LX27901
TM
®
High Performance LED Controller
P
RODUCTION
D
ATASHEET
DESCRIPTION
KEY FEATURES
Three Selectable Drive Modes to
Support Wide Range of Topologies
Optimized Solution for Off PFC LED
Drive Applications
0.8A Source & Sink Drive Capacity
On Chip 5.25V Regulator
Direct Connection to Input Voltage
up to 27V
ACU_BRITE Algorithm to Provide
Accurate Brightness Control at
Highest Operating Efficiency Over
Wide Dimming Duty Range
User Selectable Dimming Mode
Control
Synchronous Regulation Function
to Minimize Power Conversion
Stage
Accurate LED Current Matching In
Combination With Microsemi’s Non-
dissipative Balancing Technology
Robust Operation at Open and
Short LED Conditions
Compensated External and Internal
PWM Dimming Operation
Dedicated Fault Indication
LED Current and Voltage
Monitoring
RoHS Compliant & Halogen Free
APPLICATIONS
LCD LED Backlight TV
LCD LED Backlight Monitors
Other LED Backlight Systems
The LX27901 is a high performance LED
controller optimized for LCD-TV and other
multi-lamp LCD display systems and
provides a cost competitive solution for off-
PFC LED drive applications.
The controller provides a pair of PWM
drive signals with adequate capacity to drive
a half bridge or other double ended DC to
DC converter circuit with the addition of
simple external circuitry to provide safety
isolation.
The device includes a regulated 5.25V
output (VDD) activated by the ENABLE
input working from the VIN input voltage of
up to 27V. VDD is capable of providing
5mA source current to external circuitry
while supplying the on chip driver stage and
the control circuitry.
The controller allows the user to utilize
two different converter drive schemes, LLC
resonant or PWM control mode, selected by
the signal level at ENABLE pin, to best fit a
wide spectrum of application needs.
In addition, a proprietary ACU_BRITE
algorithm is incorporated in the dimming
control operation to allow the user to
adaptively adjust the PWM dimming duty to
achieve accurate brightness control over a
wide dimming duty cycle range while
maintaining the highest operating efficiency.
The LX27901 supports analog LED current
control utilizing the BRTA pin as the analog
LED current setting function.
Direct PWM input dimming operation can
be realized by inputting a dimming pulse
train to DIM pin directly.
The ACU_BRITE algorithm allows the DC
to DC converter to be turned off when the
digital dimming operation is at off state to
eliminate no-load power losses.
The
controller also provides a proprietary
synchronous regulation function to allow the
user to control the LED current and dimming
operation from an un-regulated switching
power supply source to minimize the number
of power conversion stages and the
associated losses and component costs. In
synchronous regulation mode, AOUT and
BOUT form a pair of drive signals to control
the LED string pairs in AC current mode,
and COUT provides another option to
control the LED in DC current mode.
A versatile set of system monitoring and
protection functions are incorporated in the
LX27901 design to provide reliable
protection of the device and the system.
Three input pins, ISNS, VSNS and
UVS/SYN are assigned to support the
regulation and protection sense feedback
needs over wide range of application
configurations. Likewise a dedicated fault
indication pin allows the user to generate a
direct fault report to the system and
configure addition protection features with
simple external circuits.
The device is available in 16 lead SOIC
surface mount package in the industrial
temperature range.
W W W.
Microsemi
.CO
M
IMPORTANT:
For the most current data, consult
MICROSEMI’s
website:
http://www.microsemi.com
PACKAGE ORDER INFO
THERMAL DATA
LX27901
LX27901
T
A
(C)
-40 to +85
D
Plastic SOIC 16 Pin
RoHS Compliant / Pb-free
JA
= 82.2
C/W
THERMAL RESISTANCE
-
JUNCTION TO AMBIENT
LX27901ID
Note: Available in Tape & Reel. Append the letters “TR” to the part number.
(i.e. LX27901ID-TR)
Junction Temperature Calculation: T
J
= T
A
+ (P
D
x
JA
).
