LM3492HC/LM3492HCQ Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator General Description Features The LM3492HC integrates a boost converter and a two-channel current regulator to implement a high efficient and cost effective LED driver for driving two individually dimmable LED strings with a maximum power of 15W and an output voltage of up to 65V. The boost converter employs a proprietary Projected-On-Time control method to give a fast transient response with no compensation required, and a nearly constant switching frequency programmable from 200 kHz to 1 MHz. The application circuit is stable with ceramic capacitors and produces no audible noise on dimming. The programmable peak current limit and soft-start features reduce current surges at startup, and an integrated 190 mΩ, 3.9A N-Channel MOSFET switch minimizes the solution size. The fast slew rate current regulator allows high frequency and narrow pulse width dimming signals to achieve a very high contrast ratio of 10000:1. The LED current is programmable from 50 mA to 250 mA by a single resistor. To maximize the efficiency, Dynamic Headroom Control (DHC) automatically adjusts the output voltage to a minimum. DHC also facilitates a single BOM for different number of LED in a string, which is required for backlight panels of different size, thereby reducing overall development time and cost. The LM3492HC comes with a versatile COMM pin which serves as a bi-directional I/O pin interfacing with an external MCU for the following functions: power-good, over-temperature, IOUT over- and under-voltage indications, switching frequency tuning, and channel 1 disabling. Other supervisory functions of the LM3492HC include precise enable, VCC under-voltage lock-out, current regulator over-power protection, and thermal shutdown protection. The LM3492HC is available in the thermally enhanced eTSSOP-20 package. Boost Converter: ■ LM3492HCQ is an Automotive Grade Product that is AEC Q100 grade 1 qualified ■ Very wide input voltage ranged from 4.5V-65V ■ Programmable soft-start ■ No loop compensation required ■ Stable with ceramic and other low ESR capacitors with no audible noise ■ Nearly constant switching frequency programmable from 200 kHz to 1 MHz Current Regulators: ■ Programmable LED current from 50 mA to 250 mA ■ 10000:1 contrast ratio, 300 ns minimum pulse width ■ Two individual dimmable LED strings up to 65V, total 15W (typically 28 LEDs @ 150 mA) ■ Dynamic Headroom Control maximizes efficiency ■ Over-Power protection ■ ±3% current accuracy Supervisory Functions: ■ Precision enable ■ COMM I/O pin for diagnostic and commands ■ Thermal shutdown protection ■ Thermally enhanced eTSSOP-20 package Applications ■ Ultra-high contrast ratio 6.5”-10” LCD display backlight up to 28 LEDs ■ Automotive or marine GPS display © 2012 Texas Instruments Incorporated 301705 SNVS797 www.ti.com LM3492HC/LM3492HCQ Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator March 28, 2012 LM3492HC/LM3492HCQ Typical Application 30170528 Connection Diagram 30170502 Top View 20-Lead Plastic eTSSOP (MXA20A) Ordering Information Order Number Package Type NSC Package Drawing Supplied As LM3492HCMH 73 Units per Anti-Static Tube LM3492HCMHX 2500 Units on Tape and Reel LM3492HCQMH Exposed Pad TSSOP-20 MXA20A 73 Units per Anti-Static Tube LM3492HCQMHX 2500 Units on Tape and Reel Feature AEC-Q100 Grade 1 qualified. Automotive Grade Production Flow* *Automotive Grade (Q) product incorporates enhanced manufacturing and support processes for the automotive market, including defect detection methodologies. Reliability qualification is compliant with the requirements and temperature grades defined in the AEC-Q100 standard. Automotive grade products are identified with the letter Q. For more information go to http://www.national.com/automotive. www.ti.com 2 Pin Name Description Application Information 1 EN Enable Internally pull-up. Connect to a voltage higher than 1.63V to provide precision enable for the device. 2 VIN Input Supply Voltage Supply pin to the device. Input range is 4.5V to 65V. 3, 4 SW Switch Node Internally connected to the drain of the integrated MOSFET. 5 VOUT Output Voltage Sense Sense the output voltage for nearly constant switching frequency control. 6 RT Frequency Control An external resistor from the VOUT pin to this pin sets the switching frequency. 7 FB Output Voltage Feedback The output voltage is connected to this pin through a feedback resistor divider for output voltage regulation. The dynamic range of this pin is from 1.05V to 2.0V. 8 GND Analog Ground Signal Ground 9 IOUT2 Current Regulator Input of Channel 2 Input of the current regulator of channel 2. The regulated current is programmable (refer to the IREF pin). 