LX1745-CPWT [MICROSEMI]
LED Driver, 1-Segment, PDSO20, TSSOP-20;
| 型号： | LX1745-CPWT |
| 厂家： | 
Microsemi |
| 描述： | LED Driver, 1-Segment, PDSO20, TSSOP-20 驱动器 CD |
| 文件： | 总16页 (文件大小：439K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LX1745
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
PRODUCTION DATASHEET
KEY FEATURES
DESCRIPTION
The LX1745 is
a
compact high guaranteed at a VIN equal to 1.6V with
> 90% Maximum Efficiency
Low Quiescent Supply Current
Externally Programmable Peak
Inductor Current Limit For
Maximum Efficiency
efficiency step-up boost regulator for sustained operation as low as 1.1V.
driving white or color LEDs in LCD
The maximum LED drive current is
lighting applications while supplying the easily programmed using one external
necessary LCD bias voltages with an current sense resistor in series with the
additional two integrated boost converters. LEDs. In this configuration, LED current
Designed for maximum efficiency and provides a feedback signal to the FB pin,
Logic Controlled Shutdown
< 1µA Shutdown Current
Dynamic Output LED Current
and Two LCD Bias Voltage
Adjustments Via Analog
Reference Or Direct PWM
Input
featuring
a
psuedo-hysteretic PFM maintaining constant current regardless of
topology (that decreases output voltage varying LED forward voltage (VF).
ripple), the LX1745 minimizes system cost Depending on the MOSFET selected, the
and condenses layout area making it ideal LX1745 is capable of achieving an LED
for PDA, smart-phone, and digital camera drive in excess of 1.0W.
applications.
The LX1745 provides simple dynamic
20-Pin TSSOP Package
While the LCD Bias generation is adjustment of the LED drive current (0% to
implemented using an internal N-Channel 100% full range dimming) and the LCD
APPLICATIONS
MOSFET for LCD Bias generation, the
LED driver utilizes an external N-Channel
MOSFET in order to maintain maximum
efficiency along with flexible power
requirements..
Bias output voltages (up to ±15% typ)
through separate IC interfaces.
Each
Pagers
interface has an internal RC filter allowing
designers to make these adjustments via a
direct PWM input signal or an analog
Smart Phones
PDAs
Handheld Computers
General LCD Bias Applications
LED Driver
The LX1745’s control circuitry is
optimized for portable systems with a
shutdown current of less than 1µA. The
input voltage range of 1.6V to 6.0 allows
for a wide selection of system battery
reference signal.
Further, any PWM
amplitude is easily accommodated using a
single external resistor.
The LX1745 is available in the low-
profile 20-Pin TSSOP.
voltages
and
start-up
is
IMPORTANT: For the most current data, consult MICROSEMI’s website: http://www.microsemi.com
PRODUCT HIGHLIGHT
ILED = 20mA to 0mA
L1
VBAT = 1.6V to 6.0V
47µH
1206 Case Size
C1
4.7µF
- VLCD
DRV
IN
SRC
SW1
VLCD1 = VIN to 25V
OVP
LFB
FB1
CS
LX1745
ON OFF
LSHDN
SW2
RSET
BRT
REF
15Ω
ON OFF
FB2
SHDN1
GND
ADJ1
ADJ2
SHDN2
VLCD2 = VIN to 25V
ON OFF
LX1745 Evaluation Board
PACKAGE ORDER INFO
Plastic TSSOP
20-Pin
TA (°C)
PW
-40 to 85
LX1745-CPW
Note: Available in Tape & Reel.
Append the letter “T” to the part number.