The
JA
numbers are guidelines for the thermal
performance of the device/pc-board system. All of the
above assume no ambient airflow.
JA
number above is
with 4-layer pcb board.
Copyright
2011
Rev. 1.1
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
LX27901
TM
®
High Performance LED Controller
P
RODUCTION
D
ATASHEET
PACKAGE PIN OUT
VIN
ENABLE
C_R
BRTA
DIM
SST/
BCOM
FAULT
UVS/
SYN
VDD
AOUT
GND
BOUT
C_OUT
COMP
ISNS
VSNS
AABSOLUTE
MAXIMUM RATINGS
Supply Input Voltage, VIN ............................................................................... 0.3V to 36V
Signal Outputs, VDD, AOUT, BOUT, C_OUT..............................................-0.3V to 6.5V
Signal Inputs, C_R, BRTA .................................................................. -0.3V to VDD+0.3V
Signal Inputs, ENABLE, DIM, ISNS, VSNS, UVS/SYN ................... -0.3V to VDD+0.3V
Signal Outputs COMP, SST/BCOM, FAULT ............................................... -0.3V to VDD
Maximum Junction Temperature ............................................................................... 150°C
Storage Temperature Range ......................................................................... -65C to 150C
Peak Package Solder Reflow Temperature (40 seconds maximum exposure) ........... 260°C
Lead Temperature. (Soldering 10 seconds)................................................................ 300°C
Notes: Exceeding these ratings could cause damage to the device. All voltages are with respect to
GND. Currents are positive into, negative out of specified terminal. These are stress ratings
only and functional operation of the device at these or any other conditions beyond those
indicated under “Recommended Operating Conditions” are not implied. Exposure to
“Absolute Maximum Ratings” for extended periods may affect device reliability.
W W W.
Microsemi
.CO
M
LX27901ID
D P
ACKAGE
(Top View)
RoHS / Pb-free Matte Tin Pin Finish
MSC
LX27901ID
xxxx
ELECTRICAL CHARACTERISTICS
Unless otherwise listed, the following specifications apply over the operating ambient temperature of -40
°
C < Temp < 85
°
C, and VIN = 7.5V to 27V, except
where otherwise noted and the following test conditions: VIN = 12V, C
C_R
= 330pF, R
C_R
= 15k
Parameters
Symbol
Test Conditions/Comments
MIN
TYP
MAX
Units
Power
Controller Input Voltage
Controller Operating Voltage
Power Supply Input Current
Sleep Mode Current
VDD Off Voltage
VIN
VDD
IIN
IDD
SLEEP
1µF bypass cap on VDD pin,
I
VDD
= 0 to 5mA
AOUT, BOUT = no load, VDD = no load
VIN = 24V, V
ENABLE
< 0.8V
ENABLE < 0.8V
7.5
5.0
5.25
7
200
0.1
27
5.5
10
400
V
V
mA
µA
V
Enable
ENABLE On
Shut Down
LLC Resonant Mode
Threshold
PWM Switching Mode
Threshold
Synchronous LED Drive Mode
Threshold
Input High Current
Input Low Current
EN
H
EN
L
V
LLCTH
V
PWMTH
V
SYN
EN
IIH
EN
IIL
LLC resonant drive mode, VDD =
①
5.25V0.25V
PWM switching drive mode, VDD =
5.25V0.25V
Synchronous LED drive mode, VDD =
5.25V0.25V
ENABLE = 2V
0V < V
ENABLE
< 0.8V
2
3.2
4.3
3.5
1.7
0.8
2.8
3.7
VDD
10
5
V
V
V
V
V
LX27901
LX27901
µA
µA
Under Voltage Lockout
VDD Startup Threshold
VDD UVLO Threshold
VDD UVLO Hysteresis
①
VDD
UVLOR
VDD
UVLOF
VDD
UVLOH
Rising Threshold.
Falling Threshold.