10 IOUT1 Current Regulator Input of Channel 1 Input of the current regulator of channel 1. The regulated current is programmable (refer to the IREF pin). 11 CDHC Dynamic Headroom Control An external capacitor connected to this pin sets the DHC sensitivity. At startup, a 120 µA internal current source charges an external capacitor to provide a soft-start function. 12 IREF Current Setting of the Current Regulator An external resistor connected from this pin to ground programs the regulated current of the current regulator of channels 1 and 2. 13 COMM Bi-directional Logic Communication This pin is open drain for various indications (power-good, over-temperature, IOUT over- and under-voltage) and command sending (switching frequency tuning and channel 1 disabling). 14 LGND Ground of the Current Regulator Current regulator ground. Must be connected to the GND pin for normal operation. The LGND and GND pins are not internally connected. 15 DIM1/CLK Dimming Control of Channel 1 Control the on/off of the current regulator of channel 1. This pin is internally pulled low by a 5 µA current. This pin also serves as a clock signal for latching input/output data of the COMM pin. 16 DIM2 Dimming Control of Channel 2 Control the on/off of the current regulator of channel 2. This pin is internally pulled low by a 5 µA current. 17, 18 PGND Power Ground Integrated MOSFET ground. Must be connected to the GND pin for normal operation. The PGND and GND pins are not internally connected. 19 VCC LDO Regulator Output Nominally regulated to 5.5V. Connect a capacitor of larger than 0.47 µF between the VCC and GND pins. 20 ILIM Peak Current Limit Adjust Connect an external resistor from the ILIM pin to the VCC pin reduces peak current limit. Connect the ILIM pin to the ground to obtain the maximum current limit. DAP DAP Exposed Pad Thermal connection pad. Connect to a ground plane. 3 www.ti.com LM3492HC/LM3492HCQ Pin Descriptions LM3492HC/LM3492HCQ ESD Rating (Note 2) Human Body Model Storage Temperature Range JunctionTemperature (TJ) Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and specifications. VIN, RT, VOUT to GND SW to GND SW to GND (Transient) ILIM to GND FB to GND COMM, DIM1, DIM2, to GND −0.3V to 67V −0.3V to 67V −2V (<100 ns) −0.3V to 1V −0.3V to 5V −0.3V to 6V Operating Ratings ±2kV −65°C to +150°C 150°C (Note 1) Supply Voltage (VIN) Operation Temperature Range (TA) 4.5V to 65V −40°C to +125°C Thermal Resistance (θJA) (Note 3) 32.7°C/W Electrical Characteristics Specification with standard type are for TA = TJ = +25°C only; limits in boldface type apply over the full Operating Junction Temperature (TJ) range. Minimum and Maximum are guaranteed through test, design or statistical correlation. Typical values represent the most likely parametric norm at TJ = +25°C, and are provided for reference purposes only. Unless otherwise stated the following conditions apply: VIN = 12V. Symbol Parameter Conditions Min Typ Max Units CVCC = 0.47 µF, no load 4.7 5.5 6.3 V Start-Up Regulator, VCC VCC VCC pin output voltage ICC = 2 mA 4.7 5.5 6.3 V VCC-UVLO VCC pin under-voltage lockout threshold (UVLO) VCC increasing 3.56 3.78 4.00 V VCC-UVLO-HYS VCC pin UVLO hysteresis VCC decreasing 310 IIN IIN operating current No switching, VFB = 0V 3.6 5.2 IIN-SD IIN operating current, Device shutdown VEN = 0V 30 95 IVCC VCC pin current limit (Note 4) VCC = 0V VCC-VOUT mV 18 30 VCC pin output voltage when supplied by VIN = Open, ICC = 1 mA, VOUT = 18V VOUT 3.5 4.1 4.7 VEN EN pin input threshold VEN rising 1.55 1.63 1.71 VEN-HYS EN pin threshold hysteresis VEN falling IEN-SHUT Enable Pull-up Current at shutdown IEN-OPER Enable Pull-up Current during operation VIREF IREF pin voltage VDHC50 mA µA mA V Enable Input V 194 mV VEN = 0V 2 µA VEN = 2V 40 µA VIN = 4.5V to 65V 1.231 1.256 1.281 V VIOUT under DHC at IOUT = 50 mA RIREF = 25 kΩ 0.160 0.225 0.290 V VDHC100 VIOUT under DHC at IOUT = 100 mA RIREF = 12.5 kΩ 0.38 0.48 0.58 V VDHC200 VIOUT under DHC at IOUT = 200 mA RIREF = 6.25 kΩ 0.81 0.99 1.17 V VDHC250 VIOUT under DHC at IOUT = 250 mA RIREF = 5 kΩ 0.81 1.21 1.44 V IOUT50 Current Output under DHC at VIOUT = VDHC50 RIREF = 25 kΩ, VIOUT = VDHC50 47.5 50 52.5 mA RIREF = 25 kΩ, VIOUT = VDHC50 46.5 50 53.5 mA IOUT100 Current Output under DHC at VIOUT = VDHC100 RIREF = 12.5 kΩ, VIOUT = VDHC100 97 100 103 mA RIREF = 12.5 kΩ, VIOUT = VDHC100 96 100 104 mA IOUT200 Current Output under DHC at VIOUT = VDHC200 RIREF = 6.25 kΩ, VIOUT = VDHC200 194 200 206 mA RIREF = 6.25 kΩ, VIOUT = VDHC200 192 200 208 mA IOUT250 Current Output under DHC at VIOUT = VDHC250 