Hisense LCD 2031 LED37T28KV – SMPS schematic – Circuit description OZ9957 CP1396A

  LED driver circuit is a unique LED LCD TV circuit; its function is the output level LED strip light after the required DC voltage, through a variety of over-voltage, over current, short-circuits protection circuit to control the LED light of the current to prevent damage to the LED.

  LED driver circuit is the core device is the LED backlight control application specific integrated circuit, such as embossing company 0Z9957, 0Z9986, 0Z9902. BCD's AP3039. Roma's BD9204, these works basically the same chip. Chip output drive signal and an external MOS switch, inductor and rectifier diodes boost circuit to drive the LED lights of the work.

 Schematic

  2031 power LED driver circuit board using the control chip is 029,957, which is the single-channel LED driver IC, the internal block diagram pictured above, because 32-inch and 37 inch TV with a four LED lights, so they need four 029 957 to drive. This four-drive circuit is independent work, the circuit node 

[Feet > Terminal]

(1) foot: synchronous signal input for multiple chips simultaneously. 

(2) feet: the oscillator frequency set foot. 

(3) feet: synchronization signal output for multiple chips simultaneously. 

(4) feet: the analog ground.

(5) feet: the phase setting for multiple chips simultaneously. 

(6) feet: PWM dimming signal input.

(7) feet: LED current sensing. 

(8) feet: MOS tube current step-up testing. 

(9) feet: soft-start and compensation feet. 

(10) feet: over-voltage protection test. 

(11) feet: OCP, OVP, OLP protection delay setting. 

(12) feet: the beginning to end. 

(13) feet: the input voltage. 

(14) feet: reference level output. 

(15) feet: the external MOS drive signal output tube. 

(16) feet: the power to structure almost identical. The following describes an example to them all the way.

Drive circuit
  LED light bar under the picture all the way to the drive circuit. Sent from the LLC switching power supply circuit voltage of 12V, added to the N901 (13) pin power supply terminal. When the backlight necessary when lit, backlit high output from the CPU switch control signal SW, added to the N901's (12) feet enable, when the pin when the voltage is greater than 2v. N901 to work, the internal oscillator (2) feet to set the operating frequency oscillations, amplified by the drive circuit, from (15) feet for the 5V output signal amplitude of the PWM switch drive signal to drive the gate MOS tube V901 very. MOS switch connected to the drain pipe V901 inductor L901, composed of a rectifier diode VD901 typical boost circuit, it works with the PFC boost circuit described above is basically the same. This, 84V voltage superimposed on the L901 self-inductance of the voltage stored, and then rectified through VD901, C908 filter, the output light of the drive voltage LED lights.

Output voltage level is driven by the current size of the LED light bar for feedback control. In order to ensure the stability of light-emitting LED lights, need constant working conditions, so its operating current is critical. Different types of LED lights, it’s not the same nominal operating current, and some of 120mA, and some only 60mA. We LED32T28KV 60mA lamps used in the analysis of, for example.

  When the LED lights lit, the drive voltage after the light bar, V902, sampling resistor, to form a current. This kind of resistance the current job R70l, R702, R703 to form the current sample voltage. By simple calculation, we can calculate the equivalent resistance of three sampling resistor is 83Ω, when the current of 60mA, the voltage drop across the shunt resistor is exactly 0.5V, this voltage is set in light of the N901 during the normal work standard test voltage. 0.5v voltage into the N901 (7) feet into the interior of the current manager, and o.5v reference voltage for comparison. When the input voltage error, the output control signals to adjust the output of the

  PWM switch foot drive signal duty cycle to adjust the boost LED driver circuit output voltage ⑩ level to ensure the LED lights of the current stable at 60mA, the backlight brightness to meet the requirements. At this point, measure the drive voltage should be 168V or so. As the single LED lights lit voltage of about 3V, the LED light can be calculated on a total of 56 series has LED lights.

  LED37T28KV uses 120mA of LED light bar. In order to ensure the N901 (7) feet to the foot current detection control o.5v, you will need sampling resistor R701, R702, R713, respectively, the resistance is set to 20Ω, 2.2Ω, 2.2Ω. The equivalent resistance of 4.2Ω. The LED drive voltage of 132V models around, using a 44 LED light bar.

