Microsemi LX1745 Triple output boost - led driver / lcd bia Datasheet

LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
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.
ƒ > 90% Maximum Efficiency
ƒ Low Quiescent Supply Current
ƒ Externally Programmable Peak
Inductor Current Limit For
Maximum Efficiency
ƒ Logic Controlled Shutdown
ƒ < 1µA Shutdown Current
ƒ Dynamic Output LED Current
and Two LCD Bias Voltage
Adjustments Via Analog
Reference Or Direct PWM
Input
ƒ 20-Pin TSSOP Package
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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
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
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
LSHDN
SW2
ON OFF
RSET
15Ω
BRT
REF
FB2
GND
VLCD2 = VIN to 25V
SHDN2
SHDN1
ON OFF
ADJ1
ADJ2
LX1745 Evaluation Board
ON OFF
LX1745
PACKAGE ORDER INFO
TA (°C)
-40 to 85
Plastic TSSOP
PW 20-Pin
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
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
ABSOLUTE MAXIMUM RATINGS
PACKAGE PIN OUT
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
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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
Lead Temperature (Soldering 10 seconds) .................................................. 300°C
GND
FB1
REF
OVP
BRT
CS
LFB
ADJ2
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
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. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 2
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
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. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 3
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
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. 1.1a, 2004-02-06
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
I N T E G R A T E D
P R O D U C T S
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
SIMPLIFIED BLOCK DIAGRAM
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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. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
APPLICATION CIRCUITS
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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. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
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. 1.1a, 2004-02-06
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
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Page 7
APPLICATIONS
VOUT
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
APPLICATION NOTE
CPWM =
50
π ⋅ fPWM ⋅ RPWM _ 1
eq. 3
where
eq. 4
RPWM _ 1 << 2.5MΩ
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:
RLED =
VBRTx(MAX )
eq. 5
ILED(MAX)
D1
L1
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.
For PWM frequencies greater than 100kHz, the external
BRT input resistor is calculated using the following
equation.
 V (DCMAX ) − VBRT(MAX) 

RBRT _ 1 = 2.5MΩ ⋅  PWM


VBRT(MAX)


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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.
eq. 6
where VBRT 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.
VBAT = 1.6V to 6.0V
ROVP_1
C1
4.7µF
 V (DCMAX ) − VBRT(MAX) 

RBRT _ 1 = RBRT _ 2 ⋅  PWM


VBRT(MAX)


DRV
SRC
ROVP_2
eq. 7
OVP
where RBRT_2 is selected and VBRT(MAX) is the selected
maximum LED current sense feedback threshold.
LFB
LX1745
RCS
CS
RBRT_1
RLED
15Ω
CBRT =
BRT
CBRT
RBRT_2
R
+ RBRT _ 2 

⋅  BRT _ 1
R

 BRT _ 1 ⋅ RBRT _ 2 
eq. 8
where VRIPPLE is selected to be 10% of VBRT, and fPWM is
the PWM signal frequency.
Figure 6 – LED Current Programming
DIODE SELECTION
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
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Page 8
APPLICATIONS
LED DRIVER – LED BRIGHTNESS ADJUSTMENT
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.
It is recommended that a maximum signal voltage of
Copyright  2000
Rev. 1.1a, 2004-02-06
5
π ⋅ fPWM
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
APPLICATION NOTE
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.
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.
ROVP _ 1 = ROVP _ 2
VOVP - VREF
VREF
POUT
η ⋅ VIN
eq. 10
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
eq. 11
1000
800
600
400
200
0
0
5
10
15
20
RCS (kΩ )
Figure 7 – Peak Current Programming Resistor
This graph characterizes the relationship between peak
inductor current, the inductance value, and the RCS
programming resistor.
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.
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:
 eq.
 L ⋅ I ⋅ I 
1
IPK ⋅ IOUT
VRIPPLE ≅  PK OUT 
+