RIREF = 5 kΩ, VIOUT = VDHC250 241.3 250 258.8 mA RIREF = 5 kΩ, VIOUT = VDHC250 238 250 262 mA IOUTOFF Leakage at Maximum Work Voltage VDIM = 0, VIOUT = 65V 5 µA VIOUT50-MIN Minimum Work Voltage, 50 mA RIREF = 25 kΩ, IOUT = 0.98 x IOUT50 0.1 0.15 V VIOUT100-MIN Minimum Work Voltage, 100 mA RIREF = 12.5 kΩ, IOUT = 0.98 x IOUT100 0.2 0.35 V VIOUT200-MIN Minimum Work Voltage, 200 mA RIREF = 6.25 kΩ, IOUT = 0.98 x IOUT200 0.4 0.65 V Current Regulator www.ti.com 4 Parameter Conditions VIOUT250-MIN Minimum Work Voltage, 250 mA RIREF = 5 kΩ, IOUT = 0.98 x IOUT250 VDIM-HIGH DIM Voltage HIGH VDIM-LOW DIM Voltage LOW Min Typ Max Units 0.5 0.82 V V 1.17 V 0.7 Boost Converter ICDHC-SRC CDHC pin source current VCDHC = 1.6V, VFB = 3V, VIOUT = 0V, DIM = High 60 µA ICDHC-SINK CDHC pin sink current VCDHC = 1.6V, VFB = 3V, VIOUT = 3V, DIM = High 56 µA ICDHC-PULLUP CDHC pin pull-up current DIM = Low, VCDHC = 2.3V, VFB = 3V ICL-MAX Integrated MOSFET peak current limit threshold 10 200 500 nA 3.3 3.9 4.5 A ICL-HALF Half integrated MOSFET peak current limit RILIM = 11 kΩ threshold 2.0 RDS(on) Integrated MOSFET RDS(on) 0.19 VFBTH-PWRGD Power-Good FB pin threshold VFB-OVP IFB ton ISW = 500 mA A 0.43 2.25 V FB pin over-voltage protection threshold VFB rising, VCDHC = 4V 2.64 2.76 FB pin OVP hysteresis VFB falling 0.1 0.215 0.323 Feedback pin input current VFB = 3V ON timer pulse width Ω 2.88 1 V V µA VIN = 12V, VOUT = 65V, RRT = 300 kΩ 1460 ns VIN = 24V, VOUT = 32.5V, RRT = 300 kΩ 800 ns VIN = 12V, VOUT = 65V, RRT = 100 kΩ 550 ns VIN = 24V, VOUT = 32.5V, RRT = 100 kΩ 350 ns ns ton-min-ILIM ON timer minimum pulse width at current limit 145 toff OFF timer pulse width 145 350 ns 6.7 7.8 V 0.7 V 5 µA COMM PIN VIOUT-OV IOUT pin over-voltage threshold COMM goes LOW during VIOUT rising, other VIOUT = 1.2V VCOMM-LOW COMM pin at LOW 5 mA into COMM ILEAK-FAULT COMM pin Open Leakage VCOMM = 5V 5.6 Thermal Protection TOTM Over-temperature indication TJ rising 135 °C TOTM-HYS Over-temperature indication hysteresis TJ falling 15 °C TSD Thermal shutdown temperature TJ rising 165 °C TSD-HYS Thermal shutdown temperature hysteresis TJ falling 20 °C Note 1: Absolute Maximum Ratings are limits which damage to the device may occur. Operating ratings are conditions under which operation of the device is intended to be functional. For guaranteed specifications and test conditions, see the electrical characteristics. Thermal shutdown might occur within ambient operating temperature range as junction temperature rises above TSD level, customer should refer to efficiency data and thermal resistance data to estimate the junction temperature to ambient temperature delta. Note 2: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Note 3: The θJA is measured on a 4-layer standard JEDEC thermal test board with 12 vias, no air flow and 1W power dissipation. Thermal shutdown will occur if the junction temperature exceeds 165°C. The maximum power dissipation is a function of TJ(MAX), θJA and TA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(MAX) – TA) /θJA. Note 4: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading. 5 www.ti.com LM3492HC/LM3492HCQ Symbol LM3492HC/LM3492HCQ Typical Performance Characteristics Unless otherwise specified the following conditions apply: TJ = 25°C, VIN = 12V with configuration in typical application circuit for ILED = 250 mA shown in this datasheet. Quiescent Current, IIN vs VIN VCC vs IVCC 30170511 30170512 VCC vs VIN Switching Frequency, fSW vs VIN 30170513 30170514 ILED Regulation vs Temperature RDS(on) vs Temperature 30170515 www.ti.com 30170516 6 ILED Regulation vs VIN (ILED = 0.25A) 100 1.00 0.75 -40°C 0.50 90 85 ΔILED (%) EFFICIENCY (%) 95 LM3492HC/LM3492HCQ Efficiency vs VIN (ILED = 0.25A) 25°C 0.25 0.00 -0.25 80 25°C -40°C -0.50 75 125°C -0.75 70 125°C -1.00 10 15 20 INPUT VOLTAGE (V) 25 10 15 20 INPUT VOLTAGE (V) 25 30170532 30170533 Power Up (ILED = 0.25A) Enable Transient (ILED = 0.25A) 30170534 30170535 Steady State Operation (ILED = 0.25A) LED 50% Dimming (ILED = 0.25A, Dimming frequency = 200Hz) 30170536 30170537 7 www.ti.com LM3492HC/LM3492HCQ 1000:1 LED Dimming (ILED = 0.25A, Dimming frequency = 200Hz) 10000:1 LED Dimming (ILED = 0.25A, Dimming frequency = 200Hz) 30170539 30170538 Simplified Functional Block Diagram 30170503 www.ti.com 8 The LM3492HC integrates a boost converter and a two-channel current regulator to implement a high efficient and cost effective LED driver for driving two individually dimmable LED strings with a maximum power of 15W and an output voltage of up to 65V. The boost converter provides power for the LED strings, and the current regulator controls the