4.3
3.95
4.6
4.25
0.4
4.9
4.55
V
V
V
NOTE: The LX27901 operates at LLC mode when the Enable voltage
1.7V @ VIN=12V
Copyright
2011
Rev. 1.1
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 2
LX27901
TM
®
High Performance LED Controller
P
RODUCTION
D
ATASHEET
ELECTRICAL CHARACTERISTICS
Unless otherwise listed, the following specifications apply over the operating ambient temperature of -40
°
C < Temp < 85
°
C, and VIN = 7.5V to 27V, except
where otherwise noted and the following test conditions: VIN = 12V, C
C_R
= 330pF, R
C_R
= 15k
W W W.
Microsemi
.CO
M
Parameters
Symbol
Test Conditions/Comments
MIN
TYP
MAX
Units
Output
Output Resistance
Output Resistance
Dead Time
C_OUT Output Resistance
R
SINK
R
SOURCE
OUT
DT
R
C_OUT
I
SINK
, AOUT and BOUT, T
A
= 25°C
I
SOURCE
, AOUT and BOUT, T
A
= 25°C
T
A
= 25°C, at full duty
Source and sink
270
5
5
350
10
6
6
430
20
Ω
Ω
ns
Ω
Switching Frequency Oscillator
Switching Frequency
Tolerance
LLC Frequency Control
Current Source
Ramp Valley Voltage
Ramp Peak Voltage
Synchronization Range
OSCIN
T
A
= 25°C, Frequency of AOUT, BOUT in PWM
mode, C
C_R
=330pF, R
C_R
= 15K.
V
ENABLE
< 2.8V
OSCVV
OSCPV
Ratio of UVS/SYN input over C_R oscillator
free run frequency
T
A
= 25°C, input, Corresponds to 2V after
4.75X internal amplifier.
Linear operation range for internal circuit
4.75X of ISNS signal. Input outside the range
is clamped at upper or lower limit internally.
K
PRAMP
Source and Sink
V
DIM
V
PDMODETH
Effective for duty from 0 to 100%
Voltage level of FAULT pin, in ACU_BRITE
PWM dimming mode when below threshold; in
conventional PWM dimming mode when
above threshold.
0.5
2.8
3.0
1.05
138
0
150
160
0.5
2.5
3
162
kHz
µA
V
V
Ratio
LED Dimming Control
ISNS Reference
ISNS Linear Input Range
BRTA Input Range
Pre-Amplifier Gain
Error Amp Output Capacity
DIM Input Voltage Range
PWM Dimming Mode Control
Threshold
ISNS
REF
414.6
600
0.5
4.75
100
2.0
3.2
µA
V
V
2.0
421
427.4
mV
mV
V
Fault Monitoring and Protection
VSNS Threshold
VCOMP
Voltage
VSNS
TH
V
VCOMP_diff
Vcomp2 when the Aout present @ Vsns=0V,
ISNS=0.15V; Vcomp1 when the Aout disabled
@ Vsns=3V, ISNS=0V; T
A
= 25°C
VCOMP
Voltage=Vcomp2 – Vcomp1
Fault when below the threshold, positive and
negative input
Functional operation range, positive and
negative input
Fault when below the threshold
Upper threshold for ISNS input
Upper threshold for ISNS input
1.8
0.1
2.0
2.2
V
V
LX27901
LX27901
UVS/SYN Threshold
UVS/SYN Input Range
ISNS Fault Threshold
Over Current Threshold
Short Circuit Threshold
UVS/SYN
TH
V
UVS/SYN
V
ISNSTH
OC
TH
SC
TH
0.33
-0.5
90
580
940
0.4
0.45
4
V
V
mV
mV
mV
110
630
1000
130
680
1060
Copyright
2011
Rev. 1.1
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 3
LX27901
TM
®
High Performance LED Controller
P
RODUCTION
D
ATASHEET
ELECTRICAL CHARACTERISTICS
Unless otherwise listed, the following specifications apply over the operating ambient temperature of -40
°
C < Temp < 85
°
C, and VIN = 7.5V to 27V, except
where otherwise noted and the following test conditions: VIN = 12V, C
C_R
= 330pF, R
C_R
= 15k
W W W.
