LT3517 Full-Featured LED Driver with 1.5A Switch CurrentFEATURES
■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■DESCRIPTION
The LT®3517 is a current mode DC/DC converter with an internal 1.5A, 45V switch specifi cally designed to drive LEDs. The LT3517 operates as a LED driver in boost, buck mode and buck-boost mode. It combines a tradi-tional voltage loop and a unique current loop to operate as a constant-current source or constant-voltage source. Programmable switching frequency allows optimization of the external components for effi ciency or component size. The switching frequency of LT3517 can be synchronized to an external clock signal. The LED current is externally programmable with a 100mV sense resistor. The external PWM input provides up to 5000:1 LED dimming. The CTRL pin provides further 10:1 dimming ratio. The LT3517 is available in the tiny footprint 16-Lead QFN (4mm × 4mm) and the16-Pin TSSOP packages. The LT3517 provides a complete solution for both constant-voltage and constant-current applications. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation.True Color PWM is a trademark of Linear Technology Corporation. Patent Pending.All other trademarks are the property of their respective owners.*Patents Pending.
5000:1 True Color PWMTM Dimming Ratio1.5A, 45V Internal Switch100mV High Side Current SenseOpen LED ProtectionAdjustable Frequency: 250kHz to 2.5MHzWide Input Voltage Range: Operation from 3V to 30V Transient Protection to 40VOperates in Boost, Buck Mode and Buck-Boost ModeGate Driver for PMOS LED Disconnect*Constant-Current and Constant-Voltage RegulationCTRL Pin Provides 10:1 Analog DimmingLow Shutdown Current: <1μAAvailable in (4mm × 4mm) 16-Lead QFN and 16-Pin TSSOP Packages APPLICATIONS
Display Backlighting■ Automotive and Avionic Lighting■ Illumination■ Scanners■TYPICAL APPLICATION
1A Buck Mode LED Driver100
PVIN24V100mΩ2.2μFISPVINSHDNLT3517VREFPWMCTRLPWMFBSS5040
16.9k1MHz0.1μF3517 TA01aEffi ciency10μF90EFFICIENCY (%)CTRL = VREF1AISNTGSW15μH807060
VIN3.3V2.2μFRTSYNCTGENVREFVCGND0.1μF020
406080PWM DUTY CYCLE (%)
100
3517 TA01b
3517fa1
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LT3517ABSOLUTE MAXIMUM RATINGS
VIN, SHDN, PWM, TGEN (Note 3)..............................40VSW, ISP, ISN, TG ........................................................45VTG Pin Below ISP Pin ................................................10VFB, SYNC, SS, CTRL ...................................................6VVC, RT, VREF .................................................................3VOperating Junction Temperature Range (Notes 2,4) LT3517E ..............................................–40ºC to 125ºC LT3517I ...............................................–40ºC to 125ºC LT3517H .............................................–40ºC to 150ºC(Note 1)Storage Temperature Range QFN .....................................................–65ºC to 125ºC TSSOP ................................................–65ºC to 150ºCLead Temperature (Soldering, 10 sec) TSSOP ..............................................................300ºCPIN CONFIGURATION
TOP VIEWVINSHDNVREFRTSYNCSSPWMCTRL1234567817GND16SW15SW14TG13ISP12ISN11TGEN10FB9VCSW1SW2VIN3SHDN45VREF6RT7SYNC8SS17TOP VIEWTGEN12FB11VC10CTRL9PWMISNISPTG16151413FE PACKAGE16-LEAD PLASTIC TSSOPTJMAX = 150°C, QJA = 40°C/W, QJC(PAD) = 10°C/WUF PACKAGE16-LEAD (4mm × 4mm) PLASTIC QFNTJMAX = 125°C, θJA = 36°C/WEXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCBORDER INFORMATION
