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參數(shù)資料
| 型號(hào)： | LX1745 |
| 廠商： | Microsemi Corporation |
| 元件分類： | LED驅(qū)動(dòng)器 |
| 英文描述： | Triple Output Boost ? LED Driver / LCD Bias |
| 中文描述： | 三路輸出升壓？ LED驅(qū)動(dòng)器/ LCD偏置 |
| 文件頁數(shù)： | 7/16頁 |
| 文件大?。?/td> | 439K |
| 代理商： | LX1745 |
LX1745
P
RODUCTION
D
ATASHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
W
M
.
C
Triple Output Boost – LED Driver / LCD Bias
I N T E G R A T E D P R O D U C T S
Copyright
2000
Rev. 1.1a, 2004-02-06
APPLICATION NOTE
F
UNCTIONAL
D
ESCRIPTION
The LX1745 is a triple output Pulse Frequency
Modulated (PFM) boost converter that is optimized for
large step-up voltage applications like LCD biasing and
LED drive.
Operating in a pseudo-hysteretic mode with a fixed
switch “off time” of 300ns, converter switching is enabled
when the feedback voltage (V
FB
) falls below the bandgap
reference voltage or the ADJ pin voltage managed by the
reference logic block (see Block Diagram). When this
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
CS
) in the case of the LED driver or by an internal
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. Typical converter
switching behavior is shown in Figure 14.
LCD
B
IAS
–
O
UTPUT
V
OLTAGE
P
ROGRAMMING
Selecting the appropriate values for LCD Bias output
voltage divider (Figure 3), connected to the feedback pin,
programs the output voltage.
V
BAT
= 1.6V to 6.0V
LX1745
SWx
V
OUT
R
1
R
2
FBx
Figure 3 –
LCD Bias Output Voltage
Using a value between 40k
and 75k
for R
2
works well
in most applications. R
1
can be determined by the
following equation (where V
REF
= 1.19V nominal):
OUT
2
1
R
R
=
REF
REF
V
V
-
V
eq. 1
LCD
B
IAS
–
O
UTPUT
V
OLTAGE
A
DJUSTMENT
The LX1745 allows for the dynamic adjustment of each
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.
Each of these pins includes an internal 50pF capacitor to
ground (Figure 4) that works with an external resistor to
create a low-pass filter. This allows a direct PWM (f
PWM
≥
100KHz) signal input to be used for the voltage adjustment
signal. (Consequently a DC bias signal can also be used).
LX1745
ADJx
Reference
Logic
50pF
2.5M
R
ADJx_1
Figure 4 –
LCD Bias Adjustment Input
Different PWM signal levels can be accommodated by
selecting a value for R
PWM
such that the filtered V
ADJX
value is equal to the reference voltage (eq. 2)
=
PWM
ADJx
Cycle
Duty
V
V
+
R
1
_
PWM
M
5
M
5
eq. 2
LX1745
ADJx
C
ADJx
R
ADJx_1
R
ADJx_2
Figure 5 –
LCD Bias Adjustment Input Filter
Ideally the resultant ripple on the ADJx pin should be
approximately 1% or 40dB down from the nominal
reference. When using a PWM with a frequency that is
A
P
P
L
I
C
A
T
I
O
N
S
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