(i.e. LX1745-CPWT)
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 1
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
ABSOLUTE MAXIMUM RATINGS
PACKAGE PIN OUT
Supply Input Voltage...........................................................................-0.3V to 7V
Feedback Input Voltage (VFBx) ...............................................-0.3V to VIN + 0.3V
Shutdown Input Voltage (VSHDN x) ..........................................-0.3V to VIN + 0.3V
PWM Input Amplitude (ADJx, BRT).....................................-0.3V to VIN + 0.3V
Analog Adjust Input Voltage (VADJx, VBRT)............................-0.3V to VIN + 0.3V
SRC Input Current ................................................................................ 500mARMS
Operating Temperature Range.........................................................-40°C to 85°C
Maximum Operating Junction Temperature ................................................ 150°C
Storage Temperature Range...........................................................-65°C to 150°C
Lead Temperature (Soldering 10 seconds) .................................................. 300°C
GND
FB1
REF
OVP
BRT
CS
SW1
1
2
3
4
5
6
7
20
19
18
17
16
15
14
13
12
11
ADJ1
SHDN1
IN
LSHDN
DRV
LFB
ADJ2
SRC
GND
8
9
10
SHDN2
SW2
FB2
GND
PW PACKAGE
(Top View)
Note: Exceeding these ratings could cause damage to the device. All voltages are with respect to
Ground. Currents are positive into, negative out of specified terminal.
x denotes respective pin designator 1, 2, or 3
THERMAL DATA
Plastic TSSOP 20-Pin
PW
THERMAL RESISTANCE-JUNCTION TO AMBIENT, θJA
90°C/W
Junction Temperature Calculation: TJ = TA + (PD x θJA).
The θJA numbers are guidelines for the thermal performance of the device/pc-board system. All of the
above assume no ambient airflow.
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 2
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
FUNCTIONAL PIN DESCRIPTION
Name
Description
Unregulated IC Supply Voltage Input – Input range from +1.6V to 6.0V. Bypass with a 1µF or greater capacitor
IN
for operation below 2.0V.
DRV
SRC
LED MOSFET Gate Driver – Connects to an external N-Channel MOSFET.
LED MOSFET Current Sense Input - Connects to the External N-Channel MOSFET Source.
Over Voltage Programming Pin – Connects to a resistor divider between the output load and GND to set the
OVP
maximum output voltage. OVP has a voltage threshold of 1.2V
LED Current Feedback Input – Connects to a current sense resistor between the LED output load and GND to
set the LED drive current.
LFB
GND
Common terminal for ground reference.
LED Dimming Signal Input – Provides the internal reference, via an internal filter and gain resistor, allowing for a
dynamic output LED current adjustment that corresponds to the PWM input signal duty cycle. Either a PWM
signal or analog voltage can be used. The actual BRT pin voltage range is from VIN to GND. Minimize the
current sense resistor power dissipation by selecting a range for VBRT = 0.0V to 0.5V.
BRT
REF
SWx
Buffered Reference Output – Connected to the internal bandgap reference voltage of 1.2V.
LCD Bias Inductor Switch Connection – Internally connected to the drain of a 28V N-channel MOSFET. SW is
high impedance in shutdown.
Feedback Input – Connect to a resistive divider network between the output and GND to set the output voltage
between VCC (IN) and 25V. The feedback threshold is 1.29V.
FBx
LCD Bias Adjustment PWM Signal Input – Connect to an RC filter allowing for dynamic output voltage
adjustment >±15%, corresponding to a varying duty cycle. Either a PWM signal or analog voltage can be used.
The ADJ input voltage range is from 0.9V to VIN DC. The ADJx pin should be connected to ground when the
internal reference is used.
ADJx
LED Driver Active-Low Shutdown Input – A logic low shuts down the LED driver circuitry and reduces the supply
LSHDN
current by 60µA (Typ). Pull
high for normal operation.
LSHDN
LCD Bias Active-Low Shutdown Input – A logic low shuts down the LCD Bias circuitry and reduces the supply
current by 60µA (Typ). Pull high for normal operation.
SHDNx
CS
SHDNx
Current-Sense Amplifier Input – Connecting a resistor between CS and GND sets the peak inductor current
limit.