LEAD FREE FINISHLT3517EUF#PBFLT3517IUF#PBFLT3517EFE#PBFLT3517IFE#PBFLT3517HFE#PBFTAPE AND REELLT3517EUF#TRPBFLT3517IUF#TRPBFLT3517EFE#TRPBFLT3517IFE#TRPBFLT3517HFE#TRPBFPART MARKING*351735173517FE3517FE3517HFEPACKAGE DESCRIPTION16-Lead (4mm × 4mm) Plastic QFN16-Lead (4mm × 4mm) Plastic QFN16-Lead Plastic TSSOP16-Lead Plastic TSSOP16-Lead Plastic TSSOPTEMPERATURE RANGE–40°C to 125°C–40°C to 125°C–40°C to 125°C–40°C to 125°C–40°C to 150°CConsult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed by a label on the shipping container Consult LTC Marketing for information on non-standard lead based fi nish parts.For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifi cations, go to: http://www.linear.com/tapeandreel/3517fa2
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LT3517ELECTRICAL CHARACTERISTICS The ● denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.CONDITIONSContinuous Operation (Note 3)VCTRL = 2V, VISP = 24V, VC = 1VVCTRL = 2V, VISP = 0V, VC = 1VVCTRL = 100mV, VISP = 24V, VC = 1V2V < VISP < 45VPWM > 1.5V, VC = 0VPWM = 0VSHDN = 0VRT = 16.7kRT = 4.03kRT = 91.5kSS = 0.5V, Out of PinVSYNC = 2V1.5RT = 91.5k (250kHz)SYNC = 300kHz Clock Signal, RT = 91.5kRT = 16.7k (1MHz)RT = 4.03k (2.5MHz)ISW = 1AVSW = 45V, PWM = 0VCurrent Out of Pin, VCTRL = 0.1V205501000PWM = 0V, VC = 1VCurrent Out of Pin, VFB = 0.5V●●●PARAMETERMinimum VIN Operating VoltageMaximum VIN Operating VoltageCurrent Sense Voltage (VISP – VISN)10% Scale Current Sense Voltage (VISP – VISN)Current Sense Voltage Line RegulationVIN Supply CurrentMIN3096TYPMAX3UNITSVVmVmVmV %/V10010090.0364.50.11031011.152.7270120.4mAmAμAMHzMHzkHzVμAμAVV%%%%Switching Frequency0.852.2522061.02.52501960RT VoltageSoft-Start Pin CurrentSYNC Pull-Down Current (Into the Pin)SYNC Input LowSYNC Input HighMaximum Duty Cycle9594851.5●979690741.930021002.3Switch Current LimitSwitch VCESATSwitch Leakage CurrentCTRL Input Bias CurrentError Amplifi er TransconductanceVC Output ImpedanceVC Idle Input Bias CurrentFB Pin Input Bias CurrentFB Pin ThresholdISP , ISN Idle Input Bias CurrentISP , ISN Full-Scale Input Bias CurrentSHDN Voltage HighSHDN Voltage LowSHDN Pin Bias CurrentPWM Input High VoltagePWM Input Low VoltagePWM Pin Bias CurrentPWM = 0VISP Tied to ISN, VISP = 24V, VCTRL = 2VAmVμAnAμSkΩ–200.980201.0120201001.04300nAnAVnAμAV1.50.4601.50.460120100VμAVVμA3517fa3
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LT3517ELECTRICAL CHARACTERISTICS The ● denotes the specifi cations which apply over the full operating temperature range, otherwise specifi cations are at TA = 25°C. (Note 2) VIN = 5V, SHDN = 5V, PWM = 5V unless otherwise noted.CONDITIONSMIN1.50.4TGEN = 5VIREF = –100μA3V < VIN < 40VCLOAD = 1nF Between ISP and TGCLOAD = 1nF Between ISP and TGVISP = 24V, TGEN = 5VPWM = 0V200200700.3●PARAMETERTGEN Input High VoltageTGEN Input Low VoltageTGEN Pin Bias CurrentVREF Pin VoltageVREF Pin Voltage Line RegulationGate Turn-On DelayGate Turn-Off DelayTop Gate Drive VGS (VISP – VTG)TYPMAXUNITSVVμAV%/Vnsns VV1001.9622002.040.03Note 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 LT3517E is guaranteed to meet performance specifi cations from 0°C to 125°C junction temperature. Specifi cations over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3517I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3517H is guaranteed over the full –40ºC to 150ºC operating junction temperature range. Operating lifetime is derated at junction temperatures greater than 125ºC.Note 3: Absolute maximum voltage at VIN, SHDN, PWM and TGEN pins is 40V for nonrepetitive 1 second transients and 30V for continuous operation.Note 4: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specifi ed maximum operating junction temperature may impair device reliability.TYPICAL PERFORMANCE CHARACTERISTICS
VISP – VISN Threshold vs VCTRL120
VIN = 5VVISP = 24V100VC = 1VTA = 25°C806040200
0
1.4
1.6
TA = 25°C0
20
4060DUTY CYCLE (%)
80
100
3517 G02
Switch Current Limit vs Duty Cycle2.0
10000OSCILLATOR FREQUENCY (kHz)Oscillator Frequency vs RTTA = 25°CVISP – VISN THRESHOLD (mV)SWITCH CURRENT LIMIT (A)1.5
1.0
1000
0.5
00.20.4
0.60.81.0VCTRL (V)
1.2
100
1
10RT (kΩ)
100
3518 G03
3518 G01
3517fa4
