LX1745
®
TM
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
KEY FEATURES
DESCRIPTION
guaranteed at a VIN equal to 1.6V with
sustained operation as low as 1.1V.
The maximum LED drive current is
easily programmed using one external
current sense resistor in series with the
LEDs. In this configuration, LED current
provides a feedback signal to the FB pin,
maintaining constant current regardless of
varying LED forward voltage (VF).
Depending on the MOSFET selected, the
LX1745 is capable of achieving an LED
drive in excess of 1.0W.
The LX1745 provides simple dynamic
adjustment of the LED drive current (0% to
100% full range dimming) and the LCD
Bias output voltages (up to ±15% typ)
through separate IC interfaces.
Each
interface has an internal RC filter allowing
designers to make these adjustments via a
direct PWM input signal or an analog
reference signal.
Further, any PWM
amplitude is easily accommodated using a
single external resistor.
The LX1745 is available in the lowprofile 20-Pin TSSOP.
> 90% Maximum Efficiency
Low Quiescent Supply Current
Externally Programmable Peak
Inductor Current Limit For
Maximum Efficiency
Logic Controlled Shutdown
< 1µA Shutdown Current
Dynamic Output LED Current
and Two LCD Bias Voltage
Adjustments Via Analog
Reference Or Direct PWM
Input
20-Pin TSSOP Package
WWW . Microsemi .C OM
The LX1745 is a compact high
efficiency step-up boost regulator for
driving white or color LEDs in LCD
lighting applications while supplying the
necessary LCD bias voltages with an
additional two integrated boost converters.
Designed for maximum efficiency and
featuring a psuedo-hysteretic PFM
topology (that decreases output voltage
ripple), the LX1745 minimizes system cost
and condenses layout area making it ideal
for PDA, smart-phone, and digital camera
applications.
While the LCD Bias generation is
implemented using an internal N-Channel
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..
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
voltages
and
start-up
is
APPLICATIONS
Pagers
Smart Phones
PDAs
Handheld Computers
General LCD Bias Applications
LED Driver
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
- VLCD
C1
4.7μF
DRV
IN
SRC
VLCD1 = VIN to 25V
SW1
OVP
LFB
FB1
CS
LX1745
ON OFF
LSHDN
SW2
RSET
15Ω
BRT
REF
FB2
GND
VLCD2 = VIN to 25V
SHDN2
ON OFF
SHDN1
ADJ1
ADJ2
LX1745 Evaluation Board
ON OFF
LX1745
PACKAGE ORDER INFO
PW
TA (°C)
Plastic TSSOP
20-Pin
RoHS Compliant / Pb-free
Transition DC: 0442
-40 to 85
LX1745CPW
Note: Available in Tape & Reel. Append the letters “TR”
to the part number. (i.e. LX1745CPW-TR)
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 1
LX1745
®
TM
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
PACKAGE PIN OUT
Supply Input Voltage ...........................................................................-0.3V to 7V
Feedback Input Voltage (VFBx) ...............................................-0.3V to VIN + 0.3V
Shutdown Input Voltage (V SHDN 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
RoHS / Pb-free Peak Package Solder Reflow Temperature
(40 second maximum exposure) ..................................................... 260°C (+0, -5)
SW1
ADJ1
SHDN1
IN
LSHDN
DRV
SRC
GND
SHDN2
SW2
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
WWW . Microsemi .C OM
ABSOLUTE MAXIMUM RATINGS
GND
FB1
REF
OVP
BRT
CS
LFB
ADJ2
FB2
GND
PW PACKAGE
(Top View)
RoHS / Pb-free 100% Matte Tin Lead Finish
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
PW
Plastic TSSOP 20-Pin
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.
PACKAGE DATA
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 2
LX1745
TM
®
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
FUNCTIONAL PIN DESCRIPTION
Description
IN
Unregulated IC Supply Voltage Input – Input range from +1.6V to 6.0V. Bypass with a 1μF or greater capacitor
for operation below 2.0V.