Copyright 2000
Microsemi
Page 3
Rev. 1.1a, 2004-02-06
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, the following specifications apply over the operating ambient temperature 0°C ≤ TA ≤ 70°C except where
otherwise noted and the following test conditions: VIN = 3V, = V , = V , = V
LSHDN
SHDN1
SHDN2
IN
IN
IN
LX1745
Typ
Parameter
Symbol
Test Conditions
Units
Min
Max
LED DRIVER
`
VBRT = 100mV
85
5
100
20
115
35
LFB Threshold Voltage
VLFB
mV
VBRT = 20mV
VLFB = 100mV
LFB Input Bias Current
BRT Input Voltage Range
BRT Input Bias Current
LED Driver Shutdown Input
Bias Current
ILFB
VBRT
IBRT
-100
0
100
VIN
60
nA
V
BRT = 100mv
nA
SHDN1
-100
85
100
nA
µA
I
SHDN1
0.0V ≤
≤ VIN
Current Sense Bias Current
ICS
IPK
4
170
210
100
12
R
CS = 0kΩ
RCS = 2kΩ
VIN = 5V, VDRV = 3V
Switch Peak Current
mA
DRV Sink/Source Current
DRV On-Resistance
Maximum Switch On-Time
Minimum Switch Off-Time
OVP Threshold Voltage
Reference Voltage
mA
Ω
µS
nS
RDRV(ON) VCC = 5V
15
∞
tON
VFB = 1V
VFB = 1V
tOFF
200
1.15
1.186
300
410
VOVP
VREF
1.21
1.26
V
1.21 1.234
LCD BIAS
`
Output Voltage Range
FBx Threshold Voltage
FBx Input Current
VOUT
VFB
IFB
25
V
V
1.166 1.190 1.214
200
VFB = 1.4V
nA
LCD Bias Shutdown Input Bias
Current
SHDNx
100
nA
I
SHDNx
= GND
TA = +25°C
Peak Inductor Current
ILIM
330
1.1
1.0
∞
mA
Ω
Internal NFET On-resistance
Switch Pin Leakage Current
Switch On-Time
RDS(ON)
ILEAK
tON
I
SW = 10mA, TA = +25°C, VIN = 5V
VSW = 25V
VFB = 1V
VFB = 1V
µA
µs
ns
V
Switch Off-Time
tOFF
VADJx
IADJx
150
0.9
400
1.5
1.0
ADJx Input Voltage Range
ADJx Input Bias Current
ENTIRE REGULATOR
Operating Voltage
0.3
-2
µA
`
Recommended Operating Range
TA = +25°C
VIN
1.6
1.6
6.0
1.6
V
V
Minimum Start-up Voltage
Start-up Voltage Temperature
κ
mV/°C
Coefficient
Shutdown High Input Voltage
Shutdown Low Input Voltage
VSHDNx
VSHDNx
VIN = 2V
V
V
VIN = 2V
0.4
320
VFBx = 1.4V, VLFB > VBRT – 0.1V
200
VFBx = 1.4V, VLFB > VBRT – 0.1V, VLSHDN
<
<
<
0.35
1
0.4V
VFBx = 1.4V, VLFB > VBRT – 0.1V, VSHDN1
140
80
220
120
1
Quiescent Current
IQ
0.4V
µA
VFBx = 1.4V, VLFB > VBRT – 0.1V, VSHDN2
0.4V
V
SHDN1 < 0.4V, VSHDN2 < 0.4V, VLSHDN <
0.35
0.4V
Copyright 2000
Microsemi
Page 4
Rev. 1.1a, 2004-02-06
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
SIMPLIFIED BLOCK DIAGRAM
LFB
BRT
Control
Logic
Driver
DRV
SRC
Reference
Logic
GND
Current
Limit
4µA
50pF
2.5MΩ
CS
LSHDN
Shutdown
Logic
IN
SHDNx
FBx
OVP
Control
Logic
SWx
Driver
REF
Reference
Logic
Current
Limit
ADJx
50pF
2.5MΩ
Voltage
Reference
FBx
Control
Driver
Logic
SWx
Reference
Logic
Current
Limit
ADJx
50pF
2.5MΩ
Figure 1 – Simplified Block Diagram
Copyright 2000
Microsemi
Page 5
Rev. 1.1a, 2004-02-06
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
APPLICATION CIRCUITS
ILED = 20mA to 0mA
D1
L1
VBAT = 1.6V to 6.0V
47µH
1206 Case Size
C1
R5
R6
D3
4.7µF
- VLCD
DRV
D4
D2
IN
SRC
OVP
SW1
VLCD1 = VIN to 25V
LFB
CS
FB1
(Feedforward Capacitor)
R1
R2
RCS
LX1745
ON OFF
ON OFF
LSHDN
SW2
REF
FB2
D3
RLED
15Ω (typ)
BRT
SHDN1