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LT3517TYPICAL PERFORMANCE CHARACTERISTICS
VISP – VISN Threshold vs Temperature104
VIN = 5V103VISP = 24VVC = 1VV102CTRL = 2V101100999897
96
–40–200
20406080100120140160TEMPERATURE (°C)
3517 G04
Switch Current Limit vs Temperature2.5
VIN = 5VOSCILLATOR FREQUENCY (MHz)2.5
Oscillator Frequency vs TemperatureVIN = 5VRT = 6.04kVISP – VISN THRESHOLD (mV)SWITCH CURRENT LIMIT (A)2.02.3
1.52.1
1.01.9
0.51.7
0
–40–200
20406080100120140160TEMPERATURE (°C)
3518 G05
1.5
–40–200
20406080100120140160TEMPERATURE (°C)
3517 G06
VISP – VISN Threshold vs VISP105104VISP – VISN THRESHOLD (mV)103102
VREF (V)10110099989796950
10
3020
VISP (V)
40
50
3517 G07
Reference Voltage vs Temperature2.02
VIN = 5V8
Quiescent Current vs VINTA = 25°CV7C = 0VVIN CURRENT (mA)654321
VCTRL = 2VVIN = 5VTA = 25°CVC = 1V2.01
2.00
1.99
1.98
–40–200
20406080100120140160TEMPERATURE (°C)
3517 G08
0
01020VIN (V)
3040
3517 G09
FB Pin Threshold vs Temperature1.041.03FB PIN THRESHOLD (V)1.021.0140V
1.000.990.98–40–200TG30V
VIN = 5V5VPWM
0V
PMOS Turn-OnPMOS Turn-Off5VPWM
0V
40VTG30V
20406080100120140160TEMPERATURE (°C)3517 G10VISP = 40V
200ns/DIV
3517 G11
VISP = 40V
200ns/DIV
3517 G11
3517fa5
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LT3517PIN FUNCTIONS
SW: Switch Pin. Minimize trace at this pin to reduce EMI.VIN: Input Supply Pin. Must be locally bypassed.SHDN: Shutdown Pin. Tie to 1.5V or higher to enable device or 0.4V or less to disable device. VREF: Reference Output Pin. This pin can supply up to 100μA. RT: Switching Frequency Adjustment Pin. Set switching frequency using a resistor to GND (see Typical Performance Characteristics for values). For SYNC function, choose the resistor to program a frequency 20% slower than the SYNC pulse frequency. Do not leave this pin open.SYNC: Frequency Synchronization Pin. Tie an external clock signal here. RT resistor should be chosen to pro-gram a switching frequency 20% slower than SYNC pulse frequency. Synchronization (power switch turn-on) occurs a fi xed delay after the rising edge of SYNC. Tie the SYNC pin to ground if this feature is not used. SS: Soft-Start Pin. Place a soft-start capacitor here. Leave the pin open if not in use. PWM: Pulse Width Modulated Input Pin. Signal low turns off channel, disables the main switch and makes the TG pin high. Tie the PWM pin to VREF pin or SHDN pin if not used. There is an equivalent 50k resistor from PWM pin to ground internally.CTRL: LED Current Adjustment Pin. Sets voltage across sense resistor between ISP and ISN. Connect directly to VREF for full-scale threshold of 100mV, or use signal values between GND and 1V to modulate LED current. Tie the CTRL pin to the VREF pin if not used. er Output Pin. Stabilize the loop with VC: gm Error Amplifian RC network or compensating C. FB: Voltage Loop Feedback Pin. Works as overvoltage protection for LED drivers. If FB is higher than 1V, the main switch is turned off.TGEN: Top Gate Enable Input Pin. Tie to 1.5V or higher to enable the PMOS driver function. Tie the TGEN pin to ground if TG function is not used. There is an equivalent 40k resistor from TGEN pin to ground internally.ISN: Current Sense (–) Pin. The inverting input to the current sense amplifi er.ISP: Current Sense (+) Pin. The noninverting input to the current sense amplifi er. Also serves as positive rail for TG pin driver. TG: Top Gate Driver Output. An inverted PWM sig-nal drives series PMOS device between VISP and (VISP – 7V). An internal 7V clamp protects the PMOS gate. Leave TG unconnected if not used.Ground: Exposed Pad. Solder paddle directly to ground plane. 3517fa6
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LT3517BLOCK DIAGRAM