DRV
LED MOSFET Gate Driver – Connects to an external N-Channel MOSFET.
SRC
LED MOSFET Current Sense Input - Connects to the External N-Channel MOSFET Source.
OVP
Over Voltage Programming Pin – Connects to a resistor divider between the output load and GND to set the
maximum output voltage. OVP has a voltage threshold of 1.2V
LFB
LED Current Feedback Input – Connects to a current sense resistor between the LED output load and GND to
set the LED drive current.
GND
Common terminal for ground reference.
BRT
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.
REF
Buffered Reference Output – Connected to the internal bandgap reference voltage of 1.2V.
SWx
LCD Bias Inductor Switch Connection – Internally connected to the drain of a 28V N-channel MOSFET. SW is
high impedance in shutdown.
FBx
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.
ADJx
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.
LSHDN
LED Driver Active-Low Shutdown Input – A logic low shuts down the LED driver circuitry and reduces the supply
current by 60μA (Typ). Pull LSHDN high for normal operation.
SHDNx
LCD Bias Active-Low Shutdown Input – A logic low shuts down the LCD Bias circuitry and reduces the supply
current by 60μA (Typ). Pull SHDNx high for normal operation.
CS
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Name
Current-Sense Amplifier Input – Connecting a resistor between CS and GND sets the peak inductor current
limit.
PACKAGE DATA
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 3
LX1745
®
TM
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
ELECTRICAL CHARACTERISTICS
Parameter
`
LFB Threshold Voltage
VLFB
LFB Input Bias Current
BRT Input Voltage Range
BRT Input Bias Current
LED Driver Shutdown Input
Bias Current
Current Sense Bias Current
ILFB
VBRT
IBRT
DRV Sink/Source Current
DRV On-Resistance
Maximum Switch On-Time
Minimum Switch Off-Time
OVP Threshold Voltage
Reference Voltage
Min
LX1745
Typ
Max
Units
I SHDN1
VBRT = 100mV
VBRT = 20mV
VLFB = 100mV
85
5
-100
0
BRT = 100mv
0.0V ≤ SHDN1 ≤ VIN
-100
ICS
IPK
RDRV(ON)
tON
tOFF
VOVP
VREF
100
20
RCS = 0kΩ
RCS = 2kΩ
VIN = 5V, VDRV = 3V
VCC = 5V
VFB = 1V
VFB = 1V
85
115
35
100
VIN
60
100
4
170
210
100
12
200
1.15
1.186
300
1.21
1.21
1.166
1.190
mV
nA
V
nA
nA
μA
mA
15
∞
410
1.26
1.234
mA
Ω
μS
nS
V
LCD BIAS
Output Voltage Range
FBx Threshold Voltage
FBx Input Current
LCD Bias Shutdown Input Bias
Current
Peak Inductor Current
Internal NFET On-resistance
Switch Pin Leakage Current
Switch On-Time
Switch Off-Time
ADJx Input Voltage Range
ADJx Input Bias Current
`
Test Conditions
70°C except where
LED DRIVER
Switch Peak Current
`
Symbol
≤
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Unless otherwise specified, the following specifications apply over the operating ambient temperature 0°C ≤ TA
otherwise noted and the following test conditions: VIN = 3V, LSHDN = VIN, SHDN1 = VIN, SHDN2 = VIN
ENTIRE REGULATOR
Operating Voltage
Minimum Start-up Voltage
Start-up Voltage Temperature
Coefficient
Shutdown High Input Voltage
Shutdown Low Input Voltage
