LX27902
High Performance LED Controller
Description
The LX27902 is a high performance LED controller
optimized for LCD-TV and other LCD display systems and
provides a cost competitive solution for off-PFC LED drive
applications.
The controller provides a versatile solution for both DC
power regulation and LED drive control within a single 24
pin IC chip. In addition, the power circuit architecture
supported by LX27902 allows the designer to use a single
power transformer and single primary switching stage with
Microsemi’s proprietary synchronous regulation technology
to supply both the DC outputs and LED drive power, further
saving the total cost of the power system.
The device provides on chip 5.25V output from VIN input
with the working range of up to 27V.
The controller provides two sets of drive signals: AOUT
and BOUT to drive a LLC switching stage, and COUT to
drive a synchronous regulation stage, both with independent
control loop. The drive format of COUT can be further
configured by the voltage level of ENABLE signal to support
either synchronous regulation operation or direct LED
current control to satisfy a particular PWM dimming
requirement.
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.
It also provides analog LED current control to allow the
system to change the LED current dynamically in real time
to facilitate particular application requirement such as
display mode switching between 2D and 3D. Direct PWM
input dimming operation can be realized by inputting a
dimming pulse train to DIM pin directly.
The synchronous regulation technology and ACU_BRITE
algorithm changes the traditional approach in LED backlight
control. The synchronous regulation scheme helps user to
eliminate the DC-DC converter stage and ACU_BRITE
algorithm maintains accurate brightness control without the
need of a dimming control switch to turn on and off the LED
current directly, providing the benefits of both cost saving
and efficiency improvement without compromise of
performance.
A versatile set of system monitoring and protection
functions are also incorporated in the LX27902 design to
provide reliable protection of the device and the system.
In addition the LX27902 supports standby power control
thus providing a single system controller.
The device is available in 24 lead SOIC surface mount
package in the industrial temperature range.
Features
Single Chip to Support DC Output Regulation
and LED Lighting Control
Simplified Power Circuit Architecture with High
Operating Efficiency and Reduced System Cost
Supports Single Transformer Architecture for
Regulated DC Output and LED Drive Without the
Need for Secondary Side DC to DC Converter
ACU_BRITE Algorithm Provides Accurate
Brightness Control over Wide Dimming Duty
Range Regardless the Circuit Topology and LED
Current Wave Shape
0.6A Source & Sink Drive & On Chip 5.25V
Regulator
Input Supply Voltage up to 27V
Versatile LED Current and Voltage Monitoring
and Reliable Fault Detection and Protection
Accurate LED Current Control Without
Involvement of Dissipative Power Regulation
Stage
Robust Operation at Open and Short LED
Conditions
Dedicated Fault Indication Signal
RoHS Compliant & Halogen Free
Applications
LCD LED Backlight TV
LCD LED Backlight Monitors
Other LED Backlight Systems
March 2013 Rev. 1.0
www.microsemi.com
© 2013 Microsemi Corporation- Analog Mixed Signal Group
1
High Performance LED Controller
Block Diagram
Figure 1 ·
Functional Block Diagram
2
Pin Configuration
Pin Configuration
Figure 2 ·
Pinout
•
Top view XX XX= Encoded date code
•
24 Pin Wide body SOIC package
Ordering Information
Ambient
Temperature
Type
Package
Part Number
LX27902IDW
LX27902IDW-TR
Packaging Type
Bulk / Tube
Tape and Reel
-40°C to 85°C
RoHS2 compliant,
Pb-free
SOIC24
3
High Performance LED Controller
Pin Description
Pin Number
Pin Designator
Description
Input voltage pin. A built-in regulator is connected from this pin to supply a
regulated voltage to VDD pin to feed the internal circuits. This pin can accept
input voltage from 7.5V to 27V providing the flexibility of interfacing with either
12V or 24V system supplies. A low ESR decoupling capacitor of not less
than 2.2µF should be connected from this pin to AGND with low impedance
traces.
A signal >2V activates the LED operation of the controller. The DC signal
level on this pin also sets the dimming operation mode. When 2V < V
ENABLE
<
3.3V dimming operation is performed in ACU_BRITE dimming mode. When
3.7V < V
ENABLE
< VDD dimming operation is performed in regular dimming
mode. The dimming mode selection is active throughout the entire operation
cycle of the controller.
Frequency programming terminal for LLC drive of AOUT and BOUT. A pull-
up resistor from this pin to VDD, and a capacitor connected from this pin to
AGND determines the operating frequency of the inverter. A frequency
control circuit is connected to this pin internally for LLC modulation control.
Current source setting terminal for limiting the maximum switching frequency
of AOUT and BOUT. A reference voltage of 2V is provided internally for
setting the current with an external resistor.
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. Effective operating range is 0.2V to 2V. A clamping circuit is built-in to
limit the signal feeding to the internal circuit within the effective range.