dimming of the LED strings individually. The LM3492HC integrates an Nchannel MOSFET switch and a two-channel current regulator in order to minimize the component count and solution size. The boost converter of the LM3492HC employs a Projected On-Time (POT) control method to determine the on-time of the MOSFET with respect to the input and output voltages and an external resistor RRT. During the on-period, the boost inductor is charged up, and the output capacitor is discharged to provide power to the output. A cycle-by-cycle current limit (which is 3.9A typically and programmable by an external resistor) is imposed on the MOSFET for protection. After the onperiod, the MOSFET is turned off such that the boost inductor is discharged. The next on-period is started when the voltage of the FB pin is dropped below a threshold which is determined by Dynamic Headroom Control (DHC) and is ranged from 1.05V to 2.0V (DHC affects the threshold only when the DIM1 and/or DIM2 pins are high). The boost converter under POT control can maintain the switching frequency nearly constant so that the switching frequency depends on only RRT Figure 1. Also, POT control requires no compensation circuit and gives a fast transient response of the output voltage. LDO Regulator A 5.5V LDO regulator is integrated in the LM3492HC. For stability, an external capacitor CVCC of more than 0.47 µF should be connected between the VCC and GND pins. The current limit of the LDO is typically 30 mA. It can be used to pull-up the open-drain COMM pin with an external resistor, and inject current to the ILIM pin to adjust the current limit of the integrated MOSFET. When the voltage on the VCC pin (VCC) is higher than the under-voltage lock-out (UVLO) threshold of 3.78V, the LM3492HC is enabled and the CDHC pin sources a current to charge up an external capacitor CCDHC to provide a soft-start function. Enable and Disable To enable the LM3492HC, the voltage on the EN pin (VEN) must be higher than an enable threshold of typically 1.63V. If VEN is lower than 1.43V, the LM3492HC is shutdown. In this case, the LDO regulator is turned off and the CDHC pin is internally grounded. The EN pin is internally pulled up. After enable, the EN pin is pulled up by a 40 µA current source. If the EN pin is connected to low such that the LM3492HC is shutdown, the pull-up current is reduced to 2 µA. These take the advantages that the LM3492HC can effectively avoid false disabling by noise during operation, and minimize power consumption during shutdown. The enable threshold is precise such that it can be used to implement an UVLO function for the input voltage as shown in Figure 2. The input voltage can be connected to the EN pin through a resistor divider consists of REN1 and REN2. This can ensure that the LM3492HC is operated after the input voltage reaches a minimum require value VIN(EN), which can be calculated by the following equation: VIN(EN) = 1.63V(1 + REN1/ REN2) (1) A zener diode DEN should be placed between the EN and GND pins to keep VEN below its absolute maximum caused by the increase of the input voltage. 30170525 FIGURE 1. Switching Frequency The two-channel current regulator of the LM3492HC is fast response so that it can allow very high contrast ratio of 10000:1. The two channels are dimmable individually. Channel 1 of the current regulator can be disabled by a digital command send through the COMM pin. In this case, the DIM1 pin can serve only as a clock signal for the data flow of the COMM pin. The power dissipated by the current regulator is adaptively minimized by Dynamic Headroom Control in order to maximize efficiency. The LM3492HC can be applied in numerous applications like automotive LCD backlight panels. It can operate efficiently for inputs as high as 65V. Diagnostic functions including power good indication, over-temperature indication, IOUT over- and under-voltage indications facilitate the interface of the LM3492HC application circuit with external micro-processors 30170504 FIGURE 2. Input Voltage UVLO Implemented by Precision Enable Once the EN pin is pulled low, the LM3492HC will perform the following functions: reset IOUT over- and under-voltage indications and the corresponding COMM bit pattern, resume the switching frequency tuning to the normal frequency, and resume channel 1 of the current regulator if it is disabled. Pulling the EN pin low for a short period of about 200 ns can achieve 9 www.ti.com LM3492HC/LM3492HCQ (MCUs). The LM3492HC will not latch off and continue to operate in the presence of the indications. Other useful features include thermal shutdown, VCC under-voltage lock-out, and precision