DEMO MANUAL DC2788A LT3950 60V 1.5A LED Driver with … · 2020. 9. 30. · ITOUT EMI ILTE IT EMI...

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1 DEMO MANUAL DC2788A Rev. 0 PARAMETER CONDITIONS MIN TYP MAX Input Voltage Range (V IN ) I LED Running 6.6 24 LT3950 IC Input Voltage Range (V IN ) 3V 60V Full-Scale LED Current R1 = 0.75Ω, CTRL Turret = FLOAT 330mA LED Voltage Range R7 = 1MΩ, R8 = 41.2kΩ 7.5 28V Open LED Voltage (V OUT ) R7 = 1MΩ, R8 = 41.2kΩ, LEDs Open 30V LT3950 IC Output Voltage Maximum 60V Switching Frequency R5 = 49.9kΩ, SSFM Off 2.0MHz SSFM Switching Frequency R5 = 49.9kΩ, SSFM On 2.0MHz 2.5MHz Typical Efficiency with EMI Filters FB1, FB2, C14, C21 Installed 89% Typical Efficiency with EMI Filters Removed FB1 and FB2 Shorted, C14 and C21 Removed 90% V IN Turn-On Threshold (Rising) R2 = 124kΩ, R3 = 499kΩ 7.5V V IN UVLO Threshold (Falling) Under Voltage Lockout R2 = 124kΩ, R3 = 499kΩ 6.6V PWM Frequency Internal PWM Dimming R5 = 49.9kΩ, JP2 = INTV CC 460Hz DESCRIPTION LT3950 60V 1.5A LED Driver with Internal Log-Scale Dimming Demonstration circuit 2788A is a boost LED driver fea- turing the LT ® 3950. This demonstration circuit powers a string of LEDs at 330mA. The step-up topology can be used to drive a string of up to 28V of LEDs as assembled. The maximum output voltage capability of the LT3950 is 60V. DC2788A runs from an input voltage range of 6V to 24V as built. It also runs at 2MHz and has the capability to turn on spread spectrum frequency modulation (SSFM) for a frequency range of 2.0MHz to 2.5MHz. Dimming control can be achieved with analog dimming or PWM dimming—either from an external or internally-generated clock source. DC2788A features undervoltage lockout (UVLO) set at 6.6V with a 1.0V hysteresis for turn-on. The UVLO voltage, LED current, output voltage range, switching frequency, brightness control, SSFM, and the topology can all be adjusted with simple modifications to the demonstration circuit. LT3950 is a monolithic 1.5A peak switch current, 60V LED driver. The guaranteed peak switch current rating of the IC is 1.5A and this is important to know when calculat- ing maximum output current at a given LED voltage and input voltage for a boost converter. The LT3950 features SSFM and a well-controlled SW node for low emissions. All registered trademarks and trademarks are the property of their respective owners. PERFORMANCE SUMMARY A frequency range of 200kHz to 2MHz and a high-side PWMTG PWM-dimming MOSFET makes this a very ver- satile IC for many applications. It can be used for boost, buck-boost mode and buck mode LED driver applications. The PWMTG MOSFET not only provides high PWM dim- ming ratio capability, but it also serves as a short-circuit protection device. The FAULT flag indicates when there is either a short-circuit or open-LED fault at the output. The demo circuit is designed to be easily reconfigured to suit other applications, including the example schematics in the data sheet. Consult technical support for assistance. High voltage operation, 3V input voltage operation, multi- ple topologies, small-and-compact size, fault protection, low EMI, and multiple brightness control options make the LT3950 flexible and powerful for compact, noise-sen- sitive LED driver solutions. The LT3950JMSE featured on this demo circuit is available in a thermally enhanced 16-lead plastic MSOP package. The LT3950 data sheet must be read in conjunction with this demo manual to properly use or modify demo circuit DC2788A. Design files for this circuit board are available. Specifications are at T A = 25°C

Transcript of DEMO MANUAL DC2788A LT3950 60V 1.5A LED Driver with … · 2020. 9. 30. · ITOUT EMI ILTE IT EMI...

