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Electrical Specifications Subject to Change
LT3799 Offline Isolated Flyback LED Controller with Active PFC FEATURES
n n n n n n n n n
DESCRIPTION
The LT(R)3799 is an isolated flyback controller with power factor correction specifically designed for driving LEDs. The controller operates using critical conduction mode allowing the use of a small transformer. Using a novel current sensing scheme, the controller is able to deliver a well regulated current to the secondary side without using an opto-coupler. A strong gate driver is included to drive an external high voltage MOSFET. Utilizing an onboard multiplier, the LT3799 typically achieves power factors of 0.97. The FAULT pin provides notification of open and short LED conditions. The LT3799 uses a micropower hysteretic start-up to efficiently operate at offline input voltages, with a third winding to provide power to the part. An internal LDO provides a well regulated supply for the part's internal circuitry and gate driver.
Isolated PFC LED Driver with Minimum Number of External Components TRIAC Dimmable VIN and VOUT Limited Only by External Components Active Power Factor Correction (Typical PFC > 0.97) Low Harmonic Content No Opto-Coupler Required Accurate Regulated LED Current (5% Typical) Open LED and Shorted LED Protection Thermally Enhanced 16-lead MSOP Package
APPLICATIONS
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Offline 4W to 100W+ LED Applications High DC VIN LED Applications
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and True Color PWM is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Patents pending.
TYPICAL APPLICATION
TRIAC Dimmable 20W LED Driver
1.20
LED Current vs Input Voltage
90V TO 150V AC
0.22F 0.1F 200 499k 499k
100k 100k
20 4.7pF 10F VIN
1.15 4:1:1 1.10 1.05 ILED (A)
2k DCM FB 100k 1A
1.00 0.95 0.90 0.85
VIN_SENSE 6.34k VREF 100k 32.4k 40.2k CTRL3 CTRL2 100k NTC 16.2k FAULT CTRL1 LT3799
4.99k
560F x2
GATE SENSE VINTVCC
20
20W LED POWER
0.05
0.80
90
100
110
120 130 VIN (VAC)
140
150
3799 TA01b
4.7F GND FAULT CT COMP+ 0.1F COMP-
2.2nF
3799 TA01a
0.1F
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LT3799 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW CTRL1 CTRL2 CTRL3 VREF FAULT CT COMP+ COMP- 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 VIN_SENSE SENSE GATE INTVCC NC VIN DCM FB
VIN, FAULT .................................................................32V GATE, INTVCC ...........................................................18V CTRL1, CTRL2, CTRL3, VIN_SENSE, COMP- ................4V FB, CT, VREF, COMP+,...................................................3V SENSE ......................................................................0.4V DCM .......................................................................3mA Maximum Junction Temperature .......................... 125C Operating Temperature Range (Note 2) LT3799E ............................................ -40C to 125C LT3799I ............................................. -40C to 125C Storage Temperature Range .................. -65C to 150C
17 GND
MSE PACKAGE 16-LEAD PLASTIC MSOP JA = 50C/W, JC = 10C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3799EMSE#PBF LT3799IMSE#PBF TAPE AND REEL LT3799EMSE#TRPBF LT3799IMSE#TRPBF PART MARKING* 3799 3799 PACKAGE DESCRIPTION 16-Lead Plastic MSOPE 16-Lead Plastic MSOPE TEMPERATURE RANGE -40C to 125C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER VIN Turn-On Voltage VIN Turn-Off Voltage VIN Hysteresis VIN Shunt Regulator Voltage VIN Shunt Regulator Current Limit VIN Quiescent Current INTVCC Quiescent Current VIN_SENSE Threshold VIN_SENSE Linear Range VREF Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance 0A Load 200A Load Before Turn-On After Turn-On Before Turn-On After Turn-On Turn-Off CONDITIONS
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 18V, INTVCC = 11V, unless otherwise noted.
MIN 22.2 11.8 VTURNON - VTURNOFF I = 1mA 15 55 12 1.5 30 0
l l
TYP 23 12.3 10.7 25.0 65 70 16 1.2 65 2 1.98 100 50
MAX 24.2 13.0
UNITS V V V V mA
75 20.0 2.6 90 1.3 2.02 2.02
A A A mA mV V V V V/V mhos
1.975 1.9555
VCOMP+/VCOMP-, CTRL1 = 1V, CTRL2 = 2V, CTRL3 = 2V I = 5A
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LT3799 ELECTRICAL CHARACTERISTICS
PARAMETER FB Pin Bias Current CTRL1/CTRL2/CTRL3 Pin Bias Current SENSE Current Limit Threshold SENSE Input Bias Current Current Loop Voltage Gain CT Pin Charge Current CT Pin Discharge Current CT Pin Low Threshold CT Pin High Threshold CT Pin Low Hysteresis FB Pin High Threshold DCM Current Turn-On Threshold Maximum Oscillator Frequency Minimum Oscillator Frequency Back-Up Oscillator Frequency Linear Regulator INTVCC Regulation Voltage Dropout (VIN - INTVCC) Current Limit Current Limit Gate Driver tr GATE Driver Output Rise Time tf GATE Driver Output Fall Time GATE Output Low (VOL) GATE Output High (VOH) Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3799E is guaranteed to meet performance specifications from 0C to 125C junction temperature. Specifications over the -40C INTVCC - 0.05 to 125C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3799I is guaranteed to meet performance specifications from -40C to 125C operating junction temperature. Note 3: Current flows out of the FB pin. CL = 3300pF 10% to 90% , CL = 3300pF 90% to 10% , 20 20 0.05 ns ns V V INTVCC = -10mA Below Undervoltage Threshold Above Undervoltage Threshold 17 80 9.8 10 500 25 120 10.4 900 V mV mA mA Current Out of Pin COMP+ = 1.2V, V COMP+ = 0V, V
IN_SENSE = 1V
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 18V, unless otherwise noted.