C
V
(
V
V
)
V
V
V
−
+
+
−
OUT
SW
L
OUT
F
IN 
 IN

12
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.
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Page 9
APPLICATIONS
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.
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:
IPK − 0.185
30 ⋅ 10 −6
which is taken from the following graph (Figure 7).
eq. 9
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.
IPK = 2 ⋅
RCS ≅
Peak Inductor Current
(mA)
POWER MOSFET SELECTION
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.
can be chosen from the following equation:
WWW . Microsemi .C OM
negative voltage transition that is greater than the output
voltage.
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
APPLICATION NOTE
VBAT = 1.6V to 6.0V
D3
- VOUT
D4
LX1745
VOUT
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.
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.
SWx
FBx
R1
R2
Figure 8 – Negative Bias Generation
This negative output is a mirror of the positive output
voltage. However, it is unregulated.
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).
D3
SHDN
0
0
1
1
1
0
1
0
SHDN2
0
1
0
1
1
1
0
0
SHDN1
1
1
1
1
0
0
0
0
LCD1
1
1
1
1
0
0
0
0
LCD2
0
1
0
1
0
0
0
0
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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).
LED
0
0
1
1
0
0
0
0
Table 1 – Enable Logic
VBAT = 1.6V to 6.0V
VNEG_LCD
D4
R3
R4
LX1745
LX6431,
LX432
VOUT
R5
SWx
FBx
R1
R2
Figure 9 – Regulated Negative Bias
 R 
VNEG _ LCD = VREF ⋅ 1 + 4 
 R5 
APPLICATIONS
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:
eq. 13
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
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 10
LX1745
I N T E G R A T E D
P R O D U C T S
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
EVALUATION BOARD
Name
VIN
0 to 6V
GND
0V
ADJ1
0 to VIN-100mV
Description
Main power supply for outputs.
Common ground reference.
Apply a DC voltage or a PWM voltage to this pin to adjust the LCD1
output voltage. PWM inputs should be greater than 120Hz.
ADJ2
Apply a DC voltage or a PWM voltage to this pin to adjust the LCD2
output voltage. PWM inputs should be greater than 120Hz.
SHDN
Pulled up to VIN on board (10KΩ), Ground to inhibit the LED driver
output (VOUT).
SHDN1
0 to VIN
SHDN2
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Table 2: Input and Output Pin Assignments
Input/Output
Range
Pulled up to VIN on board (10KΩ), Ground to inhibit the VLCD1.
Pulled up to VIN on board (10KΩ), Ground to inhibit the VLCD2.
VLCD1
≤25V
Output voltage test point. Programmed for 18V output, adjustable up to
25V.
-VLCD
≥-25V
Output voltage mirror of VLCD1
VLCD2
≤25V
Output voltage test point. Programmed for 22V output, adjustable up to
25V.
VOUT
≤25V
LED drive voltage probe point.
FDBK
0 to VIN
LED current sense feedback.
BRT
0 to 350mV
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.
REF
1.19V Typ.
Buffered IC reference output.
Note: All pins are referenced to ground.
APPLICATIONS
Copyright  2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 11
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
EVALUATION BOARD
Position
JU1
N/A
JU2
N/A
JU3
N/A
JU4
N/A
WWW . Microsemi .C OM
Table 3: Jumper Position Assignments
Jumper
Function
Remove the factory installed jumper and insert a 4~6cm wire loop
(optional) to observe the inductor current waveform using a current
probe.
Remove jumper to test open-circuit over-voltage protection implemented
with R1 and R2
Note: All pins are referenced to ground.
Table 4: Factory Installed Component List
Ref
Part Description
C1
CAPACITOR, 4.7µF, 1210, 6.3V