Page 1: DEMO MANUAL DC2788A LT3950 60V 1.5A LED Driver with … · 2020. 9. 30. · ITOUT EMI ILTE IT EMI ILTE VIN V 0 5 10 15 20 25 30 60 65 70 75 80 85 90 95 100 EICIENC DC2788A 02a UVLO

1

DEMO MANUAL DC2788A

Rev. 0

PARAMETER CONDITIONS MIN TYP MAXInput Voltage Range (VIN) ILED Running 6.6 24LT3950 IC Input Voltage Range (VIN) 3V 60VFull-Scale LED Current R1 = 0.75Ω, CTRL Turret = FLOAT 330mALED Voltage Range R7 = 1MΩ, R8 = 41.2kΩ 7.5 28VOpen LED Voltage (VOUT) R7 = 1MΩ, R8 = 41.2kΩ, LEDs Open 30VLT3950 IC Output Voltage Maximum 60VSwitching Frequency R5 = 49.9kΩ, SSFM Off 2.0MHzSSFM Switching Frequency R5 = 49.9kΩ, SSFM On 2.0MHz 2.5MHzTypical Efficiency with EMI Filters FB1, FB2, C14, C21 Installed 89%Typical Efficiency with EMI Filters Removed FB1 and FB2 Shorted, C14 and C21 Removed 90%VIN Turn-On Threshold (Rising) R2 = 124kΩ, R3 = 499kΩ 7.5VVIN UVLO Threshold (Falling) Under Voltage Lockout R2 = 124kΩ, R3 = 499kΩ 6.6VPWM Frequency Internal PWM Dimming R5 = 49.9kΩ, JP2 = INTVCC 460Hz

DESCRIPTION

LT3950 60V 1.5A LED Driver with Internal Log-Scale Dimming

Demonstration circuit 2788A is a boost LED driver fea-turing the LT®3950. This demonstration circuit powers a string of LEDs at 330mA. The step-up topology can be used to drive a string of up to 28V of LEDs as assembled. The maximum output voltage capability of the LT3950 is 60V. DC2788A runs from an input voltage range of 6V to 24V as built. It also runs at 2MHz and has the capability to turn on spread spectrum frequency modulation (SSFM) for a frequency range of 2.0MHz to 2.5MHz. Dimming control can be achieved with analog dimming or PWM dimming—either from an external or internally-generated clock source. DC2788A features undervoltage lockout (UVLO) set at 6.6V with a 1.0V hysteresis for turn-on.

The UVLO voltage, LED current, output voltage range, switching frequency, brightness control, SSFM, and the topology can all be adjusted with simple modifications to the demonstration circuit.

LT3950 is a monolithic 1.5A peak switch current, 60V LED driver. The guaranteed peak switch current rating of the IC is 1.5A and this is important to know when calculat-ing maximum output current at a given LED voltage and input voltage for a boost converter. The LT3950 features SSFM and a well-controlled SW node for low emissions. All registered trademarks and trademarks are the property of their respective owners.

PERFORMANCE SUMMARY

A frequency range of 200kHz to 2MHz and a high-side PWMTG PWM-dimming MOSFET makes this a very ver-satile IC for many applications. It can be used for boost, buck-boost mode and buck mode LED driver applications. The PWMTG MOSFET not only provides high PWM dim-ming ratio capability, but it also serves as a short-circuit protection device. The FAULT flag indicates when there is either a short-circuit or open-LED fault at the output.

The demo circuit is designed to be easily reconfigured to suit other applications, including the example schematics in the data sheet. Consult technical support for assistance.

High voltage operation, 3V input voltage operation, multi-ple topologies, small-and-compact size, fault protection, low EMI, and multiple brightness control options make the LT3950 flexible and powerful for compact, noise-sen-sitive LED driver solutions. The LT3950JMSE featured on this demo circuit is available in a thermally enhanced 16-lead plastic MSOP package. The LT3950 data sheet must be read in conjunction with this demo manual to properly use or modify demo circuit DC2788A.

Design files for this circuit board are available.

Specifications are at TA = 25°C

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DEMO MANUAL DC2788A

Rev. 0

The DC2788A is easy to set up to evaluate the performance of the LT3950JMSE. Refer to Figure 1 for proper measure-ment equipment setup and follow the procedure below.