CONDITIONS (Note 3), FB = 1V CTRL/CTRL2/CTRL3 = 1V 96 Current Out of Pin, SENSE = 0V VCTRL /VSENSE, 1000pF Cap from COMP+ to COMP- 100 15 21 10 200 Falling Threshold Rising Threshold 1.22 240 1.25 100 1.25 45 300 25 20 1.29 MIN TYP 100 MAX 600 30 106 UNITS nA nA mV A V/V A nA mV V mV V A kHz kHz kHz
IN_SENSE
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LT3799 TYPICAL PERFORMANCE CHARACTERISTICS
VIN Start-Up Voltage vs Temperature
24.0
VIN IQ vs Temperature
140 120
Input Voltage Hysteresis vs Temperature
12.0
23.5 INPUT VOLTAGE (V)
100 IQ (A) 80 60 40 20 VIN = 24V VIN = 12V
HYSTERESIS VOLTAGE (V)
11.6
11.2
23.0
10.8
22.5
10.4
22.0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G01
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G02
10.0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G03
VREF vs Temperature
2.100 2.075 2.050 VREF (V) VREF (V) 2.025 2.000 1.975 1.950 1.925 1.900 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125
3799 G05
2.100 2.075 2.050 2.025 2.000 1.975 1.950 1.925 1.900
VREF vs VIN
120 100 THRESHOLD (mV) 80 60 40 20
SENSE Pin Threshold Current vs Temperature
MAX ILIM
NO LOAD 200A LOAD
NO LOAD 200A LOAD
MIN ILIM
14
16
18
20
22 24 VIN (V)
26
28
30
32
3799 G05
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G06
Maximum Oscillator Frequency vs Temperature
375 350 FREQUENCY (kHz) FREQUENCY (kHz) 125
3799 G07
Minimum Oscillator Frequency vs Temperature
70 60 50 40 30 20 10 -50
325 300 275 250 225 -50
-25
50 25 0 75 TEMPERATURE (C)
100
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G08
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LT3799 TYPICAL PERFORMANCE CHARACTERISTICS
CT Pin Charge Current vs Temperature
12 10 CT CHARGE CURRENT (A) 8 6 4 2 0 -50 200
CT Pin Discharge Current vs Temperature
0.4
CT Pin Low Threshold vs Temperature
CT DISCHARGE CURRENT (nA)
190 CT PIN VOLTAGE (V) -25 50 25 0 75 TEMPERATURE (C) 100 125
3799 G10
0.3
180
0.2
170
160
0.1
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G09
150 -50
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G11
CT Pin High Threshold vs Temperature
1.5 10.6 10.4 10.2 INTVCC (V) 10.0
INTVCC vs Temperature
10.25 10.20 NO LOAD INTVCC (V) 10.15 10.10 10.05 10.00 9.95
INTVCC vs VIN
1.4 CT PIN VOLTAGE (V)
PART ON
1.3
1.2
10mA LOAD 9.8
1.1
9.6 9.4 -50
PART OFF 10 12 14 16 18 20 22 24 26 28 30 34 VIN (V)
1.0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G12
-25
50 0 25 75 TEMPERATURE (C)
100
125
3799 G13
3799 G14
26.00 25.75 VIN SHUNT VOLTAGE (V) 25.50 25.25 25.00 24.75
VIN Shunt Voltage vs Temperature
30 25 SHUNT CURRENT (mA) 20 15 10 5
Maximum Shunt Current vs Temperature
1.2 1.0 0.8 ILED (A)
LED Current vs TRAIC Angle
PAGE 17 SCHEMATIC
220V APPLICATION 0.6 0.4 0.2 0 120V APPLICATION
ISHUNT = 10mA
24.50 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G15
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
3799 G16
0
30
120 90 60 TRIAC ANGLE (C)
150
180
3799 G17
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LT3799 TYPICAL PERFORMANCE CHARACTERISTICS
LED Current vs Input Voltage
1.20 1.15 1.10 1.05 ILED (A) ILED (A) 1.00 0.95 0.90 0.85 0.80 90 100 110 120 130 VIN (VAC) 140 150
3799 G18
LED Current vs Input Voltage
1.20 1.15 1.10 1.05 ILED (A) 1.00 0.95 0.90 0.85 0.80 170 180 190 200 210 220 230 240 250 260 270 VIN (VAC) PAGE 17 SCHEMATIC: OPTIMIZED FOR 220V 1.20 1.15 1.10 1.05 1.00 0.95 0.90 0.85 0.80
LED Current vs Input Voltage
PAGE 17 SCHEMATIC: UNIVERSAL
PAGE 17 SCHEMATIC: OPTIMIZED FOR 120V
3799 G19
90 110 130 150 170 190 210 230 250 270 VIN (VAC)
3799 G20
Power Factor vs Input Voltage
1.00 0.99 0.98 POWER FACTOR ( ) POWER FACTOR ( ) 0.97 0.96 0.95 0.94 0.93 0.92 0.91 0.90 PAGE 17 SCHEMATIC: OPTIMIZED FOR 120V 90 100 110 120 130 VIN (VAC) 140 150
3799 G21
Power Factor vs Input Voltage
1.00 0.99 0.98 POWER FACTOR ( ) 0.97 0.96 0.95 0.94 0.93 0.92 0.91 0.90 170 180 190 200 210 220 230 240 250 260 270 VIN (VAC) PAGE 17 SCHEMATIC: OPTIMIZED FOR 220V 1.00 0.99 0.98 0.97 0.96 0.95 0.94 0.93 0.92 0.91 0.90
Power Factor vs Input Voltage
PAGE 17 SCHEMATIC: UNIVERSAL
3799 G22
90 110 130 150 170 190 210 230 250 270 VIN (VAC)
3799 G23
Efficiency vs Input Voltage
100 95 90 EFFICIENCY (%) EFFICIENCY (%) 85 80 75 70 65 60 90 100 110 120 130 VIN (VAC) 140 150
3799 G24
Efficiency vs Input Voltage
100 95 90 85 80 75 70 65 60 170 180 190 200 210 220 230 240 250 260 270 VIN (VAC) EFFICIENCY (%) PAGE 17 SCHEMATIC: OPTIMIZED FOR 220V 100
Efficiency vs Input Voltage
PAGE 17 SCHEMATIC: 95 UNIVERSAL
PAGE 17 SCHEMATIC: OPTIMIZED FOR 120V
90 85 80 75 70 65 60 90 110 130 150 170 190 210 230 250 270 VIN (VAC)
3799 G25
3799 G26
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LT3799 PIN FUNCTIONS
VIN (Pin 11): Input Voltage. This pin supplies current to the internal start-up circuitry and to the INTVCC LDO. This pin must be locally bypassed with a capacitor. A 25V shunt regulator is internally connected to this pin. INTVCC (Pin 13): Regulated Supply for Internal Loads and GATE Driver. Supplied from VIN and regulates to 10V (typical). INTVCC must be bypassed with a 4.7F capacitor placed close to the pin. COMP+, COMP- (Pin 7, Pin 8): Compensation Pins for Internal Error Amplifier. Connect a capacitor between these two pins to compensate the internal feedback loop. DCM (Pin 10): Discontinuous Conduction Mode Detection Pin. Connect a capacitor and resistor in series with this pin to the third winding. VIN_SENSE (Pin 16): Line Voltage Sense Pin. The pin is used for sensing the AC line voltage to perform power factor correction. Connect the output of a resistor divider from the line voltage to this pin. The voltage on this pin should be between 1.25V to 1.5V at the maximum input voltage. CTRL1, CTRL2, CTRL3 (Pin 1, Pin 2, Pin 3): Current Output Adjustment Pins. These pins control the output current. The lowest value of the three CTRL inputs is compared to the negative input of the operational amplifier. Due to the unique nature of the LT3799 control loop, the maximum current does not directly correspond to the VCTRL voltages. SENSE (Pin 15): The Current Sense Input for the Control Loop. Kelvin connect this pin to the positive terminal of the switch current sense resistor, RSENSE, and the source of the N-channel MOSFET. The negative terminal of the current sense resistor should be connected to the GND plane close to the IC. GATE (Pin 14): N-Channel MOSFET Gate Driver Output. Switches between INTVCC and GND. This pin is