LX1745(2006) データシートの表示（PDF） - Microsemi Corporation
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LX1745 Datasheet PDF : 16 Pages
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LX1745
TM
®
Triple Output Boost – LED Driver / LCD Bias
PRODUCTION DATASHEET
APPLICATION NOTE
NEGATIVE LCD BIAS GENERATION
For applications that require it, a negative bias can be
easily generated using an inductorless charge pump
consisting of only four additional discrete components
(Figure 8).
- VOUT
VOUT
D3
D4
R1
VBAT = 1.6V to 6.0V
LX1745
SWx
FBx
R2
Figure 8 – Negative Bias Generation
This negative output is a mirror of the positive output
voltage. However, it is unregulated.
If a regulated negative bias is desired then this is also
possible with some additional components. A low current
shunt regulator (LX6431 or LX432) and a bipolor pass
element can form a simple negative voltage LDO (Figure
9).
achieved by placing a feed-forward capacitor across the
feedback resistor connected to the output (Figure 2). A
recommended value of 1nF should be used.
PCB LAYOUT
Minimizing trace lengths from the IC to the inductor,
diode, input and output capacitors, and feedback
connection (i.e. pin 3) are typical considerations.
Moreover, the designer should maximize the DC input and
output trace widths to accommodate peak current levels
associated with this circuit.
SHDN SHDN2 SHDN1 LCD1 LCD2
LED
0
0
1
1
0
0
0
1
1
1
1
0
1
0
1
1
0
1
1
1
1
1
1
1
1
1
0
0
0
0
0
1
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
Table 1 – Enable Logic
VNEG_LCD
R4
R5
R3
LX6431,
LX432
VOUT
D3
D4
R1
VBAT = 1.6V to 6.0V
LX1745
SWx
FBx
R2
Figure 9 – Regulated Negative Bias
R3 is sized to meet the minimum shunt current required for
regulation while R4 and R5 are calculated. If R5 is selected
to be 100kΩ then R4 is calculated using the following
equation:
VNEG _ LCD
=
VREF
⋅ ⎜⎜⎝⎛1+
R4
R5
⎟⎟⎠⎞
eq. 13
where VREF is a -2.5V in the case of the LX6431.
FEED-FORWARD CAPACITANCE
Improved efficiency and ripple performance can be
Copyright © 2000
Rev. 1.2, 2006-02-27
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 10
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