CINRSENSEPVINLED ARRAYCFILTISPISNCURRENTSENSEAMPLIFIERTGVISPTGENPWMSWX10VISP – 7VSHDNCTRL1V–++MOSFET DRIVERA1–++A4RMAIN SWITCHDRIVERQQ1POWERSWITCH+A3ERRORAMPLIFIER1.01VFBVC+A2+–S–+–PWMCOMPARATORA8GNDSSRAMPGENERATORVIN2.5MHz TO 250kHzOSCILLATOR100μAVREFSYNC+A51VVIN10μASS1V–++–+A6FREQADJUSTVIN3517 F01Q22VA7–RTFigure 1. Buck Mode LED Driver3517fa7
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LT3517OPERATION
The LT3517 is a constant frequency, current mode regula-tor with an internal power switch. Operation can be best understood by referring to the Block Diagram in Figure 1. At the start of each oscillator cycle, the SR latch is set, which turns on the Q1 power switch. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A4. When this voltage exceeds the level at the negative input of A4, the SR latch is reset, turning off the power switch. The level at the negative input of A4 is set by the error amplifi er A3. A3 has two inputs, one from the voltage feedback loop and the other one from the current loop. Whichever feedback input is lower takes precedence, and forces the converter into either constant-current or constant-voltage mode. The LT3517 is designed to transi-tion cleanly between these two modes of operation. The current sense amplifi er senses the voltage across RSENSE and provides a pre-gain to amplifi er A1. The output of A1 is simply an amplifi ed version of the difference between the voltage across RSENSE and the lower of VCTRL/10 or 100mV. In this manner, the error amplifi er sets the correct peak switch current level to regulate the current through RSENSE. If the error amplifi er’s output increases, more current is delivered to the output; if it decreases, less current is delivered. The current regulated in RSENSE can be adjusted by changing the input voltage VCTRL. The current sense amplifi er provides rail-to-rail current sense operation. The FB voltage loop is implemented by the amplifi er A2. When the voltage loop dominates, the error amplifi er and the amplifi er A2 regulate the FB pin to 1.01V (constant-voltage mode).Dimming of the LED array is accomplished by pulsing the LED current using the PWM pin. When the PWM pin is low, switching is disabled and the error amplifi er is turned off so that it does not drive the VC pin. Also, all internal loads on the VC pin are disabled so that the charge state of the VC pin will be saved on the external compensation capacitor. This feature reduces transient recovery time. When the PWM input again transitions high, the demand current for the switch returns to the value just before PWM last transitioned low. To further reduce transient recovery time, an external PMOS is used to disconnect the LED array current loop when PWM is low, stopping CFILT from discharging.APPLICATIONS INFORMATION
Dimming ControlThere are two methods to control the current source for dimming using the LT3517. The fi rst method uses the PWM pin to modulate the current source between zero and full current to achieve a precisely programmed aver-age current. To make this method of current control more accurate, the switch demand current is stored on the VC node during the quiescent phase. This feature minimizes recovery time when the PWM signal goes high. To further improve the recovery time, a disconnect switch is used in the LED current path to prevent the output capacitor from discharging in the PWM signal low phase. The minimum PWM on or off time will depend on the choice of operating frequency through RT input pin or SYNC pin. When using the SYNC function, the SYNC and PWM signals must have the aligned rising edges to achieve the optimized high PWM dimming ratio. For best current accuracy, the minimum PWM low or high time should be at least four switching cycles (2μs for fSW = 2MHz). Maximum PWM period is determined by the system and is unlikely to be longer than 12ms. The maximum PWM dimming ratio (PWMRATIO) can be calculated from the maximum PWM period (tMAX) and the minimum PWM pulse width (tMIN) as follows: PWMRATIO=