pulled to GND during shutdown state. FB (Pin 9): Voltage Loop Feedback Pin. FB is used to detect open LED conditions by sampling the third winding voltage. An open LED condition is reported if the CT pin is high and the FB pin is higher than 1.25V. CT (Pin 6): Timer Fault Pin. A capacitor is connected between this pin and ground to provide an internal timer for fault operations. During start-up, this pin is pulled to ground and then charged with a 10A current. Faults related to the FB pin will be ignored until the CT pin reaches 1.25V. If a fault is detected, the controller will stop switching and begin to discharge the CT capacitor with a 200nA pull-down current. When the pin reaches 240mV, the controller will start to switch again. FAULT (Pin 5): Fault Pin. An open-collector pull-down on FAULT asserts if FB is greater than 1.25V with the CT pin higher than 1.25V. VREF (Pin 4): Voltage Reference Output Pin, Typically 2V. This pin drives a resistor divider for the CTRL pin, either for analog dimming or for temperature limit/compensation of LED load. Can supply up to 200A. GND (Exposed Pad Pin 17): Ground. The exposed pad of the package provides both electrical contact to ground and good thermal contact to the printed circuit board. The exposed pad must be soldered to the circuit board for proper operation.
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LT3799 BLOCK DIAGRAM
D2 R4 C3
R3 R1 R2 C2
VIN T1 L1A N:1
D1 VOUT + L1B C7 VOUT -
C1 R10
L1C
R5
9 FB 6 C4 5 CT S&H FAULT DETECTION A3
10 DCM
16 VIN_SENSE 1.22V
11 VIN
+ A8 -
R7
FAULT
INTVCC
13 C5
+ A2 +- -
7 C6 8 1 2 3 COMP
-
ONE SHOT
600mV
CURRENT COMPARATOR
R8
COMP+ SW1
A7
- A1 +
S S
R
Q
DRIVER
GATE
14
M1
1M
MASTER LATCH
SENSE A4 GND
15 R6 17
CTRL1 CTRL2 CTRL3
- + A5 + +
A6 MULTIPLIER
LOW OUTPUT CURRENT OSCILLATOR
4 VREF
3799 BD
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LT3799 OPERATION
The LT3799 is a current mode switching controller IC designed specifically for generating an average current output in an isolated flyback topology. The special problem normally encountered in such circuits is that information relating to the output voltage and current on the isolated secondary side of the transformer must be communicated to the primary side in order to maintain regulation. Historically, this has been done with an opto-isolator. The LT3799 uses a novel method of using the external MOSFETs peak current information from the sense resistor to calculate the output current of a flyback converter without the need of an opto-coupler. In addition, it also detects open LED conditions by examining the third winding voltage when the main power switch is off. Power factor has become an important specification for lighting. A power factor of one is achieved if the current drawn is proportional to the input voltage. The LT3799 modulates the peak current limit with a scaled version of the input voltage. This technique provides power factors of 0.97 or greater. The Block Diagram shows an overall view of the system. The external components are in a flyback topology configuration. The third winding senses the output voltage and also supplies power to the part in steady-state operation. The VIN pin supplies power to an internal LDO that generates 10V at the INTVCC pin. The novel control circuitry consists of an error amplifier, a multiplier, a transmission gate, a current comparator, a low output current oscillator and a master latch, which will be explained in the following sections. The part also features a sample-and-hold to detect open LED conditions, along with a FAULT pin. A comparator is used to detect discontinuous conduction mode (DCM) with a cap connected to the third winding. The part features a 1.9A gate driver. The LT3799 employs a micropower hysteretic start-up feature to allow the part to work at any combination of input and output voltages. In the Block Diagram, R3 is used to stand off the high voltage supply voltage. The internal LDO starts to supply current to the INTVCC when VIN is above 23V. The VIN and INTVCC capacitors are charged by the current from R3. When VIN exceeds 23V and INTVCC is in regulation at 10V, the part will began to charge the CT pin with 10A. Once the CT pin reaches 340mV, switching begins. The VIN pin has 10.7V of hysteresis to allow for plenty of flexibility with the input and output capacitor values. The third winding provides power to VIN when its voltage is higher than the VIN voltage. A voltage shunt is provided for fault protection and can sink up to 15mA of current when VIN is over 25V. During a typical cycle, the gate driver turns the external MOSFET on and a current flows through the primary winding. This current increases at a rate proportional to the input voltage and inversely proportional to the magnetizing inductance of the transformer. The control loop determines the maximum current and the current comparator turns the switch off when the current level is reached. When the switch turns off, the energy in the core of the transformer flows out the secondary winding through the output diode, D1. This current decreases at a rate proportional to the output voltage. When the current decreases to zero, the output diode turns off and voltage across the secondary winding starts to oscillate from the parasitic capacitance and the magnetizing inductance of the transformer. Since all windings have the same voltage across them, the third winding rings too. The capacitor connected to the DCM pin, C1, trips the comparator, A2, which serves as a dv/dt detector, when the ringing occurs. This timing information is used to calculate the output current (description to follow). The dv/dt detector waits for the ringing waveform to reach its minimum value and then the switch turns back on. This switching behavior is similar to zero volt switching and minimizes the amount of energy lost when the switch is turned back on, improving efficiency as much as 5%. Since this part operates on the edge of continuous conduction mode and discontinuous conduction mode, this operating mode is called critical conduction mode (or boundary conduction mode). Primary-Side Current Control Loop The CTRL1/CTRL2/CTRL3 pins control the output current of the flyback controller. To simplify the loop, assume the VIN_SENSE pin is held at a constant voltage above 1V, eliminating the multiplier from the control loop. The error amplifier, A5, is configured as an integrator with the external capacitor, C6. The COMP+ node voltage is