enable. The LM3492HC is available in the thermally enhanced eTSSOP-20 package. Overview Current Limit The current limit ICL of the integrated MOSFET of the LM3492HC provides a cycle-by-cycle current limit for protection. It can be decreased by injecting a small signal current IILIM into the ILIM pin, and the relationship between ICL and IILIM is ICL = ICL(MAX) – 4290 IILIM (2) Current Regulator where I CL(MAX) is the maximum current limit. Its typical value is 3.9A. As shown in Figure 3, IILIM can be provided by connecting a resistor RILIM from the VCC pin to the ILIM pin. The typical voltage on the ILIM pin is 0.7V. To obtain the maximum current limit, connect the ILIM pin to the ground. The LM3492HC integrates a two-channel current regulator for controlling the current of two LED strings. The two LED strings are dimmable individually by dimming signals applied to the DIM1 and DIM2 pins for LED strings 1 and 2, which are connected from the VOUT pin to the IOUT1 and IOUT2 pins. The DIM1 and DIM2 pins are internally pulled low. The lowest contrast ratio is 10000:1. The finest pulse width of the dimming signal for the DIM1 and DIM2 pins is 300 ns. The current of an LED string (ILED) is programmable from 50 mA to 250 mA by an external resistor RIREF connecting from the IREF pin to the ground. The relationship between ILED and RIREF is shown in Figure 4. The two channels of the current regulator can work in parallel for only one LED string by connecting the IOUT1 and IOUT2 pins together to provide an LED current of up to 500 mA. In this case, the DIM1 and DIM2 pins should also be connected together. 30170505 FIGURE 3. Programmable Current Limit 250 Thermal Protection 200 Thermal protection is implemented by an internal thermal shutdown circuit, which activates at 165°C (typically) to disable the LM3492HC. In this case, the LDO regulator is turned off and the CDHC pin is internally grounded. Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction temperature of the LM3492HC falls back below 145°C (typical hysteresis = 20° C), the LM3492HC resumes normal operation. ILED (mA) LM3492HC/LM3492HCQ During startup, the voltage of the CDHC pin is rised from 0V to 2.25V at a speed depends on CCDHC. This makes the voltage of the FB pin as well as the output voltage ramps up in a controlled manner, and effectively a soft-start function is implemented. The soft-start can be programmable by the CCDHC. An internal switch grounds the CDHC pin if any of the following cases happens: (i) VCC is below the VCC UVLO threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is pulled low. The CDHC pin cannot be connected to the ground externally. the above functions with nearly no effect on the operation of the boost converter and the current regulator. 100 50 Dynamic Headroom Control, Overridding, and Soft-start 0 5 Dynamic headroom control (DHC) is implemented in the LM3492HC to adjust the output voltage VOUT of the boost converter in order to reduce the power loss of the current regulator to maximize efficiency. Let VLED,n and VIOUT,n be the forward voltage of an LED string connecting to the IOUTn pin and the voltage of the IOUTn pin, where n is 1, 2 for channels 1, 2 of the current regulator. Since VLED,n is normally decreasing gradually (in terms of minutes) owing to the rise of LED die temperature during operation, DHC adjust VOUT by adjusting a threshold which is reflected on the voltage of the FB pin with reference to VIOUT,n, which is the difference between VOUT and VLED,n. The sensitivity of DHC, which is the response time on adjusting VOUT, is set by CCDHC. If CCDHC is small, VOUT is more sensitive to the variation of VLED,n. DHC over-ridding can be implemented by internal pull-up or external pull-up (by connecting the CDHC and VCC pins with a resistor, e.g. 10 MΩ). In this case, the voltage of the CDHC pin will rise over 2.5V, and the voltage of the FB pin will rise until over-voltage protection is hit. Since the pull-up is weak, DHC over-ridding will occur only at low contrast ratio (e.g. <1%). www.ti.com 150 10 15 RIREF (kΩ) 20 25 30170531 FIGURE 4. ILED vs RIREF 10 LM3492HC/LM3492HCQ 30170506 FIGURE 6. Output Voltage Feedback Circuit Over-Voltage Protection When VFB is higher than the FB pin over-voltage protection (OVP) threshold VFB-OVP (typically 2.76V and maximum 2.88V), the on-period of the integrated MOSFET stop immediately, and the MOSFET keeps off until VFB falls back below below 2.545V (typical hysteresis 0.215V). An alternative method to implement OVP is to directly monitor VOUT instead of VFB. An external circuit as shown in Figure 7 is required. Current is injected to the ILIM