LX1745CPW Microsemi Analog Mixed Signal Group, LX1745CPW Datasheet - Page 9

• 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
negative voltage transition that is greater than the output
voltage.
P
The LX1745 can source up to 100mA of gate current.
An logic level N-channel MOSFET with a low turn on
threshold voltage, low gate charge and low R
required to optimize overall circuit performance.
O
configuration, it may be desirable to protect the output from
an over-voltage condition in the event the load is removed
or not present.
easily programmed with two external resistors (Figure 6).
This feature eliminates the need for a Zener Diode clamp on
the output.
and then calculating R
where V
This voltage should be selected to accommodate the
maximum forward voltage of all the LEDs, over
temperature,
Conversely, it may also be selected according to the
maximum V
I
aspect of the PFM constant off-time architecture; it
determines the maximum output power capability and has a
marked effect on efficiency.
to approximately two times the expected maximum DC
input current. This setting will minimize the inductor size,
the input ripple current, and the output ripple voltage. Care
should be taken to use inductors that will not saturate at the
peak inductor current level. The desired peak inductor
current can be estimated by the following equation:
where P
conversion efficiency, and V
NDUCTOR
OWER
VER
Since the output of the LED Driver is a current mode
The LX1745 includes an over voltage monitor that is
Programming is accomplished by first selecting R
Setting of the peak inductor current limit is an important
It is recommended that the peak inductor current be set
From the calculated desired I
TM
V
M
OLTAGE
OVP
OUT
OSFET
C
DS
is the desired maximum voltage on the output.
is the total output power, η is the expected
URRENT
plus
voltage of the output MOSFET.
R
P
S
OVP
ROTECTION
ELECTION
_
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
1
I
the
PK
OVP_1
L
=
IMIT
R
=
OVP
2
maximum
using the following equation.
⋅
IN
P
⋅ η
_
P
2
ROGRAMMING
OUT
is the input voltage.
P
V
V
IN
PK
ROGRAMMING
OVP
V
an R
REF
-
V
REF
feedback
CS
resistance value
A P P L I C A T I O N N O T E
Integrated Products Division
®
DS(ON)
voltage.
Microsemi
eq. 10
eq. 9
OVP_2
is
Triple Output Boost – LED Driver / LCD Bias
can be chosen from the following equation:
which is taken from the following graph (Figure 7).
This graph characterizes the relationship between peak
inductor current, the inductance value, and the R
programming resistor.
I
good results. Choosing a lower value emphasizes peak
current overshoot, effectively raises the switching
frequency, and increases the dissipative losses due to
increased currents.
O
parameters: inductor value, output capacitance value, peak
switch current, load current, input voltage, and the output
voltage. All of these factors can be summarized by the
following equation:
where V
forward voltage of the output catch diode, and V
voltage drop across the power switch. V
approximated at 0.4V and V
NDUCTOR
UTPUT
An inductor value of 47μH has been show to yield very
Output voltage ripple is a function of the several
V
RIPPLE
1000
Figure 7 – Peak Current Programming Resistor
800
600
400
200
L
C
0
is the voltage drop across the inductor, V
APACITOR
S
≅
P
0
ELECTION
⎛
⎜ ⎜
⎝
L
RODUCTION
⋅
I
C
PK
OUT
⋅
I
OUT
R
S
CS
5
⎞
⎟ ⎟
⎠
ELECTION
⎛
⎜ ⎜
⎝
V
≅
IN
I
−
PK
D
30
F
R
(
V
can be approximated at 0.4V.
−
ATASHEET
CS
1
SW
⋅
. 0
10
10
(k Ω )
+
185
−
V
6
L
)
+
V
OUT
15
I
PK
+
L
⋅
V
I
+V
OUT
F
−
SW
V
LX1745
IN
SW
⎞
⎟ ⎟
⎠
F
20
can be
eq. 11
eq. 12
is the
is the
CS
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