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LT3799 OPERATION
converted to a current into the multiplier with the V/I converter, A6. Since A7's output is constant, the output of the multiplier is proportional to A6 and can be ignored. The output of the multiplier controls the peak current with its connection to the current comparator, A1. The output of the multiplier is also connected to the transmission gate, SW1. The transmission gate, SW1, turns on when the secondary current flows to the output capacitor. This is called the flyback period (when the output diode D1 is on). The current through the 1M resistor gets integrated by A5. The lowest CTRL input is equal to the negative input of A5 in steady state. A current output regulator normally uses a sense resistor in series with the output current and uses a feedback loop to control the peak current of the switching converter. In this isolated case the output current information is not available, so instead the LT3799 calculates it using the information available on the primary side of the transformer. The output current may be calculated by taking the average of the output diode current. As shown in Figure 1, the diode current is a triangle waveform with a base of the flyback time and a height of the peak secondary winding current. In a flyback topology, the secondary winding current is N times the primary winding current, where N is the primary to secondary winding ratio. Instead of taking the area of the triangle, think of it as a pulse width modulation (PWM) waveform. During the flyback time, the average current is half the peak secondary winding current and zero during the rest of the cycle. The equation for expressing the output current is: IOUT = 0.5 * IPK * N * D where D is equal to the percentage of the cycle represented by the flyback time. The LT3799 has access to both the primary winding current, the input to the current comparator, and when the flyback time starts and ends. Now the output current can be calculated by averaging a PWM waveform with the height of the current limit and the duty cycle of the flyback time over the entire cycle. In the feedback loop previously described, the input to the integrator is such a waveform. The integrator adjusts the peak current until the calculated output current equals the control voltage. If the calculated output current is low compared to the control pin, the error amplifier increases the voltage on the COMP+ node, thus increasing the current comparator input. When the VIN_SENSE voltage is connected to a resistor divider of the supply voltage, the current limit is proportional to the supply voltage if COMP+ is held constant. The output of the error amplifier is multiplied with the VIN_SENSE pin voltage. If the LT3799 is configured with a fast control loop, slower changes from the VIN_SENSE pin will not interfere with the current limit or the output current. The COMP+ pin will adjust to the changes of the VIN_SENSE. The only way for the multiplier to function properly is to set the control loop to be an order of magnitude slower than the fundamental frequency of the VIN_SENSE signal. In the offline case, the fundamental frequency of the supply voltage is 120Hz, so the control loop unity gain frequency needs to be set less than approximately 120Hz. Without a large amount of energy storage on the secondary side, the output current is affected by the supply voltage changes, but the DC component of the output current is accurate. TRIAC Dimming Features
TFLYBACK TPERIOD
3799 F01
SECONDARY DIODE CURRENT
IPK(sec)
SWITCH WAVEFORM
Figure 1. Secondary Diode Current and Switch Waveforms
The LT3799 incorporates some special features that aid in the design of an offline LED current source when used with a TRIAC dimmer. TRIAC dimmers are not ideal switches when turned off and allow milliamps of current to flow through them. This is an issue if used with a low quiescent part such as the LT3799. Instead of turning the main power
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LT3799 OPERATION
MOSFET off when the TRIAC is off, this power device is kept on and sinks the current to properly load the TRIAC. When the TRIAC turns on, the VIN_SENSE pin detects this and enables the loop, but the current comparator is always enabled and turns the switch off if it is tripped. Start-Up The LT3799 uses a hysteretic start-up to operate from high offline voltages. A resistor connected to the supply voltage protects the part from high voltages. This resistor is connected to the VIN pin on the part and also to a capacitor. When the resistor charges the part up to 23V and INTVCC is in regulation at 10V, the part begins to charge the CT pin to 340mV and then starts to switch. The resistor does not provide power for the part in steady state, but relies on the capacitor to start-up the part, then the third winding begins to provide power to the VIN pin along with the resistor. An internal voltage clamp is attached to the VIN pin to prevent the resistor current from allowing VIN to go above the absolute maximum voltage of the pin. The internal clamp is set at 25V and is capable of 28mA (typical) of current at room temperature. But, ideally, the resistor connected between the input supply and the VIN pin should be chosen so that less than 10mA is being shunted by this internal clamp. CT Pin and Faults The CT pin is a timing pin for the fault circuitry. When the input voltages are at the correct levels, the CT pin sources 10A of current. When the CT pin reaches 340mV, the part begins to switch. The output voltage information from the FB pin is sampled but ignored until the CT pin reaches 1.25V. When this occurs, if the FB pin is above 1.25V, the fault flag pulls low. The FAULT pin is meant to be used with a large pull-up resistor to the INTVCC pin or another supply. The CT pin begins to sink 200nA of current. When the CT pin goes below 240mV, the part will re-enable itself, begin to switch, and start to source 10A of current to the CT pin but not remove the fault condition. When the CT pin reaches 1.25V and FB is below 1.25V, the FAULT pin will no longer pull low and switching will continue. If not below 1.25V, the process repeats itself. Programming Output Current The maximum output current depends on the supply voltage and the output voltage in a flyback topology. With the VIN_SENSE pin connected to 1V and a DC supply voltage, the maximum output current is determined at the minimum supply voltage, and the maximum output voltage using the following equation: N IOUT(MAX) = 2 * (1- D) * 42 * R SENSE where D= VOUT * N VOUT * N + VIN