  Thus, the current sampling resistor directly affects the size of the output drive voltage level, if the resistance goes up, will result in reduced sample current, drive voltage is also reduced, and thus appear dimmed LED back-light failure.

Protection circuit
In order to prevent the LED light bar for over-current, over voltage damage and other reasons, but also to avoid the light of the impact damaged the circuit, LED drive circuit in a perfect protection circuit design. The following are introduced one by one.

LED light bar over-current protection

  (OCP) when short-circuit fault LED Strip, LED lights, or other causes abnormal increases of current through the current sense resistor R701, R702, R703 back to the N901 (7) feet high voltage also will change. 029 957 (7) In addition to the internal connection of the current manager, it also connects with multiple voltage comparators, one of which is over-current protection (OCP) comparator. When (7) feet above the o.55v voltage, the comparator output high level of protection from the control signal, added to the delay protection. Protection in the short delay after delay, turnoff control signal output, added to the drive output circuit, the control drive circuit does not output, enabling the LED light bar over-current protection.

  N901 internal delay protector pin external capacitance of a C902, as received from the protection ⑩ circuit control voltage sent to protect the action does not immediately, but to control voltage from the charge of the C902. When the charging voltage reaches the set time delay protection threshold, delay protection circuit output stage driver was back off the control signal, in order to achieve delay protection. The circuit can effectively avoid the phenomenon of circuit protection error occurs, that is, only when the persistent voltage protection only when the implementation of protection actions.

Step-up circuit over-current detection and protection.

  (IS) step-up MOS switch V901 work, its source in the formation of several hundred mA operating current, the current through R733, R734, the form reflects the current size of the dropout voltage.  The voltage sent to the N901's (8) feet, added to the internal comparator positive input of the comparator's inverting input termination is 0.5V reference voltage. When the V901 source when the current exceeds 1A, the sense resistor voltage will be more than 0.5v, thereby compare the working conditions change. At this point, the comparator output high voltage output directly to the drive circuit, disable the PWM drive signal from (15) feet output, MOS tube does not work Ran against V91) 1 due to over-current damage.

LED driving voltage output over-voltage protection.

  (OVP) boost circuit output voltage of the LED driver if out of control, will directly burn LED light strip, so the circuit design of the corresponding over-voltage protection circuit. After driving voltage output by the voltage divider resistors R909, R910, R911, R912 to partial pressure, the R912 to form a detection voltage, and sent to the N901's (10) foot side overvoltage detection. To LED32T28KV for example, 168V driving voltage is normal, pin voltage is 2.3V or so. If for ⑩ some reason causes the LED driving voltage increases, the pin detection voltage also will ⑩ increase. When the driver when the voltage exceeds 216V, R912 on the partial pressure of up to 3V or more, (10) feet within the 3V voltage comparator operation, high output of OVP control signal, into the delay protection, and ultimately control chip driver circuit no longer work, complete overvoltage protection.

  For LED37T28KV machine, because the driving voltage of 132V, so the four resistor divider are different, namely, 200kΩ,, 200kΩ, 220kΩ,, lOkΩ,. The N901 (10) feet of the partial pressure of voltage 2.1V. Can be obtained by calculating the overvoltage protection circuit threshold voltage 189V. When the voltage continues to rise, over-voltage protection circuit will perform the action.

LED light bar circuit protection

  (OLP) occurs when the internal circuit of LED lights or LED driver circuit board output sockets and light of the poor contact between the time, LED light bar is no current flow, the current sampling resistor R701, R702, R703 there is no voltage is generated. At this point, in order to prevent the N901 (7) feet within the current manager for the LED current lack of miscarriage of justice, avoid driving voltage further increases in (7) feet within the design of a circuit protection (OLP) comparator. When (7) pin voltage lower than 0.4V, the comparator output high of OLP control signal, high and then through the door into the delay with the protection, control drive signal is not output to achieve the light of the open circuit protection.

LED light strip part of the short-circuit protection.