R3
R4
GND
ADJ1
ADJ2
SHDN2
VLCD2 = VIN to 25V
ON OFF
Figure 2 – LED Driver with Full-Range Dimming plus LCD Bias With Contrast Adjustment Via PWM Input
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 6
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
APPLICATION NOTE
Using a value between 40kΩ and 75kΩ for R2 works well
in most applications. R1 can be determined by the
following equation (where VREF = 1.19V nominal):
FUTNhCeTIOLNXA1L7D45ESCisRIPaTIOtrNiple output Pulse Frequency
Modulated (PFM) boost converter that is optimized for
large step-up voltage applications like LCD biasing and
LED drive.
VOUT - VREF
VREF
eq. 1
R1 = R2
Operating in a pseudo-hysteretic mode with a fixed
switch “off time” of 300ns, converter switching is enabled
when the feedback voltage (VFB) 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 (RCS) 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 BIAS – OUTPUT VOLTAGE ADJUSTMENT
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 (fPWM
≥
100KHz) signal input to be used for the voltage adjustment
signal. (Consequently a DC bias signal can also be used).
LX1745
ADJx
RADJx_1
Reference
Logic
50pF
2.5MΩ
Figure 4 – LCD Bias Adjustment Input
Different PWM signal levels can be accommodated by
selecting a value for RPWM such that the filtered VADJX
value is equal to the reference voltage (eq. 2)
LCD BIAS – OUTPUT VOLTAGE PROGRAMMING
Selecting the appropriate values for LCD Bias output
voltage divider (Figure 3), connected to the feedback pin,
programs the output voltage.
2.5MΩ
2.5MΩ + RPWM _1
eq. 2
VADJx = VPWM ⋅Duty Cycle⋅
VBAT = 1.6V to 6.0V
LX1745
LX1745
RADJx_1
SWx
VOUT
FBx
ADJx
CADJx
RADJx_2
R1
R2
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
Figure 3 – LCD Bias Output Voltage
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 7
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
APPLICATION NOTE
less than 100kHz, an external filter capacitor will be
needed (Figure 5). The value of CPWM is easily calculated
based on the PWM frequency and RPWM_1 using the
following equation.
300mV (VBRT) be used in order to minimize dissipative
losses in the LED current sense resistor (RLED).
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.
50
eq. 3
CPWM
=
π⋅fPWM ⋅RPWM _1
where
eq. 4
R
PWM _1 << 2.5MΩ
For PWM frequencies greater than 100kHz, the external
BRT input resistor is calculated using the following
equation.
LED DRIVER – OUTPUT CURRENT PROGRAMMING
Maximum LED current is easily programmed by
choosing the appropriate value for RLED (Figure 6). It is
recommended that a minimum value of 15Ω 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:
VPWM(DCMAX )− V
BRT(MAX)
eq. 6
R
BRT _1 = 2.5MΩ⋅
VBRT(MAX)
where VBRT is the selected maximum LED current sense
feedback threshold.
VBRTx(MAX)
ILED(MAX)
eq. 5
RLED
=
For PWM frequencies less than 100kHz, the external
BRT input resistors and filter capacitor (Figure 4) are
calculated using the following equations.