tMAXtMIN (1)Example:tMAX = 10ms, tMIN = 2μs (fSW = 2MHz)PWMRATIO = 10ms/2μs = 5000:1The second method of dimming control uses the CTRL pin to linearly adjust the current sense threshold during the PWM high state. When the CTRL pin voltage is less 3517fa8
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LT3517APPLICATIONS INFORMATION
than 1V, the LED current is:ILED= VCTRL10•RSENSE
(2) For a buck or a buck-boost confi guration, the output voltage is typically level-shifted to a signal with respect to GND as illustrated in the Figure 4. The output can be expressed as: VOUT=
R1
•1.01V+VBE(Q2)R2
+VOUTWhen VCTRL is higher than 1V, the LED current is clamped to be:ILED=
100mVRSENSE (3)(5) RSENSELEDARRAY
R1The LED current programming feature possibly increases total dimming range by a factor of ten. The CTRL pin should not be left open (tie to VREF if not used). The CTRL pin can also be used in conjunction with a PTC thermistor to provide overtemperature protection for the LED load.VREF
45.3k2V49.9kCTRLLT3517FBQ2–R23517 F04Figure 4Inductor SelectionThe inductor used with the LT3517 should have a satura-tion current rating of 2A or greater. For buck mode LED drivers, the inductor value should be chosen to give a ripple current “ΔI” of ~30% to 40% of the LED current. In the buck mode, the inductor value can be estimated using the formula:L(µH)=
DBUCK•tSW(µs)•(VIN–VLED)ΔIVLEDVIN
(6)5kPTC3517 F02Figure 2Setting Output VoltageFor a boost application, the output voltage can be set by selecting the values of R1 and R2 (see Figure 3) according to the following equation:⎛R1⎞
VOUT=⎜+1⎟•1.01V
⎝R2⎠ VOUTLT3517FBR23517 F03DBUCK= (4)VLED is the voltage across the LED string, VIN is the input voltage to the converter, and tSW is the switching period. In the boost confi guration, the inductor can be estimated using the formula:L(µH)=
DBOOST•tSW(µs)•VIN
ΔIVLED–VINVLED
(7)R1DBOOST=
Figure 33517fa9
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LT3517APPLICATIONS INFORMATION
Table 1 provides some recommended inductor vendors. Table 1. Inductor ManufacturersVENDORSumidaTokoCooperVishayPHONE(408) 321-9660(408) 432-8281(561) 998-4100(402) 563-6866WEBwww.sumida.comwww.toko.comwww.cooperet.comwww.vishay.comInput Capacitor SelectionFor proper operation, it is necessary to place a bypass capacitor to GND close to the VIN pin of the LT3517. A 1μF or greater capacitor with low ESR should be used. A ceramic capacitor is usually the best choice.In the buck mode confi guration, the capacitor at the input to the power converter has large pulsed currents due to the current returned though the Schottky diode when the switch is off. For best reliability, this capacitor should have low ESR and ESL and have an adequate ripple current rating. The RMS input current is: IIN(RMS)=ILED•(1–D)•D
(8)To achieve the same LED ripple current, the required fi lter capacitor value is larger in the boost and buck-boost mode applications than that in the buck mode applications. For LED buck mode applications, a 1μF ceramic capacitor is usually suffi cient. For the LED boost and buck-boost mode applications, a 2.2μF ceramic capacitor is usually suffi cient. Very high performance PWM dimming appli-cations may require a larger capacitor value to support the LED voltage during PWM transitions.Use only ceramic capacitors with X7R, X5R or better dielec-tric as they are best for temperature and DC bias stability of the capacitor value. All ceramic capacitors exhibit loss of capacitance value with increasing DC voltage bias, so it may be necessary to choose a higher value capacitor to get the required capacitance at the operation voltage. Always check that the voltage rating of the capacitor is suffi cient. Table 2 shows some recommended capacitor vendors.Table 2. Ceramic Capacitor