The maximum control voltage to achieve this maximum output current is 2V * (1-D). It is suggested to operate at 95% of these values to give margin for the part's tolerances. When designing for power factor correction, the output current waveform is going to have a half sine wave squared shape and will no longer be able to provide the above currents. By taking the integral of a sine wave squared over half a cycle, the average output current is found to be half the value of the peak output current. In this case, the recommended maximum average output current is as follows: IOUT(MAX) = (1- D) * where D= VOUT * N VOUT * N + VIN N * 47.5% 42 * R SENSE
The maximum control voltage to achieve this maximum output current is (1-D) * 47.5%. For control voltages below the maximum, the output current is equal to the following equation: IOUT = CTRL * N 42 * R SENSE
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LT3799 OPERATION
The VREF pin supplies a 2V reference voltage to be used with the control pins. To set an output current, a resistor divider is used from the 2V reference to one of the control pins. The following equation sets the output current with a resistor divider: 2N R1= R2 - 1 *R 42 * I
OUT SENSE
with AC, the following equation should be used with the correction factor: N IOUT = CTRL * 42 * R SENSE - CF 2N R1= R2 - 1 (42 * IOUT * RSENSE * CF) where CR is the output current correction factor on the Y-axis in Figure 3. Setting Control Voltages for LED Over Temperature and Brownout Conditions Critical Conduction Mode Operation Critical conduction mode is a variable frequency switching scheme that always returns the secondary current to zero with every cycle. The LT3799 relies on boundary mode and discontinuous mode to calculate the critical current because the sensing scheme assumes the secondary current returns to zero with every cycle. The DCM pin uses a fast current input comparator in combination with a small capacitor to detect dv/dt on the third winding. To eliminate false tripping due to leakage inductance ringing,
where R1 is the resistor connected to the VREF pin and the CTRL pin and R2 is the resistor connected to the CTRL pin and ground. When used with an AC input voltage, the LT3799 senses when the VIN_SENSE goes below 65mV and above 65mV for detecting when the TRIAC is off. During this low input voltage time, the output current regulation loop is off but the part still switches. This helps with output current regulation with a TRIAC but introduces a line regulation error. When VIN_SENSE is low, very little power is being delivered to the output and since the output current regulation loop is off, this time period needs to be accounted for in setting the output current. This time period slightly varies with line voltage. Figure 2 shows the correction factor in selecting the resistor divider resistors. When used
1.16 OUTPUT CURRENT CORRECTION FACTOR 1.14 1.12 1.10 1.08 1.06 1.04 1.02 0 0.5 1 1.5
3799 F03
PEAK VIN_SENSE
Figure 2. Correction Factor in Selecting the Resistor Divider Resistors
Figure 3. Output Current Correction Factor
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12
LT3799 OPERATION
a blanking time of between 600ns and 2.25s is applied after the switch turns off, depending on the current limit shown in the Leakage Inductance Blanking Time vs Current Limit curve in the Typical Performance Characteristics section. The detector looks for 40A of current through the DCM pin due to falling voltage on the third winding when the secondary diode turns off. This detection is important since the output current is calculated using this comparator's output. This is not the optimal time to turn the switch on because the switch voltage is still close to VIN + VOUT * N and would waste all the energy stored in the parasitic capacitance on the switch node. Discontinuous ringing begins when the secondary current reaches zero and the energy in the parasitic capacitance on the switch node transfers to the input capacitor. This is a secondorder network composed of the parasitic capacitance on the switch node and the magnetizing inductance of the primary winding of the transformer. The minimum voltage of the switch node during this discontinuous ring is VIN - VOUT * N. The LT3799 turns the switch back on at this time, during the discontinuous switch waveform, by sensing when the slope of the switch waveform goes from negative to positive using the dv/dt detector. This switching technique may increase efficiency by 5%. Sense Resistor Selection The resistor, RSENSE, between the source of the external N-channel MOSFET and GND should be selected to provide an adequate switch current to drive the application without exceeding the current limit threshold . For applications without power factor correction, select a resistor according to: RSENSE = where D= VOUT * N VOUT * N + VIN 2(1- D)N * 95% IOUT * 42 where D= VOUT * N VOUT * N + VIN
Minimum Current Limit The LT3799 features a minimum current limit of approximately 7% of the peak current limit. This is necessary when operating in critical conduction mode since low current limits would increase the operating frequency to a very high frequency. The output voltage sensing circuitry needs a minimum amount of flyback waveform time to sense the output voltage on the third winding. The time needed is 350ns. The minimum current limit allows the use of smaller transformers since the magnetizing primary inductance does not need to be as high to allow proper time to sample the output voltage information. Errors Affecting Current Output Regulation There are a few factors affecting the regulation of current in a manufacturing environment along with some systematic issues. The main manufacturing issues are the winding turns ratio and the LT3799 control loop accuracy. The winding turns ratio is well controlled by the transformer manufacturer's winding equipment, but most transformers do not require a tight tolerance on the winding ratio. We have worked with transformer manufacturers to specify 1% error for the turns ratio. Just like any other LED driver, the part is tested and trimmed to eliminate offsets in the control loop and an error of 3% is specified at 80% of the maximum output current. The error grows larger as the LED current is decreased from the maximum output current. At half the maximum output current, the error doubles to 6%. There are a number of systematic offsets that may be eliminated by adjusting the control voltage from the ideal voltage. It is difficult to measure the flyback time with complete accuracy. If this time is not accurate, the control voltage needs to be adjusted from the ideal value to eliminate the offset but this error still causes line regulation errors. If the supply voltage is lowered, the time error becomes a smaller portion of the switching cycle period so the offset becomes smaller and vice versa. This error may be compensated for at the primary supply voltage, but this does