D1
L1
VBAT = 1.6V to 6.0V
C1
4.7µF
ROVP_1
VPWM(DCMAX )− V
DRV
SRC
OVP
BRT(MAX)
eq. 7
R
BRT _1 = RBRT _ 2
⋅
VBRT(MAX)
ROVP_2
LFB
where RBRT_2 is selected and VBRT(MAX) is the selected
maximum LED current sense feedback threshold.
LX1745
RCS
CS
RLED
BRT _ 2
R
BRT _1 + R
15Ω
5
RBRT_1
eq. 8
CBRT
=
⋅
BRT
π⋅ fPWM
RBRT _1 ⋅RBRT _ 2
CBRT
RBRT_2
where VRIPPLE is selected to be 10% of VBRT, and fPWM is
the PWM signal frequency.
Figure 6 – LED Current Programming
LED DRIVER – LED BRIGHTNESS ADJUSTMENT
DIODE SELECTION
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 (RLED), 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.
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 peak inductor current
requirements. Further, the diode’s reverse breakdown
voltage characteristic must be capable of withstanding a
It is recommended that a maximum signal voltage of
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 8
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
APPLICATION NOTE
negative voltage transition that is greater than the output
voltage.
can be chosen from the following equation:
IPK − 0.185
≅
eq. 11
RCS
POWER MOSFET SELECTION
−6
30⋅10
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 RDS(ON) is
required to optimize overall circuit performance.
which is taken from the following graph (Figure 7).
OVER VOLTAGE PROTECTION PROGRAMMING
Since the output of the LED Driver is a current mode
configuration, it may be desirable to protect the output from
an over-voltage condition in the event the load is removed
or not present.
1000
800
600
400
200
0
The LX1745 includes an over voltage monitor that is
easily programmed with two external resistors (Figure 6).
This feature eliminates the need for a Zener Diode clamp on
the output.
Programming is accomplished by first selecting ROVP_2
and then calculating ROVP_1 using the following equation.
0
5
10
15
20
RCS (k
)
Ω
VOVP - VREF
VREF
eq. 9
R
OVP_1 = ROVP_ 2
Figure 7 – Peak Current Programming Resistor
This graph characterizes the relationship between peak
inductor current, the inductance value, and the RCS
programming resistor.
where VOVP is the desired maximum voltage on the output.
This voltage should be selected to accommodate the
maximum forward voltage of all the LEDs, over
temperature, plus the maximum feedback voltage.
Conversely, it may also be selected according to the
maximum VDS voltage of the output MOSFET.
INDUCTOR SELECTION
An inductor value of 47µH has been show to yield very
good results. Choosing a lower value emphasizes peak
current overshoot, effectively raises the switching
frequency, and increases the dissipative losses due to
increased currents.
INDUCTOR CURRENT LIMIT PROGRAMMING
Setting of the peak inductor current limit is an important
aspect of the PFM constant off-time architecture; it
determines the maximum output power capability and has a
marked effect on efficiency.
OUTPUT CAPACITOR SELECTION
Output voltage ripple is a function of the several
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:
It is recommended that the peak inductor current be set
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:
eq. 12
L⋅IPK ⋅I
COUT
1
IPK ⋅IOUT
OUT + VF − V
OUT
VRIPPLE
≅
+
IN
V
IN −(VSW + VL )
V
POUT
eq. 10
IPK = 2⋅
where VL is the voltage drop across the inductor, VF is the
forward voltage of the output catch diode, and VSW is the
voltage drop across the power switch. VL+VSW can be
approximated at 0.4V and VF can be approximated at 0.4V.
η⋅ V
IN
where POUT is the total output power, η is the expected
conversion efficiency, and VIN is the input voltage.
From the calculated desired IPK an RCS resistance value
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 9
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
APPLICATION NOTE
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.
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).
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.
VBAT = 1.6V to 6.0V
D3
- VOUT
D4
LX1745
SWx
VOUT
FBx
SHDN
SHDN2
SHDN1
LCD1
LCD2
LED
0
R1
0
0
1
1
1
0
1
0
0
1
0
1
1
1
0
0
1
1
1
1
0
0
0
0
1
1
1
1
0
0
0
0
0
1
0
1
0
0
0
0
0
R2
1
1
0
Figure 8 – Negative Bias Generation
0
0
This negative output is a mirror of the positive output
voltage. However, it is unregulated.