PWM dimming control input. V
DIM
> 2V sets the dimming operation in
continuous analog mode. A PWM signal amplitude low level < 0.5V, high
level > 2.0V will force the PWM dimming operation to follow the PWM signal
directly. Input level <0.5V will stop the controller output.
ACU_BRITE dimming compensation and soft start. The elapsed dimming
demand and the accumulated LED current over the active dimming period are
compared at this pin and comparison result is used to control the LED light
output to exactly match the dimming demand. Accurate LED dimming control
is guaranteed with closed loop compensation for the distortions 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
clamping circuit is connected from this pin to COMP to force the COMP
voltage to rise slowly with the charge of the capacitor at this pin. Soft start
terminates when the voltage rises to 3V 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 ACU_BRITE circuit.
Clamping the voltage of this pin below 3V will disable the ACU_BRITE
compensation and the burst dimming control will follow the conventional PWM
operation mode.
Ramp generation oscillator for COUT drive circuit. The frequency of the
oscillator should be synchronized to the input signal from SYNC pin. A
resistor from this pin to VDD and a capacitor from this pin to AGND sets the
base frequency. The base frequency should be normally lower than the
minimum operating frequency of CR1.
Synchronization signal input pin. The frequency of CR2 oscillator is
synchronized by the signal from this pin.
1
VIN
2
ENABLE
3
CR1
4
I_R
5
BRTA
6
DIM
7
SST_BCOM
8
CR2
9
SYNC
4
Pin Description
Pin Number
Pin Designator
Description
Error amplifier output of the COUT drive loop. 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. In addition, a 250µA pull down current
source is connected to this pin and will be activated when the VSNS signal
>2V, LED over current or short circuit, or ENABLE signal is off. COUT will be
shut off when COMP2 voltage drops below 0.5V.
Sense input for the COUT loop. The input signal at this pin is amplified by 5
times internally before feeding to the error amplifier to compare with the
reference signal from BRTA pin. The amplified signal is also used to detect
open LED, over current and short circuit conditions. The threshold for open
LED is set to be 50% of the reference signal divided by 5, the threshold of
over current is 600mV. The 8 bit fault counter will be activated when FB2
signal stays below the open lamp threshold or above the over current
threshold and the fault flag will be asserted if the fault persists when the fault
counter is full.
Short circuit sensing input for detection of short circuit conditions of the LED
drive circuit. A protection action will be initiated to cut off the LED supply and
activate the fault timing. The fault flag will be raised when the fault timing
counter reaches full count.
Voltage sense input pin. The sense signal at this pin feeds the over voltage
detection circuit. A threshold of 2V is set internally for over voltage detection.
When the VSNS signal reaches the threshold of 2V, the voltage regulation
current source will be activated to pull down COMP2 to limit the amplitude of
the voltage. Meanwhile, the fault monitoring circuit will start counting with an
8 bit fault counter and raise the fault flag when the fault counter is full.
Current sense feedback of the power stage driven by AOUT, BOUT. A
threshold of 0.6V is set internally as the over current protection limit with a
negative current sense input. A 250µA internal current regulation source will
be turned on to pull down the COMP1 signal at the instant the input signal
drops below the 0.6V over current threshold. A fault flag will be raised when
an over current event occurs 32 times consecutively and a soft start process
will be initiated. The capacitor at the ST_BCOM pin will be discharged and
COMP1 signal will be forced to rise together with the charge of the
SST_BCOM capacitor until the voltage reaches 3V.
Sense feedback for AOUT, BOUT drive loop. The signal is compared with an
internal reference of 2.0V for output regulation. This signal is also used to
detect over voltage conditions. The threshold for over voltage detection is
2.5V. The fault counter will start counting when the over-voltage condition
occurs. A fault flag will be asserted if the fault condition persists when the
counter reaches 32 counts, and a soft start cycle will be initiated upon the
assertion of the fault flag. A hysteresis control loop is also equipped with this
pin to maintain the signal level at this pin between the reference level of 2V
and upper limit of FB1
REF
+0.07V at standby or open load condition. The
250µA current source will be activated to pull down COMP1 signal when FB1
reaches the upper limit of FB1
REF
+0.07V.
This pin provides a compensation function to the AOUT, BOUT drive loop to
enhance the cross regulation effect from the COUT drive loop operation.
Output of the error amplifier of AOUT, BOUT drive loop. The error amplifier is
a GM type that provides robust control behavior and simple compensation.
The signal of this pin feeds to the LLC control circuit to regulate the DC output
according to the feedback signal at FB1 pin. A 0.5V comparator is equipped
at this pin to control the on/off of the LLC drive signal. AOUT and BOUT will
10
COMP2
11
FB2
12
SCS
13
VSNS
14
ISNS
15
FB1
16
FF2
17
COMP1
5
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