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Posted on 01 August 2019

Powering LED Arrays in Backlight Applications

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32 LEDs are driven from a 12V supply

LEDs are becoming an increasingly popular backlighting option for all types of LCD displays, large and small, as more efficient and more cost-effective white LEDs become available on the market.

By David Sorlien, Applications Engineer, Intersil

 

Advantages of LED backlighting include low cost, high reliability, low voltage, low EMI, high immunity to vibration, wide operating temperature range, and wide dimming range. These features make LED backlighting particularly suitable for handheld applications, such as cellular phones, portable media players, digital still and video cameras, and GPS receivers, among others.

An LED backlight has two basic configurations: edge-lit or array-lit. Edge-lit displays use one or many side-emitting LEDs along the sides of the display. Array-lit displays employ multiple LEDs arranged in a grid pattern directly behind the display. In both configurations, the LED light source is coupled with light guides and diffusers that distribute the light evenly behind the display.

Intersil’s EL7801 is a high-power LED backlight driver with integrated 36V FET, capable of driving 1 to 8 high-power LEDs in a series from a wide range of input voltages. The 1 MHz PWM converter can be configured in boost or buck topologies, supporting a wide variety of LED backlight applications.

LED light level may be controlled by adjusting the DC bias via the LEVEL pin, or by applying an external PWM signal to the EN/PWM pin. Since LED color temperature varies with bias current, PWM dimming offers better control of color temperature because current through the LEDs is kept constant. The EL7801 provides a 5V gate driver synchronized to the EN/PWM pin that can be used to control an external FET that disconnects the LED stack during the PWM dimming signal-off period. A voltage applied to the LEVEL pin then sets the output current of the converter during the PWM on period.

A minimal BOM LED backlight application using an EL7801 in boost configuration to drive a string of eight series-connected LEDs is shown in Figure 1.

Typical EL7801 Circuit

In this application, eight series-connected LEDs are driven from a 12V supply. A logiclevel PWM dimming signal is applied to the EN/PWM input to control average LED current. Current in the LED load during the PWM on-time is determined by the value of the feedback sense resistor R3, and the target feedback regulation voltage (VFB). With MODE tied to VDC, voltage across the feedback resistor is set by VLEVEL.

ILED = VFB / RSENSE
VFB = VLEVEL / 5

The value of VFB should be kept in the 50mV to 450mV range for linear operation. With MODE pin tied to ground, VFB is set to 400mV via an internal reference, and resistors R1 and R2 can be omitted.

For applications that require more than 8 series connected LEDs, multiple strings of series-connected LEDs can be controlled using a single EL7801 device. If one simply connects the LED strings in parallel, there can be noticeable mismatch in brightness between the strings, due to variations in LED forward voltage. To illustrate this concept, the forward voltage versus forward current variation of eight white LEDs taken from the same reel is shown in Figure 2.

Forward Voltage vs. Forward Current for eight white LEDs from same reel

In this example, LED current at a specific forward voltage can differ by as much as 10mA. When two such LEDs are connected in parallel, and driven by a constant current source, the variability of the individual LED forward voltage will likely result in one LED receiving significantly more current than the other.

An additional source of LED forward voltage versus forward current mismatch is introduced by temperature variations between the individual LEDs. White LEDs of the type used in backlight applications typically have a temperature coefficient of -2 to -4mV/°K. Thus, as temperature increases, the forward voltage decreases. This effect will contribute to the current mismatch between LEDs or LED strings connected in parallel.

A better method of driving multiple LED strings from a single LED driver IC is required. One solution to provide equal current to two LED strings involves the use of a simple current mirror, constructed with a matched transistor pair. Such an application is shown in Figure 3.

Multi-Leg EL7801 Circuit, with Simple Current Mirror

In this application, Q1-Q2 is a matched pair of transistors. It is important to keep Q1 and Q2 at the same temperature to provide good current matching in each LED string. Devices that include two or more thermallycoupled matched transistors in a single package are available for this purpose.

A logic-level PWM dimming signal is applied to the EN/PWM input to control average LED current. Total current in the LED strings during the PWM on-time is controlled by the value of R4 and the target feedback voltage (VFB), which is controlled by applying a DC voltage at LEVEL.

The value of R3 must be carefully selected to guarantee that under all operating conditions, the voltage between points A and C in this circuit is greater or equal to the voltage between points B and C. If this relationship is not maintained, the LED strings will receive unequal current.

To calculate the required value at R3, the maximum (Vf_MAX) and minimum (Vf_MIN) forward voltage of the LEDs must first be determined. It is important to consider all sources of forward voltage variation, as described in the sections above. The worst-case voltage difference between the two LED strings (ΔVf_string) is then calculated.

ΔVf_string = (# of LEDs in string) · (Vf_MAX - Vf_MIN)

The minimum current in either LED string is then determined. This is nominally onehalf of the total output current, set by R4 and VFB. The accuracy of the current mirror circuit at Q1-Q2 should be considered, as well as the gain and offset error in the EL7801 internal circuitry that translates the voltage on LEVEL to the target feedback voltage VFB. When minimum LED current is known, the ideal value of R3 can be determined.

R3 = ΔVf_string / ILED_MIN

It is important to not violate the absolute maximum power rating of the components, therefore R3, Q1, and Q2 must be selected to handle the worst-case conditions, and PCB layout must be done with thermal considerations in mind.