0
Table 1 – Enable Logic
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).
VBAT = 1.6V to 6.0V
D3
VNEG_LCD
D4
R3
R4
R5
LX6431,
LX432
LX1745
SWx
VOUT
FBx
R1
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:
R
R5
eq. 13
4
VNEG_LCD = VREF ⋅ 1+
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. 1.1a, 2004-02-06
Microsemi
Page 10
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
EVALUATION BOARD
Table 2: Input and Output Pin Assignments
Input/Output
Name
Description
Range
0 to 6V
0V
VIN
GND
Main power supply for outputs.
Common ground reference.
Apply a DC voltage or a PWM voltage to this pin to adjust the LCD1
ADJ1
ADJ2
SHDN
output voltage. PWM inputs should be greater than 120Hz.
0 to VIN-100mV
Apply a DC voltage or a PWM voltage to this pin to adjust the LCD2
output voltage. PWM inputs should be greater than 120Hz.
Pulled up to VIN on board (10KΩ), Ground to inhibit the LED driver
output (VOUT).
0 to VIN
SHDN1
SHDN2
Pulled up to VIN on board (10KΩ), Ground to inhibit the VLCD1.
Pulled up to VIN on board (10KΩ), Ground to inhibit the VLCD2.
Output voltage test point. Programmed for 18V output, adjustable up to
VLCD1
-VLCD
VLCD2
≤25V
≥-25V
≤25V
25V.
Output voltage mirror of VLCD1
Output voltage test point. Programmed for 22V output, adjustable up to
25V.
VOUT
FDBK
≤25V
0 to VIN
LED drive voltage probe point.
LED current sense feedback.
Apply a DC voltage or a PWM voltage to this pin to adjust the LED
current. PWM inputs should be greater than 120Hz with a DC portion
less than 350mV.
BRT
0 to 350mV
REF
1.19V Typ.
Buffered IC reference output.
Note: All pins are referenced to ground.
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
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11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
EVALUATION BOARD
Table 3: Jumper Position Assignments
Function
Jumper
JU1
JU2
Position
N/A
Remove the factory installed jumper and insert a 4~6cm wire loop
(optional) to observe the inductor current waveform using a current
probe.
N/A
N/A
JU3
Remove jumper to test open-circuit over-voltage protection implemented
with R1 and R2
JU4
N/A
Note: All pins are referenced to ground.