Note: Make sure that the voltage applied to VIN does not exceed 45V, which is close to the maximum voltage rating for the input capacitors.

1. Set JP1 to On and JP2 to GND to disable Internal PWM Dimming and to run the LED driver at 100% duty cycle. Set JP3 to No SSFM to disable SSFM and run at 2.0MHz constant frequency. JP3 can be switched to SSFM On to evaluate the performance of the PCB with spread spectrum frequency modulation.

2. Connect the EN terminal to GND with a clip-on lead. Connect the power supply (with power off), LED load, and meters as shown.

3. After all connections are made, turn on the input power and verify that the input voltage is between 8V and 18V.

4. Remove the clip-on lead from EN. Verify that the LED current is 330mA, the VOUT voltage is between 7.5V and 28V and the FAULT terminal is not asserted low.

Note: If the output voltage is low or if the FAULT ter-minal is asserted low, temporarily disconnect the load to make sure that the LED string is connected properly and not faulted.

5. Once the proper output current and voltage are estab-lished, adjust the input voltage and load within the operating ranges and observe the output voltage reg-ulation, dimming and PWM.

QUICK START PROCEDURE

Figure 1. Test Procedure Setup Drawing for DC2788A

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DEMO MANUAL DC2788A

Rev. 0

PWM DIMMING

To evaluate internally generated PWM dimming per-formance, (with power off) set JP2 = INTVCC and JP1 to INT. PWM dimming duty cycle is set by adjusting the position of VR1 potentiometer with a small screwdriver (with power on). It is safest to switch jumper positions with the power off, and then turn power back on when positions are set.

To evaluate externally generated PWM dimming perfor-mance, (with power off) set JP2 = GND and JP1 to EXT. Place a 3.3V or 5V variable duty-cycle input on the PWM terminal to control PWM dimming. PWM dimming fre-quency should be greater than or equal to 100Hz. 120Hz is recommended for the highest dimming ratio performance without low risk of visible flicker.

ANALOG DIMMING

Constant LED current is controlled by setting the voltage of the CTRL pin on the LT3950. Either a voltage source can be placed on the CTRL turret and set between 200mV and 1.5V for LED current control, or the resistors R16 and R4 can be used to set the CTRL pin voltage with a divider from INTVCC as shown in the schematic. Analog dimming and PWM dimming can be combined for a very high dimming ratio.

EMI FILTERS

EMI input filters are placed on the PCB for low EMI testing results. This PCB passes CISPR25 class 5 conducted and radiated emissions testing for automotive vehicles. The input filter FB1 and C14 helps with high frequency noise at the input. FB2 and C21 help with high frequency noise at the output. Since this converter runs at 2MHz, large AM band (530kHz to 1.8MHz) emissions filters are not needed and the overall solution size is small. EMI filters may not be necessary in all applications, however. For the highest dimming ratio and for the highest efficiency, the input and output EMI filters can be removed.

In order to remove the EMI filters, the ferrite beads (FB1 and FB2) can be shorted out, and the capacitors (C14 and C21) should be removed. Figure 2 through Figure 6 demonstrate the difference in efficiency and

PWM dimming with the EMI filters in place or removed. Extremely high PWM dimming performance is possible without EMI filters, but even with the filters, very high PWM dimming is capable with the LT3950.

ADJUSTMENTS

Other adjustments can easily be made to the demonstra-tion circuit. The overvoltage protection voltage (OVP) can be set by changing the values of R7 and R8. Please read the data sheet for details. R7 and R8 are used to set the VOUT fault voltage when LEDs are removed from the out-put, but the running LED string voltage should remain below this point to not cause a fault.

The switching frequency can be changed over a wide range by setting the RT resistor, R5. SSFM spreads up from the RT resistor setting to fSW (RT) + 25%. SSFM is turned on and off by changing the jumper position on the PCB. It is that simple. SSFM can be turned on and off for evaluation and emissions testing.