pin to drive the LM3492HC to the current limit mode once VOUT is higher than the avalanche voltage of the zener diode DOVP plus 0.7V, the typical voltage on the ILIM pin. In this case, a maximum limit of VOUT is imposed. However, at the maximum limit of VOUT, VFB should be higher than 2.25V to avoid affecting the startup of the LM3492HC. 30170527 FIGURE 5. Over-power Protection If the voltage on the IOUTn (n = 1, 2) pin is over 24V when channel n is on, the regulated current of channel n will be reduced linearly if the voltage further increases (as shown in Figure 5). The regulated current of another channel is not affected. This over-power protection feature avoids damaging the current regulator owing to the shorting of many LEDs in one string. Output Voltage Feedback The output voltage is fed back to the FB pin through a feedback circuit consists of RFB1, RFB2, and CFB as shown in Figure 6. The value of CFB is recommended to be 10 pF. in order to help feed the AC component of the output voltage back. The DC component of the output voltage is fed back by RFB1 and RFB2. The voltage of the FB pin VFB can be adjusted by DHC. When VFB reaches VFB-OVP, the maximum output voltage of the boost converter VOUT(MAX) reaches its maximum, and it is calculated as follows: VOUT(MAX) = 2.88V (1 + RFB1/ RFB2) 30170530 FIGURE 7. External OVP circuit (3) Bi-Directional Communication Pin Under DHC, the output voltage should be maintained at a nominal voltage but not the maximum. The nominal output voltage (VOUT(NOM)) is VOUT(NOM) = max (VLED,n + VIOUT,n), n = 1, 2 The COMM pin of the LM3492HC is an open-drain bi-directional I/O pin for interfacing with an external MCU for the following functions: power-good indication, over-temperature indication, IOUT over- and under-voltage indications, switching frequency tuning, and channel 1 disabling. Except the power good indication and the over-temperature alert, all data flow through the COMM pin is serial and is latched by the falling edge of the signal applying to the DIM1 pin, even when channel 1 of the current regulator is disabled. If the DIM1 pin stays only low or only high, either by an external circuit or letting it open and pull low internally, data flow will not occur. Figure 8 and Figure 9 show a timing diagram of reading and writing a bit from and to the LM3492HC through the COMM pin. The COMM pin should be pull-up by an MCU I/O pin which has pull-up capability, or an external resistor RCOMM to the VCC pin. Otherwise, the voltage of the COMM pin will remain at zero. The rise time of the output signal of the COMM pin depends on the pull-up power. If the rise time is long (RCOMM is too large or pull-up power from the connecting MCU I/O pin is too weak), data may be ready after a longer duration after the falling edge. In this case, a longer delay between the falling edge latching and the (input or output) bit is required. (4) where VLED,n is the forward voltage of LED string n and VIOUT,n is the voltage of the IOUTn pin, where n is 1, 2 for channels 1, 2 of the current regulator). The minimum value of VIOUT,n is about 5Ω x ILED. It is recommended that the nominal voltage of the FB pin (VFB(NOM)) is between 1.05V to 2V. Hence, the equation relating VOUT(MAX), VOUT(NOM), and VFB (NOM) is as follows: VOUT(MAX) = VOUT(NOM) x 2.88V / VFB(NOM) (5) 11 www.ti.com LM3492HC/LM3492HCQ 1) can be read from the COMM pin. The LM3492HC will not latch off and continue to operate in the presence of the IOUT under-voltage indication. IOUT OVER-VOLTAGE INDICATION The LM3492HC gives an IOUTn (n = 1, 2) over-voltage indication if the voltage of the IOUTn pin when DIMn is high is higher than a threshold of typically 6.5V, and the condition lasts for 508 consecutively dimming signals applied on the DIMn pin. The IOUT over-voltage indication can be regarded as a short fault of the LED string n except the following 2 cases: i) powering up the LM3492HC at a very low dimming ratio such that VOUT maintains at a maximum and DHC is not fast enough to reduce VOUT; ii) under DHC over-ridding. A bit pattern (refer to table 1) can be read from the COMM pin. The LM3492HC will not latch off and continue to operate in the presence of the IOUT over-voltage indication. 