ManufacturersVENDORTaiyo YudenAVXMurataTDKPHONE(408) 573-4150(843) 448-9411(770) 436-1300(847) 803-6100WEBwww.t-yuden.comwww.avxcorp.comwww.murata.comwww.tdk.comwhere D is the switch duty cycle. A 2.2μF ceramic type capacitor is usually suffi cient.Output Capacitor SelectionThe selection of output capacitor depends on the load and converter confi guration, i.e., step-up or step-down. For LED applications, the equivalent resistance of the LED is typically low, and the output fi lter capacitor should be sized to attenuate the current ripple. Loop CompensationThe LT3517 uses an internal transconductance error amplifi er whose VC output compensates the control loop. The external inductor, output capacitor, and the compen-sation resistor and capacitor determine the loop stability. The inductor and output capacitor are chosen based on 3517fa10
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LT3517APPLICATIONS INFORMATION
performance, size and cost. The compensation resistor and capacitor at VC are selected to optimize control loop stability. For typical LED applications, a 10nF compensa-tion capacitor at VC is adequate and a series resistor is not required. A compensation resistor may be used to increase the slew rate on the VC pin to maintain tighter regulation of LED current during fast transients on VIN or CTRL.Diode SelectionThe Schottky diode conducts current during the interval when the switch is turned off. Select a diode rated for the maximum SW voltage. If using the PWM feature for dimming, it is important to consider diode leakage, which increases with the temperature, from the output during the PWM low interval. Therefore, choose the Schottky diode with suffi ciently low leakage current. Table 3 has some recommended component vendors.Table 3. Schottky DiodesPART NUMBERDiodes Inc.DFLS160ZetexZLLS1000TAInternational Rectifi er10MQ060N601.5401601VR (V)IAVE (A)Board LayoutThe high speed operation of the LT3517 demands careful attention to board layout and component placement. The Exposed Pad of the package is the only GND terminal of the IC and is also important for thermal management of the IC. It is crucial to achieve a good electrical and thermal contact between the Exposed Pad and the ground plane of the board. To reduce electromagnetic interference (EMI), it is important to minimize the area of the SW node. Use a GND plane under SW and minimize the length of traces in the high frequency switching path between SW and GND through the diode and the capacitors. Since there is a small DC input bias current to the ISN and ISP inputs, resistance in series with these inputs should be minimized and matched, otherwise there will be an offset. Finally, the bypass capacitor on the VIN supply to the LT3517 should be placed as close as possible to the VIN terminal of the device.Soft-StartFor many applications, it is necessary to minimize the inrush current at start-up. The built-in soft-start circuit signifi cantly reduces the start-up current spike and output voltage overshoot. A typical value for the soft-start capacitor is 0.1μF.3517fa11
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LT3517APPLICATIONS INFORMATION
Switching FrequencyThere are two methods to set the switching frequency of LT3517. Both methods require a resistor connected at RT pin. Do not leave the RT pin open. Also, do not load this pin with a capacitor. A resistor must always be connected for proper operation. One way to set the frequency is simply connecting an external resistor between the RT pin and GND. See Table 4 below or see the Oscillator Frequency vs RT graph in the Typical Performance Characteristics for resis-tor values and corresponding switching frequencies.Table 4. Switching Frequency vs RTSwitching Frequency (kHz)2505001000150020002500RT ( kΩ )90.939.216.99.536.044.02In general, a lower switching