ISL97636A is an integrated power LED driver to control LCD backlight 6 LED current applications. The ISL97636A is typically capable of driving 54 (6x9) pieces of 3.5V/30mA or 60 (6x10) pieces of 3.2V/20mA LEDs. The ISL97636A includes 6 channels of voltage-controlled current sources with typical current matching of ±1% to compensate for the effects of forward voltage variations in non-uniform LED stacks. To operate in typical multi-string, the ISL97636A has dynamic monitoring of the highest LED headroom control voltage string and its feedback signal output regulator. LED brightness can be achieved by applying a PWM signal from DC to audio noise-free 20kHz. The ISL97636A has extensive protection features including string open and short detection, OVP, OTP, thermal shutdown and optional input overcurrent main fault disconnect switch protection. Offering a 24 Ld 4MX4MM QFN, the ISL97636A operates from -40°C to +85°C with an input voltage range of 6V to 24V for high LED count applications.

feature

6 channels

6V to 24V input

Maximum output 34.5V

Drives up to 54 (3.5V/30mA each) or 60 (3.2V/20mA each) LEDs

Current matching ±1% typical

Dynamic Headroom Control

Pulse width modulated signal with up to 20kHz dimming

Protection - String Open Detection - String Short Detection with Selectable Thresholds - Over Temperature Protection - Over Voltage Protection - Optional Input Over Current Protection with Disconnect Switch

1.2 MHz switching frequency

24 Ld 4mmx4mm QFN package

Lead Free (RoHS Compliant)

application

Notebook Display LED Backlight

LCD Display LED Backlight

Automotive Display LED Backlight

Automotive or Traffic Lighting

Absolute Maximum Ratings (TA=+25°C) Thermal Information

VIN, malfunction. -0.3V to 24V

DC, Compressor, RSET, EN/PWM. -0.3V to 6.5V

OVP, IIN0-IIN5. -0.3V to 28V

LX. -0.3V to 36V

prostatitis. -0.3V to +0.3V

The above voltage ratings are all relative to the ground pin

operating conditions

temperature range. -40°C to +85°C

Thermal Resistance (Typical, Note 1, 2) θJA (°C/W) θJC (°C/W)

24Ld QFN. 39 2

Thermal Characteristics (Typical, Note 3) PSIJT (Celsius/Watt)

24-yard QFN. about 0.7

Maximum continuous junction temperature. + 125 degrees Celsius

Storage temperature. -65°C to +150°C

Lead-free reflow profile.

IMPORTANT: Guaranteed all parameters with min/max specs. Typical values are for reference only. Unless otherwise stated, all tests are pulsed at the specified temperature, thus: TJ=TC=TA

NOTE: Do not operate at or near the listed maximum ratings for extended periods of time. Exposure to these conditions may compromise product reliability and cause failures not covered by warranty.

notes:

1. θJA is measured in free air with the part mounted on a high-efficiency thermal conductivity test board with "direct connect" characteristics.

2. For θJC, the "case temperature" location is the center of the exposed metal pad on the bottom of the package assumed at ideal case temperature.

3. PSIJT is the connection point for the top thermal resistance. If the maximum temperature of the package can be measured, at that rating, the die attach temperature can be estimated more accurately than the θJC and θJC thermal resistance ratings.

4. Limits determined by characterization, without production testing.

Electrical Specifications All specifications below are tested at TA=-40°C to +85°C; VIN=12V, EN=5V, RSET=36.6kΩ; parameters are at +25°C unless otherwise specified 100% test for minimum and/or maximum limits. Temperature limits determined by characterization, not production tested

theory of operation

PWM Boost Converter Current-mode PWM boost converters generate the highest forward voltage drop operating at the programmed current. The ISL97636A uses current mode control with a fast current sense loop and a slow voltage feedback loop. Such an architecture enables fast transient response backlighting applications critical to notebook PCs, where the power source can be drained from a battery or replaced immediately with an AC/DC adapter without noticeable visual disturbance. Number The number of LEDs that can be driven by the ISL97636A depends on the type of LEDs selected in the application. The ISL97636A is a chip that can amplify to 34.5V and can typically drive 9 LEDs in series for each of the 6 channels, enabling a total of 54 3.5V/30mA type LEDs. The ENABLE and PWMIEN/PWMI pins serve two purposes; it serves as an enable signal and can be used for dimming. If a PWM signal is applied to this pin, a minimum of 40 microseconds will be used as the enable signal. If there is no signal for more than 28ms, the device will go into shutdown state. The EN/PWMI pin cannot float, so a 10kΩ pull-down may require adding a resistor. CURRENT MATCHING AND CURRENT ACCURACY Each channel of LED current is regulated by a current source circuit, as shown in Figure 17. The scaling factor of the LED peak current output is set by converting the RSET current to 733/RSET. The termination of the source current source mosfet is designed to be 100 mV to reduce power loss. The wrong source channel-to-channel current matching comes from the op amp's offset, internal layout and reference, and parameters are current matching and absolute current accuracy. Absolute accuracy is also determined by external RSET, so a 1% tolerance resistor should be used.