30170507 FIGURE 8. Read from the COMM Pin TABLE 1. COMM Bit Patterns COMM Bit Pattern 0001 IOUT2 over-voltage indication 0011 IOUT1 under-voltage indication 0101 IOUT2 under-voltage indication 0111 COMM BIT PATTERN Table 1 summarized all COMM bit patterns of IOUT over- and under-voltage indications. A bit pattern can be read from the COMM pin continuously even the condition causing IOUT over- and under-voltage disappeared. An existing COMM bit pattern will be clear if one of the following condition occurs: i) the LM3492HC is shutdown; ii) the LM3492HC is disabled by pulling the EN pin low; iii) the over-temperature indication is appearing. To clear the COMM bit pattern without affecting the operation of the boost converter and the current regulator, a reset signal which pulls the EN pin low for about 200 ns can be applied. In this case, the COMM pin will not give bit pattern any more unless a condition causing IOUT over- or under-voltage appears and lasts for 508 consecutively dimming signals. If an external MCU is used to read the COMM bit pattern, it is recommended that a reset signal should be send to clear the COMM bit pattern after the bit pattern is read. This can avoid that the existing COMM bit pattern is overwritten by another pattern appeared in a later time. In case of over-temperature, the COMM pin will be pulled low to give an over-temperature indication disregard of any existing COMM bit pattern. After that the over-temperature indication disappears, the COMM bit pattern appeared before the over-temperature indication will appear again. 30170508 FIGURE 9. Write to the COMM Pin POWER-GOOD INDICATION Upon startup, the COMM pin reads low. The output voltage of the boost converter of the LM3492HC will rise until the voltage on the FB pin (VFB) reaches 2.25V, when the COMM pin reads high to indicate power-good. The power-good indication is independent of the signal applied on the DIM1 pin. OVER-TEMPERATURE INDICATION If the junction temperature of the LM3492HC reaches 135°C, the COMM pin reads low, showing an over-temperature indication. External MCU should consider to turn off or reduce the brightness of the LED strings in order to prevent over-temperature. The over-temperature indication is independent of the signal applied on the DIM1 pin. The COMM pin reads high if the junction temperature falls below 120°C. The LM3492HC will not latch off and continue to operate in the presence of the over-temperature indication. IOUT UNDER-VOLTAGE INDICATION The LM3492HC gives an IOUTn (n = 1, 2) under-voltage indication if the voltage of the IOUTn pin when DIMn is high is lower than its minimum required voltage which can regulate ILED, and the voltage of the CDHC pin reaches its maximum, and these conditions last for 508 consecutively dimming signals applied on the DIMn pin. This means that the current of the LED string n does not reach its regulated value. In most case, the IOUT under-voltage indication can be regarded as an open fault of the LED string n. A bit pattern (refer to table www.ti.com Indication IOUT1 over-voltage indication SWITCHING FREQUENCY TUNING After power good, the switching frequency (fSW) of the LM3492HC can be tuned down 20% or 40%, or resume normal by writing commands (refer to table 2) to the COMM pin. This helps avoid interfering some sensitive devices, for example radios, working nearby the LM3492HC. Upon reset, fSW of the LM3492HC will resume normal by default. If the over-temperature indication or any COMM bit pattern has al- 12 CHANNEL 1 DISABLE After power good, channel 1 of the current regulator can be disabled by writing a command (refer to table 2) to the COMM pin. If LED string 1 is malfunctioning, channel 1 can be disabled and the signal applied on the DIM1 pin can serve as only a clock signal for the data flow of the COMM pin. Channel 1 is by default enabled after reset. If the over-temperature indication or any COMM bit pattern has already presented, no command can be written to the LM3492HC. TABLE 2. Commands Command Command Bit Pattern fSW resume normal 1111011101110111 fSW tune down by 20% 1111000100010001 fSW tune down by 40% 1111001100110011 Channel 1 disable 1111010101010101 Application Information EXTERNAL COMPONENTS The following procedures are to design an LED driver using the LM3492HC with an input voltage ranged from 10V to 24V and two LED strings consists of 10 LEDs each with a forward voltage of 3V for each LED when running at 250 mA. The output power is 15W. The switching frequency fSW is designed to be 300 kHz. RFB1, RFB2, and CFB: The nominal voltage of the LED string with 10 LEDs is 30V, and the minimum voltage of the IOUTn pin (n = 1, 2) is 1.25V when ILED is 250 mA. As a result, VOUT (NOM) is 31.25V. Design VOUT(MAX) to be 50V. From (5), VFB (NOM) is about 1.8V, which falls in the recommended operation range from 1.05V to 2V. Also, design RFB2 to be 16.2 kΩ. From (3), RFB1 is calculated to be 265.1 kΩ, and a standard resistor value of 261 kΩ is selected. CFB is selected to be 10 pF as recommended. L1: The main parameter affected by the