LED current is set with R1 and the CTRL voltage as shown in the data sheet. For the highest accuracy, use CTRL = 1.5V or higher for full-scale LED sense voltage of 250mV. Since the peak switch current rating of the LT3950 is 1.5A, theoretically, about 1A is the maximum current that can be extracted at the output for any topol-ogy (buck mode). With very, very small ripple current, 1.2A might be able to be delivered at the output of a buck mode converter, but this might not be very practical. In a boost converter topology, please note that the peak switch current is the input current of the converter plus some ripple. In a boost, the input current can be significantly higher than the LED current. Please do not expect 1.5A LED current in a boost converter with this IC. Much higher peak switch current rating is needed for that.

The converter topology can be changed from boost to buck-boost mode (LEDs returned to VIN) or buck mode. Please consult the factory applications engineers or the data sheet for details. Components Q1, R19, and R20 are used for overvoltage protection in both buck-boost mode and buck mode. They are not used for the boost topology.

QUICK START PROCEDURE

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DEMO MANUAL DC2788A

Rev. 0

TEST RESULTS

Figure 2. DC2788A at Full Load (330mA 24VLED) with and without EMI Filters, SSFM On

Figure 3. Infinite-Persist LED Current Showing PWM Dimming and SSFM Working Together for Flicker-Free Brightness Control

(a) Efficiency (b) LED Current vs Input Voltage

WITHOUT EMI FILTERWITH EMI FILTER

VIN (V)0 5 10 15 20 25 30

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Figure 4. DC2788A Achieves Dimming Ratios of 1000:1 at 100Hz with EMI Filters

Figure 5. Up to 5000:1 PWM Dimming is Possible, Even with EMI Filters on DC2788A

Figure 6. Maximum PWM Dimming Ratio is Very High with Output EMI Filters Removed

5μs/DIV DC2788A F03

ILED100mA/DIV

VIN = 12V, VLED = 24V, ILED = 330mAfSW = 2MHz + SSFM ON 50:1, 460Hz INTERNAL PWM DIMMINGINFINITE PERSIST

10μs/DIV DC2788A F04

ILED200mA/DIV

VPWM2V/DIV

VIN = 12V, VLED = 24V, ILED = 330mAfSW = 2MHz + SSFM ON 100Hz EXTERNAL PWM DIMMING

1μs/DIV DC2788A F05

ILED100mA/DIV

VPWM2V/DIV

VIN = 12V, VLED = 24V, ILED = 330mAfSW = 2MHz + SSFM ON 100Hz EXTERNAL PWM DIMMING

1μs/DIV DC2788A F06

ILED100mA/DIV

VPWM2V/DIV

VIN = 12V, VLED = 24V, ILED = 330mAfSW = 2MHz + SSFM ON 100Hz EXTERNAL PWM DIMMINGOUTPUT EMI FILTER REMOVED

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DEMO MANUAL DC2788A

Rev. 0

EMISSION RESULTS

Figure 7. Average and Peak Conducted Emissions Performance Using Current Method Both Pass CISPR25 Limits

(a) CISPR25 Conducted EMI Performance Current Method (b) CISPR25 Conducted EMI Performance Current Method

(a) CISPR25 Conducted EMI Performance Voltage Method (b) CISPR25 Conducted EMI Performance Voltage Method

(a) CISPR25 Radiated EMI Performance (b) CISPR25 Radiated EMI Performance

CLASS 5 AVERAGE LIMITSSFM ONAMBIENT

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Figure 8. Average and Peak Conducted Emissions Performance Using Voltage Method Both Pass CISPR25 Limits

Figure 9. CISPR25 Average and Peak Radiated Emissions Performance Both Pass CISPR25 Limits

