1% or 40dB down from the nominal
reference. When using a PWM with a frequency that is
The LX1745 allows for the dynamic adjustment of each
Each of these pins includes an internal 50pF capacitor to
Different PWM signal levels can be accommodated by
Ideally the resultant ripple on the ADJx pin should be
IAS
V
LX1745
ADJx
Figure 5 – LCD Bias Adjustment Input Filter
– O
Figure 4 – LCD Bias Adjustment Input
P
=
Reference
RODUCTION
UTPUT
V
Logic
PWM
LX1745
R =
⋅
ADJx
Duty
1
V
50pF
OLTAGE
R
PWM
Cycle
R
2
2.5MΩ
V
1
D
REF
OUT
ADJx
C
such that the filtered V
ATASHEET
can be determined by the
ADJx
V
⋅
REF
= 1.19V nominal):
-
⎛
⎜
⎜
⎝
A
V
2
DJUSTMENT
REF
5 .
R
ADJx_1
R
M
R
ADJx_2
2
ADJx_1
Ω
5 .
+
M
R
Ω
PWM
2
_
works well
1
⎞
⎟
⎟
⎠
LX1745
PWM
eq. 1
eq. 2
ADJX
≥
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