The application circuit depicted in Figure 3 has a few drawbacks. The circuit is rather inefficient, since worstcase LED forward voltage variations must be accommodated. Furthermore, driving additional LED strings while maintaining tight string-tostring current matching may be difficult due to the unavailability of suitable matched transistor arrays. For such applications, a better solution is required.

A more efficient backlight application using the EL7801 to drive four strings of eight series-connected LEDs is shown in Figure 4.

Multi-Leg EL7801 Circuit, with Active Current Mirror

In this application, a total of 32 LEDs are driven from a 12V supply. Similar to the application depicted in Figure 1, LED brightness is controlled by applying a logic-level PWM signal on the EN/PWM input. The LED current during the PWM on time is controlled by the voltage at LEVEL.

U2 and U3 are single supply dual op amps, configured as voltage- controlled current sinks. Examining the leftmost leg, we see that current flowing through the LED string also flows through Q1 and R3. The op amp will adjust the gate drive of Q1 to force the voltage across R3 to equal VFB. LED current during the PWM on time is determined by the voltage on the LEVEL pin.

ILED = VLEVEL / (5 · R3)

With the 10 ohm current sense resistors shown in Figure 4, a voltage of 1V applied to LEVEL will result in 20mA current per LED leg. LED current can be increased by reducing the value of the current sense resistors, or by increasing VLEVEL.

The op amps selected for this application must be able to function with input voltages near ground. The op amps will typically be single-supply type, powered from the EL7801 VDC output, therefore a rail-to-rail op amp is suggested (for instance, Intersil EL5220CY). With this circuit, leg-to-leg current matching is primarily a function of the op amp input offset error, so an op amp with a low VOS specification is preferred. The op amp output slew rate is also an important consideration to maximize system efficiency and dimming linearity, and becomes increasingly important as the frequency of the PWM dimming signal increases.

Diodes D2 though D5 identify the LED string that exhibits the greatest combined forward voltage drop. The voltage at the bottom of this LED string also appears at the cathode of D6. During the PWM dimming signal on-time, ENL is driven to 5V, turning Q5 on. The control loop of the EL7801 will then increase switching duty cycle until the VFB reaches the desired voltage level. At this time, the voltage across R8 becomes equal to the minimum drainsource voltage of the four current sink MOSFETS (Q1 through Q4). Therefore, the value of resistor R8 determines the minimum voltage that will appear across any of the current sink MOSFETs.

To increase efficiency of the system, the value of R8 can be reduced. However, the ratio of R8 to R9 must be greater than the ratio of the current sink MOSFET RDS(ON) to the current sense resistors (10 ohms in this example), in order for the circuit to generate equal current in each LED string.

Since the voltage across each of the 10 ohm resistors at R3- R6 is equal to the FB voltage, the legs that exhibit a lower combined LED forward voltage drop will see an increased drain-source voltage at the current sink MOSFETs. The designer should consider the LED forward voltage tolerance across the operating temperature and desired LED current ranges, and select current sink MOSFETs capable of handling the worst-case power dissipation condition.

During the PWM off time, the ENL signal is driven low, turning off Q5. Voltage across R9 then becomes zero. The voltage controlled current sink circuits respond in turn by driving the gate of the connected MOSFETs low, disabling the current flow through the LEDs.

Capacitors C4 and C5 allow some of the output voltage ripple to appear at the FB circuit node, and help stabilize the EL7801 control loop. The EL7801 employs a direct summing control loop with current feedback. No error amplifier is used in the system. This arrangement provides fast transient response and makes use of the output capacitor to close the loop. A combination of ceramic and low-ESR electrolytic capacitors can be used to minimize implementation costs. Generally, the higher numbers of LEDs, lower VFB voltages, and smaller values of current sense resistors will require smaller value output capacitors to achieve loop stability. In the circuit depicted above, with 20mA LED current per leg, a total of 40iF of capacitance at C3 is recommended.

It may be desirable to sense the actual light output of the LEDs, and adjust the LED current to maintain a precise level of luminous intensity. A method of controlling LED current with a light sensor IC is shown in Figure 5.

Controlling LED Current Using an EL7900 Light Senor IC

The circuit shown above can replace the fixed voltage divider on LEVEL depicted in Figure 4, to provide a method of maintaining a constant light output. The EL7900 is a light-to-current optical sensor combining a photodiode and current amplifier on a single monolithic IC. Output current is directly proportional to the light intensity on the photodiode.

EL7900_IOUT = EV · (60µA / 100Lux)
Where EV is illuminance in Lux

The op amp at U5 is configured as a current- to-voltage converter. The voltage divider formed by resistors R10 and R11 sets the output voltage when no light is present at the EL7900 sensor. With the values of R10 and R11 shown, the output of U5 is 1.5V when there is no light present. Resistor R12 is selected to provide the desired gain of the system.

VOUT = VREF - (EV · R12 · (60µA / 100Lux))
Where VREF is the voltage at U5's non-inverting input.

Luminance versus Output Voltage

The component values shown in Figure 3 were selected to provide a 1.0V output at 1000lux light input. When light intensity increases, the output voltage of this lightsensing circuit decreases, and the attached EL7801 circuit will respond by decreasing the switching duty cycle, thus reducing LED current.

In conclusion, the EL7801 is a highly versatile device, and can be used in a variety of applications, including those where more than 8 LEDs must be powered. When used in conjunction with an EL7900 light-to-current sensor, applications that require a constant light output can be easily implemented.

 

 

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