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參數(shù)資料
| 型號： | LX1745-CPW |
| 廠商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分類： | 顯示驅(qū)動器 |
| 英文描述： | Triple Output Boost ? LED Driver / LCD Bias |
| 中文描述： | LED DISPLAY DRIVER, PDSO20 |
| 封裝： | TSSOP-20 |
| 文件頁數(shù)： | 8/16頁 |
| 文件大小： | 439K |
| 代理商： | LX1745-CPW |
LX1745
P
RODUCTION
D
ATASHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 8
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
less than 100kHz, an external filter capacitor will be
needed (Figure 5). The value of C
PWM
is easily calculated
based on the PWM frequency and R
PWM_1
using the
following equation.
PWM
f
π
where
<<
R
1
_
PWM
1
_
PWM
PWM
R
50
C
=
eq. 3
M
5
eq. 4
LED
D
RIVER
–
O
UTPUT
C
URRENT
P
ROGRAMMING
Maximum LED current is easily programmed by
choosing the appropriate value for R
LED
(Figure 6). It is
recommended that a minimum value of 15
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:
BRTx
LED
I
)
MAX
(
LED
)
MAX
(
V
R
=
eq. 5
C
1
4.7μ
F
L
1
LX1745
LFB
CS
DRV
SRC
LED
15
OVP
BRT
D
1
R
OVP_1
R
OVP_2
R
CS
V
BAT
= 1.6V to 6.0V
C
BRT
R
BRT_2
R
BRT_1
Figure 6 –
LED Current Programming
LED
D
RIVER
–
LED
B
RIGHTNESS
A
DJUSTMENT
The LX1745 features a full range dimming LED driver.
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
LED
), sets the LED output current.
Dimming can be accomplished in one of two ways: by
applying a variable DC voltage, or by varying the duty
cycle (DC) of a PWM control signal, directly to the BRT
pin.
It is recommended that a maximum signal voltage of
300mV (V
BRT
) be used in order to minimize dissipative
losses in the LED current sense resistor (R
LED
).
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. This
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.
For PWM frequencies greater than 100kHz, the external
BRT input resistor is calculated using the following
equation.
where V
BRT
is the selected maximum LED current sense
feedback threshold.
For PWM frequencies less than 100kHz, the external
BRT input resistors and filter capacitor (Figure 4) are
calculated using the following equations.
where R
BRT_2
is selected and V
BRT(MAX)
is the selected
maximum LED current sense feedback threshold.
PWM
R
f
where V
RIPPLE
is selected to be 10% of V
BRT
, and f
PWM
is
the PWM signal frequency
.
=
)
MAX
(
BRT
)
MAX
(
BRT
MAX
V
PWM
V
1
_
BRT
V
)
DC
(
M
5
R
eq. 6
=
)
MAX
(
BRT
)
MAX
(
BRT
MAX
V
PWM
V
2
_
BRT
1
_
BRT
V
)
DC
(
R
R
eq. 7
+
π
=
2
_
BRT
1
_
BRT
2
_
BRT
1
_
BRT
BRT
R
R
R
5
C
eq. 8
D
IODE
S
ELECTION
A Schottky diode is recommended for most applications
(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
output
and
requirements. Further, the diode’s reverse breakdown
voltage characteristic must be capable of withstanding a
peak
inductor
current
A
P
P
L
I
C
A
T
I
O
N
S
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