inductor is the peak to peak inductor current ripple (ILR). To maintain a continuous conduction mode (CCM) operation, the average inductor current IL1 should be larger than half of ILR. For a boost converter, IL1 equals to the input current IIN. Hence, IIN = (VOUT(NOM) x 2ILED ) / VIN (6) ton = (1 – VIN/VOUT) / fSW (7) L1 = (VIN x ton) / 2IIN (8) In this example, CCDHC is recommended to be a 0.47 µF good quality and low ESR ceramic capacitor. RRT and RIREF: The resistors RRT and RIREF set the switching frequency fSW of the boost converter and the LED current ILED respectively. From Figure 1, if fSW is 300 kHz, RRT is selected to be 499 kΩ. From Figure 4, if ILED is 250 mA, RIREF is selected to be 4.99 kΩ. RCOMM: Since the COMM pin is open drain, a resistor RCOMM of 52.3 kΩ is used to connect the VCC and COMM pins to act as a pull-up function. PC Board Layout The layout of the printed circuit board is critical in order to optimize the performance of the LM3492HC application circuit. In general, external components should be placed as close to the LM3492HC and each other as possible in order to make copper traces short and direct. In particular, components of the boost converter CIN, L1, D1, COUT, and the LM3492HC should be closed. Also, the output feedback capacitor CFB should be closed to the output capacitor COUT. The ground plane connecting the GND, PGND, and LGND pins and the exposed pad of the LM3492HC and the ground connection of the CIN and COUT should be placed on the same copper layer. Good heat dissipation helps optimize the performance of the LM3492HC. The ground plane should be used to connect the exposed pad of the LM3492HC, which is internally connected to the LM3492HC die substrate. The area of the ground plane should be extended as much as possible on the same copper layer around the LM3492HC. Using numerous vias beneath the exposed pad to dissipate heat of the LM3492HC to another copper layer is also a good practice. Also, If VIN is maximum, which is 24V in this example, and only one LED string is turned on (since the two channels of the LM3492HC are individually dimmable), IIN is minimum. From (6)-(8), it can be calculated that IIN(MIN), ton, and L1 are 0.326A, 0.77 µs, and 28.5µH. On the other hand, from (6), IIN is maximum when VIN is minimum, which is 10V in this example, and the two LED strings are turned on together. Hence I IN(MAX) is 1.56A. Then, ILR is ILR = (VIN x ton) / L1 (9) From (7), ton is 2.27 µs. From (9), ILR is 0.80A. The steady state peak inductor current IL1(PEAK) is IL1(PEAK) = IL1 + ILR / 2 (10) As a result, IL1(PEAK) is 1.96A. A standard value of 27 µH is selected for L1, and its saturation current is larger than 1.96A. 13 www.ti.com LM3492HC/LM3492HCQ D1: The selection of the boost diode D 1 depends on two factors. The first factor is the reverse voltage, which equals to VOUT for a boost converter. The second factor is the peak diode current at the steady state, which equals to the peak inductor current as shown in (10). In this example, a 100V 3A schottky diode is selected. CIN and COUT: The function of the input capacitor CIN and the output capacitor COUT is to reduce the input and output voltage ripples. Experimentation is usually necessary to determine their value. The rated DC voltage of capacitors used should be higher than the maximum DC voltage applied. Owing to the concern of product lifetime, ceramic capacitors are recommended. But ceramic capacitors with high rated DC voltage and high capacitance are rare in general. Multiple capacitors connecting in parallel can be used for CIN and COUT. In this example, two 10 µF ceramic capacitor are used for CIN, and two 2.2 µF ceramic capacitor are used for COUT. CVCC: The capacitor on the VCC pin provides noise filtering and stabilizes the LDO regulator. It also prevents false triggering of the VCC UVLO. CVCC is recommended to be a 1 µF good quality and low ESR ceramic capacitor. CCDHC: The capacitor at the CDHC pin not only affects the sensitivity of the DHC but also determines the soft-start time tSS, i.e. the time for the output voltage to rise until power good. tSS is determined from the following equation: ready presented, no command can be written to the LM3492HC. LM3492HC/LM3492HCQ 30170529 www.ti.com 14 LM3492HC/LM3492HCQ Physical Dimensions inches (millimeters) unless otherwise noted 20-Lead Plastic eTSSOP Package NS Package Number MXA20A 15 www.ti.com LM3492HC/LM3492HCQ Two-Channel Individual Dimmable LED Driver with Boost Converter and Fast Current Regulator Notes www.ti.com IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. 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