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For applications with power factor correction, select a resistor according to: RSENSE = (1- D)N * 47.5% IOUT * 42
13
LT3799 OPERATION
not solve the problem completely for other supply voltages. Another systematic error is that the current comparator cannot instantaneously turn off the main power device. This delay time leads to primary current overshoot. This overshoot is less of a problem when the output current is close to its maximum, since the overshoot is only related to the slope of the primary current and not the current level. The overshoot is proportional to the supply voltage, so again this affects the line regulation. Universal Input The LT3799 operates over the universal input range of 90VAC to 265VAC . Output current regulation error may be minimized by using two application circuits for the wide input range: one optimized for 120VAC and another optimized for 220VAC . The first application pictured in the Typical Applications section shows three options: universal input, 120VAC , and 220VAC . The circuit varies by three resistors. In the Typical Performance Characteristics section, the LED Current vs VIN graphs show the output current line regulation for all three circuits. Selecting Winding Turns Ratio Boundary mode operation gives a lot of freedom in selecting the turns ratio of the transformer. We suggest to keep the duty cycle low, lower NPS, at the maximum input voltage since the duty cycle will increase when the AC waveform is decreases to zero volts. A higher NPS increases the output
VSUPPLY
current while keeping the primary current limit constant. Although this seems to be a good idea, it comes at the expense of a higher RMS current for the secondary-side diode which might not be desirable because of the primary side MOSFET's superior performance as a switch. A higher NPS does reduce the voltage stress on the secondary-side diode while increasing the voltage stress on the primaryside MOSFET. If switching frequency at full output load is kept constant, the amount of energy delivered per cycle by the transformer also stays constant regardless of the NPS. Therefore, the size of the transformer remains the same at practical NPS's. Adjusting the turns ratio is a good way to find an optimal MOSFET and diode for a given application. Switch Voltage Clamp Requirement Leakage inductance of an offline transformer is high due to the extra isolation requirement. The leakage inductance energy is not coupled to the secondary and goes into the drain node of the MOSFET. This is problematic since 400V and higher rated MOSFETs cannot always handle this energy by avalanching. Therefore the MOSFET needs protection. A transient voltage suppressor (TVS) and diode are recommended for all offline application and connected, as shown in Figure 4. The TVS device needs a reverse breakdown voltage greater than (VOUT + Vf)*N where VOUT is the output voltage of the flyback converter, Vf is the secondary diode forward voltage, and N is the turns ratio.
GATE
3799 F04
Figure 4. Clamp
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14
LT3799 OPERATION
Transformer Design Considerations Transformer specification and design is a critical part of successfully applying the LT3799. In addition to the usual list of caveats dealing with high frequency isolated power supply transformer design, the following information should be carefully considered. Since the current on the secondary side of the transformer is inferred by the current sampled on the primary, the transformer turns ratio must be tightly controlled to ensure a consistent output current. A tolerance of 5% in turns ratio from transformer to transformer could result in a variation of more than 5% in output regulation. Fortunately, most magnetic component manufacturers are capable of guaranteeing a turns ratio tolerance of 1% or better. Linear Technology has worked with several leading magnetic component manufacturers to produce predesigned flyback transformers for use with the LT3799. Table 1 shows the details of several of these transformers. Loop Compensation The current output feedback loop is an integrator configuration with the compensation capacitor between the negative input and output of the operational amplifier. This is a one-pole system therefore a zero is not needed in the compensation. For offline applications with PFC, the crossover should be set an order of magnitude lower than the line frequency of 120Hz or 100Hz. In a typical application, the compensation capacitor is 0.1F . In non-PFC applications, the crossover frequency may be increased to improve transient performance. The desired crossover frequency needs to be set an order of magnitude below the switching frequency for optimal performance. MOSFET and Diode Selection With a strong 1.9A gate driver, the LT3799 can effectively drive most high voltage MOSFETs. A low Qg MOSFET is recommended to maximize efficiency. In most applications, the RDS(ON) should be chosen to limit the temperature rise of the MOSFET. The drain of the MOSFET is stressed to VOUT * NPS + VIN during the time the MOSFET is off and the secondary diode is conducting current. But in most applications, the leakage inductance voltage spike exceeds this voltage. The voltage of this stress is determined by the switch voltage clamp. Always check the switch waveform with an oscilloscope to make sure the leakage inductance voltage spike is below the breakdown voltage of the MOSFET. A transient voltage suppressor and diode are slower than the leakage inductance voltage spike, therefore causing a higher voltage than calculated.