Microsemi
.CO
M
Parameters
Fault Count
Fault reset Interval
Fault reset Interval
Fault Output Sinking Capacity
Fault Output Open Impedance
Symbol
FC
F
RESET_1
F
RESET_R
F
SINK
R
F
Test Conditions/Comments
At clock frequency of AOUT, BOUT
Interval for resetting fault counter after POR, At
clock frequency of AOUT, BOUT
Periodic Interval for resetting fault counter, At
clock frequency of AOUT, BOUT
VFAULT ≤ 0.8V, TA = 25°C
When in open state
MIN
TYP
256
32768
65536
MAX
Units
Count
Count
Count
mA
MΩ
4
5
6
Bias & References
Soft Start Charging Source
I
SSC
C_B Charging source at start up
2
µA
Power-Up Sequencing
Start up Time Out
TSTART
After POR
32768
Count
Name
Pin #
FUNCTIONAL PIN DESCRIPTION
Description
Input voltage pin. A built-in regulator is connected to this pin to step down the input voltage to the
internal operating voltage VDD. This pin can accept input voltage from 7.5V to 27V to provide the
flexibility of interfacing with either 12V or 24V system supply. A low ESR decoupling capacitor of
not less than 1µF should be connected from this pin to GND with low impedance traces.
Enable pin to control the on/off operation of the controller. A logic low signal <0.8V on this pin
sets the controller in sleep mode to save energy, and a signal > 1.7V activates the controller
operation. The DC signal level on this pin also sets the drive mode of the controller. With
VDD=5.25V5%, when 1.7V<V
ENABLE
<2.8V the AOUT, BOUT signal is set in LLC resonant drive
mode; when 3.2V<V
ENABLE
<3.7V the controller drives the power converter in normal PWM drive
mode; and when V
ENABLE
>4.3V the controller is set in synchronous LED drive mode.
Converter operating frequency programming terminal. A pull-up resistor from this pin to VDD and
a capacitor connected from this pin to ground determines the operating frequency of the inverter.
Terminal for programming the analog LED current setting. An analog signal is accepted from this
pin to set the reference level for the LED current control loop.
PWM dimming control input. V
DIM
> 2V sets the dimming operation in continuous analog mode. A
PWM signal of low level <0.5V and high level > 2V will force the PWM dimming operation to follow
the PWM signal directly. Input level <0.5V will stop the controller output.
VIN
1
ENABLE
2
C_R
3
BRTA
4
DIM
5
LX27901
LX27901
SST/BCOM
6
For PWM dimming brightness compensation and soft start. The elapsed dimming demand and
the accumulated LED current over the active dimming period are compared at this pin. The
comparison result is used to control the LED light output to exactly match the dimming demand
with compensation for the distortion of the LED current during dimming operation. At start up this
pin is connected to a 2µA charging current source to control the soft start process. An internal
active pull down circuit is connected from this pin to COMP to force the COMP voltage rise slowly
with the charge of the capacitor at this pin. Soft start completes when the voltage rises to 2.5V
and the control enters normal operation mode. The 2µA charging source and the COMP pull
down circuit are disconnected then, and this pin is connected to the brightness compensation
circuit.
Fault indication pin with open drain output. Internally pulled down when an open LED string,
FAULT
Copyright
2011
Rev. 1.1
7
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 4
LX27901
TM
®
High Performance LED Controller
P
RODUCTION
D
ATASHEET
Name
Pin #
FUNCTIONAL PIN DESCRIPTION
Description
severely shorted LED string, over voltage of the LED strings, or over current occurs. Once
latched, the fault signal can only be reset by ENABLE or power recycling. The voltage level at
this pin during normal operation also sets the operation mode of dimming control. When the
voltage is above 3V the COMP signal is turned on and off by the digital dimming control signal
and AOUT, BOUT and C_OUT are turned on and off accordingly by the COMP signal. When the
voltage at the FAULT pin is below 3V the COMP signal is turned on and off by the control signal
from the ACU_BRITE dimming control algorithm, and AOUT, BOUT and C_OUT are turned on
and off accordingly by the COMP signal. The voltage level of this pin during normal operation can
be set by an external resistive divider.