frequency should be used where either very high or very low switching duty cycle operation is required, or high effi ciency is desired. Selection of a higher switching frequency will allow use of smaller value external components and yield a smaller solution size and profi le.Thermal ConsiderationsThe LT3517 is rated to a maximum input voltage of 30V for continuous operation, and 40V for nonrepetitive one second transients. Careful attention must be paid to the internal power dissipation of the LT3517 at higher input voltages to ensure that the maximum junction temperature is not exceeded. This junction limit is especially important when operating at high ambient temperatures. The Exposed Pad on the bottom of the package must be soldered to a ground plane. This ground should then be connected to an internal copper ground plane with thermal vias placed directly under the package to spread out the heat dissipated by the LT3517.The other way is to make the LT3517 synchronize with an external clock via SYNC pin. For proper operation, a resistor should be connected at the RT pin and be able to generate a switching frequency 20% lower than the external clock when external clock is absent. 3517fa12
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LT3517TYPICAL APPLICATIONS
Buck Mode 1A LED DriverRSENSE100mΩC22.2μFISPC12.2μFVREFPWMVINSHDNLT3517CTRLPWMFBSSIL1A/DIV
1μs/DIV
RT16.9k1MHzC50.1μF3517 TA02a3517 TA02b
PVIN24VM11AC310μFL115μHD1SWILED1A/DIVPWM5V/DIV
2000:1 PWM Dimming at 120HzVIN3.3V
ISNTGRTSYNCTGENVREFVCGNDC40.1μFC1: KEMET C0805C225K4RACC2: MURATA GRM31MR71E225KA93C3: MURATA GRM32DR71E106KA12BC4, C5: MURATA GRM21BR71H104KA01BD1: DIODES DFLS160L1: TOKO B992AS-150MLEDS: LUXEON K2 (WHITE)M1: ZETEX ZXMP6A13FTA
PVIN = 24VfOSC = 1MHzILED = 1A
350mA, 5V to 12V Boost Converter with Accurate Input Current LimitRSENSEL1100mΩ6.8μHD1VIN5V
C12.2μFVOUT12V350mAR1549kEFFICIENCY (%)Effi ciency90
ISPTGISNVINCTRLPWMSHDNSHDNTGENVREFVCR310kC410nFC1: KEMET C0805C225K4RACC2: KEMET C1206C106K4RAC
C3: MURATA GRM21BR71H104KA01BC4: MURATA GCM033R71A103KA03D1: ZETEX ZLLS1000TAL1: TOKO B992AS-6R8N
GNDLT3517SWFBC210μFR249.9k80
70
SYNCRTSSC30.1μFRT6.04k2MHz3517 TA03a60
50
50100150
200250ILOAD(mA)
300350
3517 TA03b
3517fa13
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LT3517TYPICAL APPLICATIONS
Buck-Boost Mode LED DriverL16.8μHD1R13.92MFBR2124kPWMPWMTGENC12.2μFVREFCTRLISNTGM1C24.7μFLT3517ISPRSENSE330mΩC50.22μFVIN
8V TO 16V
SHDNVINSW300mASYNCVCRT6.04k2MHzRTSSGNDC40.1μFC30.1μF3517 TA04aC1: KEMET C0806C225K4RACC2: KEMET C1206C475K3RAC
C3, C4: MURATA GRM21BR71H104KA01BC5: MURATA GRM21BR71H224KA01BD1: DIODE DFLS160L1: TOKO B992AS-6R8NLEDS: LUXEON I (WHITE)M1: ZETEX ZXMP6A13FTA
5000:1 PWM Dimming at 100Hz90
PWM5V/DIV
EFFICIENCY (%)ILED
200mA/DIV
8070605040
500ns/DIV
VIN = 12VfOSC = 2MHzILED = 300mA
3517 TA04b
Effi ciencyVIN = 12VCTRL = VREFIL1A/DIV
3020
020
406080PWM DUTY CYCLE (%)
100
3517 TA04c
3517fa14
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LT3517TYPICAL APPLICATIONS
Low Side Current Sensing Load Dump Protected Buck-Boost Mode LED DriverL16.8μHD1R13.92MFBR2124kPWMPWMTGENVREFC12.2μFCTRLSYNCVCR310kC433nFRT6.04k2MHzRTSSISNTGGNDM1300mALT3517ISPRSENSE330mΩC24.7μFD1: DIODES DFLS160L1: TOKO B992AS-6R8N
C1: KEMET C0806C225K4RACC2: KEMET C1206C475K3RAC
C3: MURATA GRM21BR71H104KA01BC4: MURATA GRM219R71H333KAQ01BC5: MURATA GRM21BR71H224KA01BM1: ZETEX ZXMP6A13FTALEDs: LUXEON I (WHITE)
C50.22μFVIN
8V TO 16V
SHDNVINSWC30.1μF3517 TA05a5000:1 PWM Dimming at 100Hz90
PWM5V/DIV
EFFICIENCY (%)ILED
200mA/DIV
8070605040
500ns/DIV
VIN = 12VfOSC = 2MHzILED = 300mA
3517 TA05b
Effi ciencyVIN = 12VCTRL = VREFIL1A/DIV
3020
020
406080PWM DUTY CYCLE (%)
100
3517 TA05c
Load Dump ResponseVISP 10V/DIV
VISP15V TO 40VVIN 15V TO 40VVISP REF GNDVIN REF GND
ILED 200mA/DIVIL 1A/DIV
5ms/DIV
VIN Raises From 15V to 40V in 5ms.