Table 1. Predesigned Transformers--Typical Specifications, Unless Otherwise Noted
TRANSFORMER PART NUMBER JA4429 7508110210 750813002 750811330 750813144 750813134 750811291 750813390 750811290 X-11181-002 SIZE (L x W x H) 21.1mm x 21.1mm x 17.3mm 15.75mm x 15mm x 18.5mm 15.75mm x 15mm x 18.5mm 43.2mm x 39.6mm x 30.5mm 16.5mm x 18mm x 18mm 16.5mm x 18mm x 18mm 31mm x 31mm x 25mm 43.18mm x 39.6mm x 30.48mm 31mm x 31mm x 25mm 23.5mm x 21.4mm x 9.5mm LPRI (H) 400 2000 2000 300 600 600 400 100 460 500 NPSA (NP:NS:NA) 1:0.24:0.24 6.67:1:1.67 20:1.0:5.0 6:1.0:1.0 4:1:0.71 8:1:1.28 1:1:0.24 1:1:0.22 1:1:0.17 72:16:10 RPRI (m) 252 5100 6100 150 2400 1850 550 150 600 1000 RSEC (m) 126 165 25 25 420 105 1230 688 560 80 MANUFACTURER Coilcraft Wurth Elektronik Wurth Elektronik Wurth Elektronik Wurth Elektronik Wurth Elektronik Wurth Elektronik Wurth Elektronik Wurth Elektronik Premo TARGET APPLICATION (VOUT /IOUT) 22V/1A 10V/0.4A 3.8V/1.1A 18V/5A 28V/0.5A 14V/1A 85V/0.4A 90V/1A 125V/0.32A 30V/0.5A
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15
LT3799 OPERATION
The secondary diode stress may be as much as VOUT + 2 * VIN /NPS due to the anode of the diode ringing with the secondary leakage inductance. An RC snubber in parallel with the diode eliminates this ringing, so that the reverse voltage stress is limited to VOUT + VIN /NPS. With a high NPS and output current greater than 3A, the IRMS through the diode can become very high and a low forward drop Schottky is recommended. Discontinuous Mode Detection The discontinuous mode detector uses AC-coupling to detect the ringing on the third winding. A 10pF capacitor with a 500 resistor in series is recommended in most designs. Depending on the amount of leakage inductance ringing, an additional current may be needed to prevent false tripping from the leakage inductance ringing. A resistor from INTVCC to the DCM pin adds this current. Up to an additional 100A of current may be needed in some cases. The DCM pin is roughly 0.7V, therefore the resistor value is selected using the following equation: R= 10V - 0.7V I Protection from Open LED and Shorted LED Faults The LT3799 detects output overvoltage conditions by looking at the voltage on the third winding. The third winding voltage is proportional to the output voltage when the main power switch is off and the secondary diode is conducting current. Sensing the output voltage requires delivering power to the output. Using the CT pin, the part turns off switching when a overvoltage condition occurs and rechecks to see if the overvoltage condition has cleared, as described in "CT Pin and Faults" in the Operation section. This greatly reduces the output current delivered to the output but a Zener is required to dissipate 2% of the set output current during an open LED condition. The Zener diode's voltage needs to be 10% higher than the output voltage set by the resistor divider connected to the FB pin. Multiple Zener diodes in series may be needed for higher output power applications to keep the Zener's temperature within the specification. During a shorted LED condition, the LT3799 operates at the minimum operating frequency. In normal operation, the third winding provides power to the IC, but the third winding voltage is zero during a shorted LED condition. This causes the part's VIN UVLO to shutdown switching. The part starts switching again when VIN has reached its turn-on voltage.
where I is equal to the additional current into the DCM pin. Power Factor Correction/Harmonic Content The LT3799 attains high power factor and low harmonic content by making the peak current of the main power switch proportional to the line voltage by using an internal multiplier. A power factor of >0.97 is easily attainable for most applications by following the design equations in this datasheet. With proper design, LT3799 applications meet IEC 6100-3-2 Class C harmonic standards.
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16
LT3799 TYPICAL APPLICATIONS
Universal TRIAC Dimmable 20W LED Driver
L2 800H L1 33mH C1 0.1F C2 0.1F BR1 C3 0.22F R1 200 R7 100k R8 100k D3 VIN VIN_SENSE R5 3.48k VREF R16 32.4k 100k NTC FAULT
BR1: DIODES, INC. HD06 D1: CENTRAL SEMICONDUCTOR CMR1U-06M D2, D3: DIODES INC. BAV20W DR: CENTRAL SEMICONDUCTOR CMR1U-02M Z1: FAIRCHILD SMBJ170A Z2: CENTRAL SEMICONDUCTOR CMZ5938B T1: COILCRAFT JA4429-AL M1: FAIRCHILD FDPF15N65
90V TO 265V AC
R6 D2 20 C4 C5 4.7pF 10F DCM FB LT3799 R15 4.99k R16 20