Dynamic Headroom Control

The ISL97636A has a proprietary dynamic headroom to detect the highest forward voltage string or the lowest voltage of any IIN pin. When the lowest IIN voltage is below the short-circuit threshold, VSC, this voltage will be used as the feedback boost regulator signal. This enhancement puts the output at the correct level to have the lowest IIN pin headroom voltage at the target location. Because all LEDs are connected to the same output voltage, the other IIN pins will have higher voltages, but each channel will ensure the same current per channel. The output voltage will be regulated periodically to always refer to the building. Dimming control The ISL97636A allows two ways to control the LED current and, therefore, the brightness. they are:

1. DC current adjustment

2. PWM chopping of the LED current as defined in step 1. The maximum DC current setting the initial brightness should be done by selecting an appropriate value for RSET. This should be chosen to fix the maximum possible LED current:

DC current adjustment RSET can be a DCP (Digitally Controlled Potentiometer) for DC current adjustment, but the minimum resistance should not be less than 21kΩ, the maximum is 35mA For example, if the required maximum LED current (ILED(max)) is 20mA , rearranging Equation 1 yields Equation 3:

PWM current control

The average LED current per channel can be determined by the external PWMI signal as shown in Equation 3:

The PWM dimming frequency can be, for example, 20kHz, but there is a minimum commute time requirement that the dimming range is between 10% and 99.5%. If the dimming frequency is lower than 5kHz, the light range can be from 1% to 99.5%. The PWM dimming off time cannot exceed 28ms or otherwise the driver will go into off state. 5V Low Dropout Regulator A 5.2V LDO regulator appears on the VDC pin for the low voltage supply required by the chip's internal control circuitry. Because VDC is an LDO pin for regulation. For applications with input voltage ≤5.5V the VIN and VDC pins can be tied together. The VDC pin serves as a rough reference and has little mA source capability. Inrush current control and soft start The ISL97636A has built-in independent inrush current control and soft start functions. The inrush current control function is built around short circuit protection FETs and can be used in applications that include this device. At startup, the output is 30µA due to the pull-down current of the fault pin. This discharge faults the gate-source capacitance of the FET, rotating on the FET in a controlled manner. When this happens the output capacitor slowly charges through the weak steering before the FET is fully boosted. This will result in low inrush current. This current can be further reduced by adding a capacitor (in the 1nF to 5nF range) on the gate to the source of the FET.

As soon as the chip detects that the failsafe FET is turned hard, it is assumed that the inrush current is full. At this point, the boost regulator will start switching and the sensor will rise. The current in the boost power switch is monitored and terminated in any cycle where the switching current exceeds the current limit. The 97636a island includes a soft-start function that limits the current from a low value (375mA). This will ramp up to a final 3A current limit to 375mA in seven steps. The steps will total more than 1ms, so the limit will be reached after 1ms. This allows the output capacitor to be charged to the desired value under low current limit, as well as preventing high input current and medium output current requirements for systems with only low input current. For systems without a primary failsafe FET, inrush current will flow to COUT when VIN is applied as determined by the VIN ramp rate and the value of COUT and L. Fault Protection and Monitoring The ISL97636A has extensive protection functions covering all perceivable fault conditions. Failure Mode An LED can be open or shorted. The behavior of this open LED can also take the form of an infinite resistance, or for some LEDs, a zener diode, and now turn on the LED. For basic LEDs (without built-in zener diodes), an open circuit failure of one LED will only cause one LED channel and will not affect the other channels. Similarly, a short-circuit condition on a channel will cause that channel to be turned off without affecting other channels unless a similar failure occurs. Since the boost response lags the output, certain transient events such as LED current steps or large step changes in LED duty cycle look like LED failure modes. The ISL97636A uses feedback from the LEDs to tell when it is operating stably at these Regions during transients and protection against apparent failure allow any LED stack to fail in the event of a failure. See Table 1 for more details.