Sense for under voltage conditions. A threshold of 0.4V is set internally to compare the sense
feedback signal to this pin when the LED strings are on. A fault will be declared if the sensed
signal stays below the threshold over certain counts set by the internal circuitry. LED string
current and voltage sense signals can be fed to this pin to detect open or short string conditions.
When the controller is set in synchronous regulation mode UVS/SYN will receive the
synchronization signal from the power supply to synchronize the switching frequency of AOUT
and BOUT. An internal rectifier circuit is included to allow an AC input signal to be connected to
this pin directly.
Voltage sense input pin. The voltage signal on this pin feeds the over voltage detection circuit. A
threshold of 2V is set internally for fault detection. When the voltage sense signal reaches the
threshold of 2V, a clamping current source will be activated to pull down COMP signal to limit the
sensed voltage. In the meanwhile, the fault monitoring circuit will start counting the accumulated
fault duration and set up the fault flag when the fault count reaches pre-determined counts.
Current sense input pin. A full wave rectifier circuit is built in to allow AC input to this pin. The
regulation point at this pin is 421mV which is amplified by 4.75 times and then fed to the error
amplifier to compare with a reference signal of 2V to regulate the LED current. This signal is also
used to detect open LED, over current and short circuit conditions. The threshold for open LED is
110mV, the threshold of over current is 630mV, and the threshold for short circuit is 1000mV. A
fault flag will be declared if ISNS signal stays below 110mV or above 630mV over defined counts,
and the operation will be cut off immediately when ISNS signal goes above 1000mV.
Output of the error amplifier. The error amplifier is a Gm type that provides robust control behavior
and simple compensation. The signal on this pin feeds to the PWM modulation circuit to control
the drive output to regulate the LED current. At start up the voltage of this pin is controlled by the
soft start circuit.
Gate signal to drive an additional MOSFET for LED current regulation. The signal is in
synchronous with the C_R oscillator frequency and is leading edge modulated by the output of the
error amplifier in LLC and synchronous regulation modes and trailing edge modulated in PWM
half bridge drive mode. This signal can be used in combination with AOUT and BOUT to extend
the regulation range in LLC operation mode or provide additional driving capabilities in
synchronous regulation mode.
Gate drive output. These driver outputs are capable of providing 0.8A source and sink current at
nominal operating voltage. Dead time is inserted to prevent possible shoot through at full duty
cycle when these signals are used for half bridge or full bridge driving. Internal pull down resistors
of 20kΩ is provided at each pin. When the controller is in resonant drive mode AOUT and BOUT
are driven to maximum duty and their switching frequency varies with COMP signal level; when
the controller is in PWM drive mode these two outputs generate pulse width modulated drive
signals at fixed frequency; and when the controller is in synchronous LED drive mode, UVS/SYN
receives the synchronization signal, AOUT outputs a NFET drive signal in synchronous with the
positive cycle of the UVS/SYN input and BOUT outputs a PFET drive signal in synchronous with
the negative cycle of the UVS/SYN input to drive respective external power devices to control the
LED current directly. Different from the PWM drive mode, the pulse width of AOUT and BOUT is
leading edge modulated in synchronous drive mode.
Ground return of the on chip circuit. It is important to provide a low impedance ground plane to
this pin and the signal control circuitry in PCB layout, and not let the ground return current of the
W W W.
Microsemi
.CO
M
UVS/SYN
8
VSNS
9
ISNS
10
COMP
11
C_OUT
12
LX27901
LX27901
AOUT
BOUT
15
13
GND
Copyright
2011
Rev. 1.1
14
Microsemi
Analog Mixed Signal Group
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
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