C2, C5, C6
CAPACITOR, 4.7µF, 1210, 35V
C3, C7, C8
CAPACITOR, 1000pF, 0805, 35V
C4, C11
CAPACITOR, (SPARE), See Note 1
C9, C10, C12,
CAPACITOR, 1µF, 0805, 35V
C13
CR1, CR2,
Microsemi UPS5819, SCHOTTKY, 1A, 40V, POWERMITE
CR3, CR4, CR5
LED1, LED2,
LED3, LED4
Microsemi UPWLEDxx, LED, Optomite
L1, L2, L3
INDUCTOR, 47µH, 480mA, SMT
Q1
FDV303N MOSFET, 30V, SOT-23
R1, R5
RESISTOR, 1M, 1/16W, 0805
R2, R6, R8
RESISTOR, 75K, 1/16W, 0805
R3
RESISTOR, 15, 1/16W, 0805
R4
RESISTOR, 4.02K, 1/16W, 0805
R7
RESISTOR, 1.25M, 1/16W, 0805
R9, R10, R11
RESISTOR, 10K, 1/16W, 0805
R12, R13
RESISTOR, 100K, 1/16W, 0805
U1
Microsemi LX1745CPW BOOST CONTROLLER
Notes
1.
Use these locations to insert additional input and/or output capacitance.
APPLICATIONS
Copyright  2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 12
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
EVALUATION BOARD
Vin
1
GND
1
JU1
L1
+C4
spare
R9 R10
10K 10K
ADJ1
1
SHDN1
1
SHDN
1
SHDN2
1
CR1
L2
C9
1uF
L3
1
2
3
4
5
6
7
8
9
10
1
JU3
2
1
UPS5819
SW1
LCDGND1
ADJ1
FB1
REF
SHDN1
VIN
OVP
BRT
SHDN
DRV
CS
SRC
LFB
LEDGND
ADJ2
SHDN2
FB2
SW2
LCDGND2
1
C6
4.7uF
35V
+
C7
1000pF
+
C2
4.7uF
35V
+
C11
Spare
R2
72K
20
19
18
17
16
15
14
13
12
11
1
GND
1
FDBK
1
ADJ2
1
BRT
LED 4
LED 3
LED 2
LED 1
R4
LX1745
VLCD2
VOUT
1
R1
C3
1M 1000pF
UPS5819 C10
1uF
CR4
4.02K
CR3
22V
1
JU4
CR5
U1
JU2
2
REF
2
UPS5819
Q1
FDV303N
R11
10K
1
R13
100K
2
47.0uH
C1
4.7uF
6.3V
+
1
-VLCD
WWW . Microsemi .C OM
C12
1uF
1
R3
15
UPS5819
R7
1.25M
R12
100K
R8
72K
C13
1uF
CR2
UPS5819
18V
C5
4.7uF
35V
+
C8
1000pF
1
VLCD1
R5
1M
R6
72K
Figure 10 – LX1745EVAL Evaluation Board Schematic
APPLICATIONS
Figure 11 – LX1745EVAL Evaluation Board
Copyright  2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 13
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
VOUT VERSUS VADJ
GATE DRIVE
6
DRV Voltage (V)
Output Voltage (V)
WWW . Microsemi .C OM
30
25
20
15
10
5
0
5
4
3
2
1
0
0
0.5
1
1.5
2
0
20
40
Adjustment Signal Voltage (VADJx)
60
80
100 120 140 160
DRV Current (mA)
Figure 12 – Output Voltage Vs. Adjustment Signal Threshold
Figure 13 – Gate Drive Voltage Vs. Drive Current
Note: The LX1745 uses the internal voltage reference
until the VADJ signal exceeds 0.5V (typ).
VIN = 5V, TA = 25°C
EFFICIENCY
Efficiency
WAVEFORM
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
Figure 14 – Typical Switching Waveform
CH1 – SWx Voltage, CH2 – Output Voltage, CH3 – Inductor Current
VIN = 5V, Four LEDs, L = 47µH, RCS = 4kΩ
VIN = 3.6V, VOUT = 5.5V, L = 47µH
VIN = 3.6V, VOUT = 18V, IOUT = 9mA
CHARTS
Copyright  2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 14
LX1745
I N T E G R A T E D
Triple Output Boost – LED Driver / LCD Bias
P R O D U C T S
P RODUCTION D ATASHEET
PACKAGE DIMENSIONS
WWW . Microsemi .C OM
PW
20-Pin Thin Small Shrink Outline Package (TSSOP)
3 21
P
E
F
D
A H
SEATING PLANE
B
Note:
MILLIMETERS
MIN
MAX
0.80
1.05
0.19
0.30
0.09
0.180
6.40
6.60
4.30
4.48
0.65 BSC
0.05
0.15
–
1.10
0.50
0.70
0°
8°
6.25
6.50
–
0.10
L
C
M
INCHES
MIN
MAX
0.032
0.041
0.007
0.012
0.0035 0.0071
0.252
0.260
0.169
0.176
0.025 BSC
0.002
0.005
–
0.0433
0.020
0.028
0°
8°
0.246
0.256
–
0.004
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
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 15
MECHANICALS
Dim
A
B
C
D
E
F
G
H
L
M
P
*LC
G
LX1745
I N T E G R A T E D
P R O D U C T S
Triple Output Boost – LED Driver / LCD Bias
P RODUCTION D ATASHEET
NOTES
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NOTES
Copyright  2000
Rev. 1.1a, 2004-02-06
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 16
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