R3 499k R4 499k
4:1:1
R13 2k R4 100k Z1 D1
C10 560F x2
D4
1A
R18 100k
R9 40.2k
CTRL3 CTRL2 CTRL1
GATE SENSE VINTVCC GND
M1
RS 0.05
Z2
20W LED POWER
R10 24.9k FAULT CT COMP+
C9 4.7F
C8 2.2nF
3799 TA02
COMP-
C7, 0.1F
Component Values for Input Voltage Ranges
R5 () Optimized for 110V Optimized for 220V Universal 6.34k 3.48k 3.48k R10 () 16.2k 24.9k 15.4k RS () 0.05 0.075 0.05 R1 () 200 1.00k 200 C2 (F) 0.1 0.033 0.1 C3 (F) 0.22 0.1 0.22
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17
LT3799 TYPICAL APPLICATIONS
Universal Input TRIAC Dimmable 4W LED Driver
L1 3.3mH R20, 10k R21, 10k L2, 3.3mH C1 33nF
BR1
L1 3.3mH C3 68nF C2 22nF R1 750 R7 100k R8 100k D3 VIN VIN_SENSE R5 3.48k VREF R9 40.2k R10 32.4k FAULT FAULT CT COMP+ C6 0.1F CTRL3 CTRL2 CTRL1 GATE SENSE VINTVCC GND COMP- C9 4.7F
RS 0.3
90V TO 265V AC
R6 D2 20 C4 C5 4.7pF 10F DCM FB LT3799 R15 4.99k R16 20
20:5:1
R3 499k R4 499k
R13 10k R4 100k Z1 D1 M1
C10 1500F
D4
1A
R18 100k
4W LED POWER
Z2
C8 2.2nF
3799 TA03
BR1: DIODES, INC. HD06 D1: CENTRAL SEMICONDUCTOR CMR1U-06M D2, D3: CENTRAL SEMICONDUCTOR CMMSHI-100 D4: CENTRAL SEMICONDUCTOR CMSH2-40L Z1: FAIRCHILD SMBJ170A Z2: CENTRAL SEMICONDUCTOR CMZ59198 T1: WURTH ELEKTRONIK WE-750813002 M1: FAIRCHILD FQU5N60
C7, 0.1F
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18
LT3799 PACKAGE DESCRIPTION
MSE Package 16-Lead Plastic MSOP Exposed Die Pad ,
(Reference LTC DWG # 05-08-1667 Rev A)
BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 0.102 (.112 .004)
0.889 0.127 (.035 .005)
2.845 0.102 (.112 .004) 1 8 0.35 REF
5.23 (.206) MIN
1.651 0.102 3.20 - 3.45 (.065 .004) (.126 - .136)
0.305 0.038 (.0120 .0015) TYP
0.50 (.0197) BSC
16 4.039 0.102 (.159 .004) (NOTE 3)
DETAIL "B" CORNER TAIL IS PART OF DETAIL "B" THE LEADFRAME FEATURE. FOR REFERENCE ONLY 9 NO MEASUREMENT PURPOSE 0.280 0.076 (.011 .003) REF
1.651 0.102 (.065 .004)
0.12 REF
RECOMMENDED SOLDER PAD LAYOUT
DETAIL "A" 0 - 6 TYP
16151413121110 9
0.254 (.010)
GAUGE PLANE
4.90 0.152 (.193 .006)
3.00 0.102 (.118 .004) (NOTE 4)
0.53 0.152 (.021 .006)
DETAIL "A"
0.18 (.007)
1.10 (.043) MAX
12345678
0.86 (.034) REF
SEATING PLANE
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
0.17 - 0.27 (.007 - .011) TYP
0.50 (.0197) BSC
0.1016 0.0508 (.004 .002)
MSOP (MSE16) 0608 REV A
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
19
LT3799 TYPICAL APPLICATION
Universal Input TRIAC Dimmable 14W LED Driver
L2 750H L1 39mH C1 47nF BR1 C3 0.22F R1 250 C2 0.1F R2 250 R7 100k R8 100k D3 VIN VIN_SENSE R5 3.48k VREF R18 100k R16 10k R9 40.2k CTRL3 CTRL2 CTRL1 PHOTOCELL R17 10k R10 23.2k FAULT CT COMP+ C6 0.1F GATE SENSE VINTVCC GND COMP- C9 4.7F
RS 0.10
90V TO 265V AC
R6 D2 20 C4 C5 4.7pF 10F DCM FB LT3799 R15 5.90k R16 20
4:1:0.71
R3 499k R4 499k
R13 2k R4 100k Z1 D1
C10 390F x2
D4
0.5A
Z2
14W LED POWER
C8 2.2nF
3799 TA04
FAULT
BR1: DIODES, INC. HD06 D1: CENTRAL SEMICONDUCTOR CMR1U-06M D2, D3: DIODES INC. BAV20W D4: DIODES INC. DFLS1150 Z1: FAIRCHILD SMBJ170A Z2: CENTRAL SEMICONDUCTOR CMZ5938B T1: WURTH ELEKTRONIK WE750813144 M1: ST MICRO STD12N65M5
C7, 0.1F
RELATED PARTS
PART NUMBER LT3755/ LT3755-1/ LT3755-2 LT3756/ LT3756-1/ LT3756-2 LT3743 LT3518 LT3517 LT3741 DESCRIPTION High Side 60V, 1MHz LED Controller with 3000:1 True Color PWMTM Dimming High Side 100V, 1MHz LED Controller with 3000:1 True Color PWM Dimming Synchronous Step-Down 20A LED Driver with Three-State LED Current Control 2.3A, 2.5MHz High Current LED Driver with 3000:1 Dimming 1.3A, 2.5MHz High Current LED Driver with 3000:1 Dimming High Power, Constant-Current, Constant-Voltage Synchronous Step-Down Controller COMMENTS VIN : 4.5V to 40V, VOUT(MAX) = 60V, Dimming: 3000:1 True Color PWM, ISD < 1A, 3mm x 3mm QFN-16 and MSOP-16E Packages VIN : 6V to 100V, VOUT(MAX) = 100V, Dimming: 3000:1 True Color PWM, ISD < 1A, 3mm x 3mm QFN-16 and MSOP-16E Packages VIN : 5.5V to 36V, Dimming: 10000:1 True Color PWM, ISD < 1A, 5mm x 8mm QFN-52 Package VIN : 3V to 30V, Dimming: 3000:1 True Color PWM, ISD < 1A, 4mm x 4mm QFN-16 Package VIN : 3V to 30V, Dimming: 3000:1 True Color PWM, ISD < 1A, 4mm x 4mm QFN-16 Package VIN : 6V to 36V, Average Current Mode Control, ISD < 1A, 4mm x 4mm QFN-20 and TSSOP-20E Packages
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20 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0211 * PRINTED IN USA
www.linear.com
LINEAR TECHNOLOGY CORPORA TION 2011

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