  LED light is a diode, short-circuit breakdown is the most common way of damage, followed by open-circuit damage. Assuming a 56 LED light bar, LED light half short-circuit damage, the rest of the 28 LED lights on the drive voltage of only 84V light that can work properly. This step-up circuit is not required to work. 84V power supply by the L901, VD901, direct light LED light bar (but relatively low brightness). If at this time the LED on the light bar to short-circuit damage, not as 84V step-down voltage, making the LED light bar on the current increases. Light-current protection circuit has been due not from the control, resulting in severe heat LED lights, LED lights eventually burn out the remaining.

  Considering the above factors, the circuit was designed by the V913, R745, R752, R751, VD913 and other components of the composition detection circuit protection. When the LED current increases, the flow through R745, R752, R751 on the pressure drop increases, the time when the big 0.7V. V913 into the conduction state by the deadline, the collector output high, the VD913, the signal output over-current protection (OCP). Since this time the control goal is to reduce or stop the output voltage of 84V, so the OCP signal directly to the LLC switching circuit. Followed through V833, N844, V803. The formation of high level of protection control signal, added to the N802 (NCP1396A) of (8) feet and (9) feet. N802 closed circuit internal motivation, LLC the power to stop working, 84V output voltage is no longer, LED light bar off, complete protection.

Backlight brightness adjustment
  Hisense LED LCD TV has energy-conversion function, depending on the environment and the user's set of TV LED backlight brightness to adjust to more comfortable viewing, and can save energy.

  When the backlight brightness adjustment, from the CPU outputs a PWM backlight brightness control signal BRI, the signal is added to the N901 (6) feet backlight brightness control side, into the on-chip PWM control circuit, the output after the output of the comparator to the oscillation the negative side. When the PWM dimming signal is low, the comparator positive input terminal of the PWM oscillation signal output can be normal; when the PWM dimming signal is high, the comparator output directly to low, PWM oscillation signal is not output, N701 ( 15) feet and no drive signal output, step-up circuit is not working, backlight off. Since the PWM dimming signal frequency is low, only 200Hz. The PWM oscillation signal is as high as 130kHz, the equivalent of the dimming signal oscillation signal of the "modulation"  the final output frequency of 200Hz from the chip backlight control signal. 200Hz for the human eye can no longer be identified, so we feel no bright backlight off conversion, only to see the dimming of the backlight.

  N901 (15) feet of the PWM drive signal output also added after the backlight all the way through VD902 under the MOS gate tube V902. Without dimming the state, the PWM signal frequency is high, while the V902 does not discharge the gate circuit, making the gate is always high. MOS tube V902 is often through the state. LED light of the current can flow through, light normal light. In dimming mode, the frequency of the PWM drive signal is only 200Hz. When the signal is low, V902 have enough time to enter the closed state, thus ensuring LED lights off; when the drive signal to high, V902 into the conduction state. LED strip light recovery. Thus, MOS tube V902 only dimmer switch under the state action, so the tube is also known as dimming control MOS.

  LED light source has two kinds of digital PWM and analog voltage dimming, the LED-backlit LCD TVs are digital PWM dimming. This is due to the adoption analog voltage dimming, LED lights with the current changes, the luminous intensity will be a corresponding change, according to the LED light-emitting properties, when the brightness changes, the color temperature changes must also occur, which will affect the performance of the color LCD image, so not suitable for analog dimming LED LCD TV. 

Other circuits

  Since the four LED lights require four 029,957 were required to drive four lamps of light in order to ensure consistency, the need to control four 029,957 simultaneous work. Chip (1), (3), (5) feet is the multi-chip set also work-related synchronization pin. In this circuit, set the backlight control to the main N901 chip, the other three as deputy chip. N901 by (1), (5) feet of the external setting, from (3) pin output synchronization control signal (SYNC). The signal sent to the N902, N903, N904's (1) foot to control the other three 029,957 simultaneous work to ensure the stability of the backlight brightness and uniformity.  N901 (9) feet to compensate for the foot, set by external capacitor C903 to filter out the signal of the clutter signal, to ensure normal drive signal output. The foot is also a soft-start pin, by external components C901, C904. You can start work on time for the chip set, to avoid the impact of the current moment of starting chips, soft-start function.