Causing the input current to exceed the electrical limits of the device will cause all output channels. Short Circuit Protection (SCP) Short circuit detection circuit monitors the voltage on each channel and disables faulty channels above 8V is detected (see Table 1 for actions taken) Open Circuit Protection (OCP) It works when one of the LEDs is open circuit Either infinite resistance or gradually increasing finite resistance resistance. The ISL97636A monitors each so that any string at least reaches the expected output current to be considered "good". If the current value subsequently falls below 50% of the target value the channel will be considered "open". Also, if the boost output of the ISL97636A reaches the OVP limit or should be reached below the over temperature threshold, all channels that are not "good" will be considered "open" immediately. "Opening" the detection circuit of the "channel" will cause the affected channel to be disabled. When the device is above the trip point it attempts to prevent over temperature.

Some users use some special types of LEDs to enhance and enable open circuit operation with a Zener diode structure in parallel with the ESD LEDs. when? This type of LED is open circuited and the effect is like the LED forward voltage is increased, but there is no lighting. Any affected strings will not be disabled unless a failure results in reaching the boost OVP limit, allowing strings to remain functional. Be careful in this case that the boost OVP limit and SCP limit are set correctly in order to ensure that multiple failures on one string do not cause all other good channels to fail. This is due to raise the forward voltage of the faulty channel and all the other channels look like short LEDs. See Table 1 for details on fault condition response. Over Voltage Protection (OVP) The integrated OVP circuit monitors the output voltage and keeps it at a safe level. The OVP threshold is set as:

These resistors should be large to minimize power loss. For example, 1MΩRUPPER and 39kΩRLOWER set OVP to 32.2V. The large OVP resistor also allows COUT to discharge slowly during the PWM off time. Undervoltage Lockout If the input voltage falls below the UVLO level of 2.45V, the device will stop switching and reset. Operation will restart when the voltage returns to the operating range. Input overcurrent protection is monitored during normal switching operation through an internal boost power FET. If the current exceeds the current limit, the internal switch will be turned off. This surveillance is in a self-preservation manner. In addition, the ISL97636A monitors the voltage of the LX and OVP pins. At startup, the fixed current is drawn from the LX pin into the output capacitor. The device will not start unless the voltage at LX exceeds 1.2V. Also, if the voltage at LX turns on during any subsequent stage when the power FET is not being switched, it immediately disables the input protection FET. This OVP pin is also monitored and if it rises and then falls below 20% of the target OVP level the input protection FET will also turn off. Over-Temperature Protection (OTP) The ISL97636A includes two over-temperature thresholds. This lower limit is set to +130°C. When this threshold is reached, any channel whose output current is significantly below the regulation target at a certain level will be considered an "open" "circuit" and disabled after a timeout. This time the period is also reduced to the 800 microsecond threshold when the period is above the lower value. The purpose of lowering the threshold is for the channel to reach the maximum temperature threshold before causing sufficient power loss (due to the voltage across other channels between them). The upper limit is set to +150°C. Each time it is reached, the boost will stop switching and the output current supply will be turned off. Once the device has cooled to approximately +100°C, the device will restart with DC power and the LED current level will be reduced to 77% of the initial setting. If the dissipation problem persists, then reaching the limit will result in the same behavior, with a progressive current reduction of 53% and finally 30%. The device is always active unless the pin is disabled via EN. See Figure 18 and Table 1 for a broad range of fault protection scenarios.

Component selection

According to the principle of secondary balance of inductor voltage, the on-time of the change of inductor current in the switching regulator is equal to the off-time of the switching regulator. Because the sensor is:

where D is the time during the switching period of the switch duty cycle defined by the turn-on. VD is the negligible voltage in the forward approximation of a Schottky diode. Rearranging the terms without considering the VD boost ratio and duty cycle are:

nput capacitor

Switching regulators require input capacitors to output peak charge currents and lower input impedance supplies. This reduces regulator and input supply, improving system stability. The high switching loop frequency causes nearly all of the ripple current to flow in the input capacitor, which must be rated accordingly. Capacitor selection with low internal series resistance minimizes heating effects and improves system efficiency, such as X5R or X7R ceramic capacitors offering smaller size and lower coefficients of temperature and voltage values compared to other ceramic capacitors. In boost mode, the input current continues to flow into the inductor, and its AC ripple component is related to the inductor charging rate only and the smaller value input can use the capacitor. A capacitor of at least 10µF is recommended for the input. Make sure the voltage-rated input capacitors are suitable to handle the full supply range.