Copyright © 2000
Rev. 1.2, 2006-02-27
VFB = 1.4V
I SHDNx
SHDNx = GND
ILIM
RDS(ON)
ILEAK
tON
tOFF
VADJx
IADJx
TA = +25°C
ISW = 10mA, TA = +25°C, VIN = 5V
VSW = 25V
VFB = 1V
VFB = 1V
VIN
150
0.9
1.6
TA = +25°C
κ
V SHDNx
V SHDNx
IQ
V
V
nA
100
nA
1.0
∞
400
1.5
1.0
mA
Ω
μA
μs
ns
V
μA
6.0
1.6
V
V
330
1.1
0.3
Recommended Operating Range
25
1.214
200
-2
VIN = 2V
VIN = 2V
VFBx = 1.4V, VLFB > VBRT – 0.1V
VFBx = 1.4V, VLFB > VBRT – 0.1V, V LSHDN <
0.4V
VFBx = 1.4V, VLFB > VBRT – 0.1V, V SHDN1 <
0.4V
VFBx = 1.4V, VLFB > VBRT – 0.1V, V SHDN2 <
0.4V
V SHDN1 < 0.4V, V SHDN2 < 0.4V, V LSHDN <
0.4V
mV/°C
1.6
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
200
0.4
320
0.35
1
140
220
80
120
0.35
1
V
V
ELECTRICALS
Quiescent Current
VOUT
VFB
IFB
μA
Page 4
LX1745
®
TM
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
SIMPLIFIED BLOCK DIAGRAM
WWW . Microsemi .C OM
LFB
Control
Logic
Driver
DRV
SRC
Reference
Logic
BRT
50pF
2.5MΩ
GND
Current
Limit
4μA
CS
Shutdown
Logic
IN
LSHDN
SHDNx
OVP
FBx
Control
Logic
REF
Reference
Logic
ADJx
50pF
2.5MΩ
Driver
SWx
Current
Limit
Voltage
Reference
FBx
Control
Logic
Driver
SWx
Reference
Logic
Current
Limit
BLOCK DIAGRAM
ADJx
50pF
2.5MΩ
Figure 1 – Simplified Block Diagram
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 5
LX1745
®
TM
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
APPLICATION CIRCUITS
WWW . Microsemi .C OM
ILED = 20mA to 0mA
D1
L1
VBAT = 1.6V to 6.0V
47μH
1206 Case Size
L2
L3
D3
- VLCD
D4
C1
4.7μF
R5
DRV
IN
SRC
SW1
D2
VLCD1 = VIN to 25V
R6
OVP
LFB
FB1
(Feedforward Capacitor)
R1
CS
LX1745
LSHDN
D3
R2
SW2
RCS
ON OFF
RLED
15Ω (typ)
BRT
REF
FB2
R3
VLCD2 = VIN to 25V
GND
SHDN2
SHDN1
ON OFF
ADJ1
ADJ2
R4
ON OFF
Figure 2 – LED Driver with Full-Range Dimming plus LCD Bias With Contrast Adjustment Via PWM Input
APPLICATIONS
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 6
LX1745
®
TM
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
APPLICATION NOTE
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.
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):
R1 = R2
VOUT - VREF
VREF
eq. 1
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
Reference
Logic
50pF
ADJx
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FUNCTIONAL DESCRIPTION
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 (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 offtime 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.
RADJx_1
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)
⎛
⎞
2.5MΩ
⎟
VADJx = VPWM ⋅ Duty Cycle ⋅ ⎜
⎜ 2.5MΩ + R
⎟
PWM _ 1 ⎠
⎝
eq. 2
VBAT = 1.6V to 6.0V
LX1745
SWx
RADJx_1
ADJx
R1
CADJx
R2
Figure 5 – LCD Bias Adjustment Input Filter
Figure 3 – LCD Bias Output Voltage
Copyright © 2000
Rev. 1.2, 2006-02-27
LX1745
FBx
RADJx_2
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
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 7
APPLICATIONS
VOUT
LX1745
®
TM
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
APPLICATION NOTE
CPWM =
50
π ⋅ fPWM ⋅ RPWM _ 1
eq. 3
where
eq. 4
RPWM _ 1
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