Table 4: Factory Installed Component List
Ref
Part Description
C1
CAPACITOR, 4.7µF, 1210, 6.3V
CAPACITOR, 4.7µF, 1210, 35V
CAPACITOR, 1000pF, 0805, 35V
CAPACITOR, (SPARE), See Note 1
C2, C5, C6
C3, C7, C8
C4, C11
C9, C10, C12,
CAPACITOR, 1µF, 0805, 35V
C13
CR1, CR2,
Microsemi UPS5819, SCHOTTKY, 1A, 40V, POWERMITE
Microsemi UPWLEDxx, LED, Optomite
CR3, CR4, CR5
LED1, LED2,
LED3, LED4
L1, L2, L3
Q1
R1, R5
R2, R6, R8
R3
INDUCTOR, 47µH, 480mA, SMT
FDV303N MOSFET, 30V, SOT-23
RESISTOR, 1M, 1/16W, 0805
RESISTOR, 75K, 1/16W, 0805
RESISTOR, 15, 1/16W, 0805
RESISTOR, 4.02K, 1/16W, 0805
RESISTOR, 1.25M, 1/16W, 0805
RESISTOR, 10K, 1/16W, 0805
R4
R7
R9, R10, R11
R12, R13
U1
RESISTOR, 100K, 1/16W, 0805
Microsemi LX1745CPW BOOST CONTROLLER
Notes
1. Use these locations to insert additional input and/or output capacitance.
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
Page 12
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
EVALUATION BOARD
1
1
-VLCD
REF
R13
100K
C12
1uF
1
1
VOUT
JU1
CR1
L1
2
1
1
1
2
Vin
JU4
1
CR5
47.0uH
UPS5819
+
+
C1
GND
C4
spare
4.7uF
6.3V
+
+
UPS5819
CR4
R1
1M
C2
C11
GND
C10
1uF
C3
C9
4.7uF Spare
35V
1000pF
LED 4
1uF
Q1
FDV303N
R11
10K
R9
R10
UPS5819
10K
10K
U1
SW1
R2
JU2
L2
LED 3
LED 2
LED 1
2
1
1
2
20
19
18
17
16
15
14
13
12
11
72K
LCDGND1
1
1
1
ADJ1
ADJ1
FB1
REF
3
SHDN1
VIN
4
OVP
BRT
5
SHDN1
SHDN
DRV
6
CS
LFB
7
SRC
8
SHDN
L3
LEDGND
SHDN2
SW2
ADJ2
JU3
9
FB2
2
1
10
LCDGND2
1
SHDN2
1
R4
FDBK
LX1745
4.02K
CR3
R3
15
22V
1
VLCD2
1
1
ADJ2
BRT
UPS5819
+
C6
R7
C7
1.25M
4.7uF
35V
R12
1000pF
100K
R8
C13
1uF
72K
CR2
18V
1
VLCD1
UPS5819
+
C5
R5
1M
C8
4.7uF
35V
1000pF
R6
72K
Figure 10 – LX1745EVAL Evaluation Board Schematic
Figure 11 – LX1745EVAL Evaluation Board
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 13
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
VOUT VERSUS VADJ
GATE DRIVE
30
6
5
4
3
2
1
0
25
20
15
10
5
0
0
0.5
1
1.5
2
0
20
40
60
80 100 120 140 160
Adjustment Signal Voltage (VADJx)
DRV Current (mA)
Figure 12 – Output Voltage Vs. Adjustment Signal Threshold
Note: The LX1745 uses the internal voltage reference
until the VADJ signal exceeds 0.5V (typ).
Figure 13 – Gate Drive Voltage Vs. Drive Current
VIN = 5V, TA = 25°C
WAVEFORM
EFFICIENCY
90%
85%
80%
75%
70%
65%
60%
55%
50%
0
5
10
15
20
25
Output Current (mA)
Figure 15 – LED Driver (Upper) and LCD Bias Efficiency
VIN = 5V, Four LEDs, L = 47µH, RCS = 4kΩ
VIN = 3.6V, VOUT = 5.5V, L = 47µH
Figure 14 – Typical Switching Waveform
CH1 – SWx Voltage, CH2 – Output Voltage, CH3 – Inductor Current
VIN = 3.6V, VOUT = 18V, IOUT = 9mA
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 14
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
PACKAGE DIMENSIONS
20-Pin Thin Small Shrink Outline Package (TSSOP)
PW
3 2 1
P
E
F
D
A
H
L
SEATING PLANE
B
C
G
M
MILLIMETERS
INCHES
Dim
MIN
0.80
0.19
0.09
6.40
4.30
MAX
1.05
0.30
MIN
MAX
0.041
0.012
A
B
0.032
0.007
C
0.180 0.0035 0.0071
D
6.60
4.48
0.252
0.169
0.260
0.176
E
F
0.65 BSC
0.025 BSC
G
H
0.05
–
0.15
1.10
0.70
8°
0.002
–
0.005
0.0433
0.028
8°
L
0.50
0°
0.020
0°
M
P
6.25
–
6.50
0.10
0.246
–
0.256
0.004
*LC
Note: Dimensions do not include mold flash or protrusions; these shall not exceed 0.155mm(.006”) on any side. Lead dimension shall
not include solder coverage.
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
Page 15
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
LX1745
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
PRODUCTION DATASHEET
NOTES
Copyright 2000
Rev. 1.1a, 2004-02-06
Microsemi
Page 16
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
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