sensor

Inductor selection should be based on its maximum current (ISAT) characteristics, power dissipation (DCR), susceptibility to electromagnetic interference (shielded vs. unshielded), and size. Inductor type and inductance value affect many key parameters, including ripple current, current limit, efficiency, transient performance, and stability. Its maximum current capability must be sufficient to handle worst-case peak currents. If the rated current of the magnetic core selected for the inductor is too low, the effective inductance value of the magnetic core will drop, resulting in an increase in peak-to-average current, and the core overheating with low efficiency. Series resistance, DCR, inside the inductor causes conduction losses and heat dissipation. Shielded inductors are generally better suited for applications susceptible to EMI, such as LED backlighting. The peak current can be displayed from the voltage across the inductor during the off period as:

Choosing 85% is just an average approximation of efficiency. The first term is the average current, which is inversely proportional to the input voltage. The second term inductor current change is inversely proportional to L and femtoseconds. Therefore, for a given switching frequency and minimum input voltage at which the system operates, the inductor must be carefully selected for ISAT. For a given inductor size, generally the larger the inductance, the higher the resistance due to the extra winding of the coil. So the higher the inductance, the lower the peak current capability. The ISL97636A current limit account may also have to be considered. Output Capacitor The output capacitor acts to smooth the output voltage and switch on the power supply. The output ripple voltage includes the discharge of the ILPEAK output capacitor during FET turn-on and the voltage drop across the ESR output capacitor. The ripple voltage can be expressed as:

The charge conservation principle in Equation 8 addresses the fact that the output capacitor charging inductor ripple current during the boost switch off period subtracts a relatively small output current in the boost topology. As a result, the user needs to choose an output capacitor with low ESD and sufficient input ripple current capability. Output ripple by adding CO or fS or using an ESR capacitor. Generally speaking, ceramic capacitors are the best choice for small to medium LCD output capacitors for backlight applications due to their cost, form factor and low ESR. Larger output capacitors will also mitigate the driver response due to longer samples, during PWM dimming off periods and maintain the effect of output droop. The driver does not need to intensify the transient current in the next shortest possible time. The output capacitor also needs to be compensated, typically a 2x4.7μF/50V ceramic capacitor for notebook backlight display applications. Schottky diodes require a high-speed rectifier diode to prevent excessive voltage overshoot, especially in boost configuration. Low forward voltage and reverse leakage current will minimize losses, making Schottky diodes preferred. Although the Schottky diode only turns on during the boost switch off period, it has the same peak value as the inductor current, so a proper current rating must be used with the Schottky diode. For high-current applications, each channel of the ISL97636A can support up to 35mA. For applications requiring higher current, multiple channels can be grouped to achieve the required current. For example, the cathode of the last LED can be connected to IIN0 to IIN2; this configuration can be thought of as a single column with 105mA current drive capability

compensate

The ISL97636A has two main elements in the system: a current mode boost regulator and an op amp multi-channel current source. The ISL97636A is included in its feedback path to allow the user some adjustment response and better regulation of transients. The ISL97636A uses a current mode control architecture with a fast current sense loop and a slow voltage feedback loop. The rapid flow feedback loop does not require any compensation. Slow For stable operation, the voltage loop must be compensated. This compensation network is Rc, Cc1 series network COMP pin to ground and optional Cc2 capacitor connected to COMP pin. Rc sets the integrator gain for high frequency fast transient response and the Cc1 set integrator to zero to ensure loop stability. For most applications, Rc is in the range of 200Ω to 3kΩ and Cc1 is in the range of 27nF to 37nF. Depending on the PCB layout, Cc2 in the 100nF range may be required to create a pole that eliminates the zero effect of the output capacitor ESR on stability. The ISL97636A evaluation board is configured with 500Ω, Cc1 is 33nF, and Cc2 is 0, which is stable. In practical applications, these values may need to be adjusted empirically, but the recommended values are usually a good starting point.