CLASS 5 PEAK LIMITSSFM ONAMBIENT

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DEMO MANUAL DC2788A

Rev. 0

PARTS LISTITEM QTY REFERENCE PART DESCRIPTION MANUFACTURER/PART NUMBER

Required Electrical Components

1 2 C1, C11 CAP., X5R, 1µF, 50V, 10% 0402 TAIYO YUDEN, UMK105CBJ105KV-F

2 1 C5 CAP., 4.7µF, X7S, 100V, 20%, 1206 MURATA, GRM31CC72A475ME11L

3 1 C9 CAP., 270pF, C0G, 50V, 5%, 0402 MURATA, GRM1555C1H271JA01

4 1 C20 CAP., 4.7µF, X7R, 50V, 10%, 1206 MURATA, GRM31CR71H475KA12L

5 1 D1 DIODE, SCHOTTKY 60V 1A SOD123F (DC) NEXPERIA, PMEG6010CEH, 115

6 1 L1 FIXED INDUCTOR, 6.8µH, PWR, 20%, 1.6A, 168mΩ, AEC-Q200 WURTH, 74438336068

7 1 M1 MOSFET P-CH 60V 1.6A SOT23-3 VISHAY, SI2309CDS-T1-GE3

8 1 R1 RES., 0.75Ω, 1%, 1/3W, 0805, SHORT-SIDE TERM, SENSE SUSUMU, RL1220S-R75-F

9 1 R5 RES., 49.9k, 1%, 1/16W, 0402, AEC-Q200 VISHAY, CRCW040249K9FKED

10 1 R6 RES., 62k, 1%, 1/16W, 0402, AEC-Q200 VISHAY, CRCW040262K0FKED

11 1 R7 RES., 1MΩ, 1%, 1/16W, 0402, AEC-Q200 VISHAY, CRCW04021M00FKED

12 1 R8 RES., 41.2k, 1%, 1/16W, 0402, AEC-Q200 VISHAY, CRCW040241K2FKED

13 1 U1 DC/DC CONVERTOR, 16-LEAD , QFN, 3mm × 3mm ADI, LT3950JMSE#PBF

Optional Electrical Components

1 1 C6 CAP., ALUM, 22µF, 50V, SMD AEC-Q200 PANASONIC, EEH-ZC1H220P

2 0 C12 CAP., OPTION, 0603

3 2 C14, C21 CAP., X7R, 0.1µF, 50V, 10% 0402 MURATA, GRM155R71H104KE14D

4 0 C22 (OPT) CAP., OPTION, 1206

5 1 FB1 FERRITE BEAD, 600Ω, 0805, 1LN WURTH, 7427920415

6 1 FB2 FERRITE BEAD, 1.5k 0805 1LN WURTH, 742792097

7 0 Q1 MOSFET, OPTION

8 1 R2 RES., 124k, 1%, 1/16W, 0402 VISHAY, CRCW0402124KFKED

9 1 R3 RES., 499k, 1%, 1/16W, 0402 VISHAY, CRCW0402499KFKED

10 2 R4, R10 RES., 100k, 1%, 1/16W, 0402 VISHAY, CRCW0402100KFKED

11 1 R9 RES., 100k, 1%, 1/10W, 0603, AEC-Q200 VISHAY, CRCW0603100KFKEA

12 0 R12 RES., OPTION, 0402

13 1 R11 RES., 0Ω, 1/16W, 0402 VISHAY, CRCW04020000Z0ED

14 0 R16, R17, R18 (OPT) RES., OPTION, 0402

15 0 R19, R20 RES., OPTION, 0603

16 1 VR1 TRIMMER 100k 0.25W SMD BOURNS, 3314J-1-104E

Hardware

1 4 E1, E2, E4, E10 TESTPOINT, TURRET, .094" PBF MILL-MAX, 2501-2-00-80-00-00-07-0

2 6 E3, E5, E6, E7, E8, E9 TESTPOINT, TURRET, .061" PBF MILL-MAX, 2308-2-00-80-00-00-07-0

3 3 JP1, JP2, JP3 HEADER 3×2 0.079 DOUBLE ROW WURTH, 62000621121

4 3 XJP1, XJP2, XJP3 SHUNT, .079" CENTER WURTH, 60800213421

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DEMO MANUAL DC2788A

Rev. 0

Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

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Page 8: DEMO MANUAL DC2788A LT3950 60V 1.5A LED Driver with … · 2020. 9. 30. · ITOUT EMI ILTE IT EMI ILTE VIN V 0 5 10 15 20 25 30 60 65 70 75 80 85 90 95 100 EICIENC DC2788A 02a UVLO

8

DEMO MANUAL DC2788A

Rev. 0

ANALOG DEVICES, INC. 2020

09/20www.analog.com

ESD Caution ESD (electrostatic discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection circuitry, damage may occur on devices subjected to high energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality.

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