3517 TA05d
VIN 10V/DIV
3517fa15
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LT3517TYPICAL APPLICATIONS
Boost 100mA LED Driver with LED Open ProtectionL122μHVIN
8V TO 16V
SHDNVINPWMTGENVREFC12.2μFCTRLSYNCVCRT16.9k1MHzD1PWMSWISPRSENSE1ΩLT3517ISNTGFBR11M100mAC30.1μFR230.1kLED103517 TA06aM1LED1LED2C22.2μFC1, C2: KEMET C1206C225K2RACC3, C4: MURATA GRM21BR71H104KA01BD1: DIODES DFLS160L1: COILCRAFT DS3316P-223LEDS: CREE XLAMP 7090M1: ZETEX ZXMP6A13FTA RTSSGNDC40.1μF3000:1 PWM Dimming at 100Hz90
PWM5V/DIVILED
100mA/DIVIL
500mA/DIV
1μs/DIV
VIN = 12VfOSC = 1MHzILED = 100mA
3517 TA06b
Effi ciency8070EFFICIENCY (%)6050403020
VIN = 12VCTRL = VREF020
406080PWM DUTY CYCLE (%)
100
3517 TA06c
3517fa16
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LT3517TYPICAL APPLICATION
5.5V SEPIC Converter with Short-Circuit ProtectionEffi ciencyVIN3VC22.2μFVINCTRLSYNCSHDNSHDNTGENVREFVCR310kC410nFC1: KEMET C0805C225K4RACC2, C5: KEMET C1206C106K4RACC3: MURATA GRM21BR71H104KA01BC4: MURATA GCM033R71A103KA03D1: ZETEX ZLLS1000TAL1, L2: TOKO B992AS-4R3N
GNDSSC30.1μFRT6.04k2MHz3517 TA07aL14.3μHC510μFD1RSENSE0.22ΩC210μFVOUT5.5V350mAR1221kEFFICIENCY (%)R249.9k90807060504030
L24.3μHPWMSWFBTGLT3517ISPISNRT050100
150200ILOAD (mA)
250300350
3517 TA07b
3517fa17
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LT3517PACKAGE DESCRIPTION
UF Package16-Lead Plastic QFN (4mm × 4mm)(Reference LTC DWG # 05-08-1692)0.72p0.054.35p 0.052.15p 0.052.90p 0.05(4 SIDES)NOTE:
1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (W2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON A5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
PACKAGE OUTLINE0.30p0.050.65 BSCRECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
BOTTOM VIEW—EXPOSED PAD
4.00p 0.10(4 SIDES)PIN 1TOP MARK(NOTE 6)2.15p 0.10(4-SIDES)0.75p 0.05R = 0.115TYPPIN 1 NOTCH R = 0.20 TYPOR 0.35 s 45o CHAMFER
15160.55p 0.2012(UF16) QFN 10040.200 REF0.00 – 0.050.30p 0.050.65 BSC3517fa18
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LT3517PACKAGE DESCRIPTION
FE Package16-Lead Plastic TSSOP (4.4mm)(Reference LTC DWG # 05-08-1663)Exposed Pad Variation BA2.74(.108) 4.90 – 5.10*(.193 – .201)2.74(.108)1615141312111096.60p0.104.50p0.10SEE NOTE 42.74(.108)0.45p0.051.05p0.100.65 BSC2.746.40(.108)(.252)BSCRECOMMENDED SOLDER PAD LAYOUT
123456781.10(.0433)MAX0° – 8° 4.30 – 4.50*(.169 – .177)0.25REF0.09 – 0.20(.0035 – .0079)0.50 – 0.75(.020 – .030)0.65(.0256)BSCNOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
MILLIMETERS
2. DIMENSIONS ARE IN
(INCHES)3. DRAWING NOT TO SCALE
0.195 – 0.30(.0077 – .0118)TYP
0.05 – 0.15(.002 – .006)FE16 (BA) TSSOP 02044. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006\") PER SIDE
3517faInformation 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 representa-tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.19
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LT3517RELATED PARTS
PART NUMBERLT1618LT3003LT3474LT3475LT3476LT3477LT3478/LT3478-1LT3479LT3486LT3496LT3518LT3590LT3595LTC®3783DESCRIPTIONConstant Current, 1.4MHz, 1.5A Boost Converter3-Channel LED Ballaster with PWM Dimming36V, 1A (ILED), 2MHz, Step-Down LED DriverDual 1.5A (ILED), 36V 2MHz Step-Down LED DriverQuad Output 1.5A, 36V, 2MHz High Current LED Driver with 1,000:1 Dimming3A, 42V, 3MHz Boost, Buck-Boost, Buck LED DriverCOMMENTSVIN: 5V to 18V, VOUT(MAX) = 36V, Dimming = Analog/PWM, ISD < 1μA, MSOP10 PackageVIN: 3V to 48V, Dimming = 3,000:1 True Color PWM, ISD < 5μA, MSOP10 PackageVIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 400:1 True Color PWM, ISD < 1μA, TSSOP16E PackageVIN: 4V to 36V, VOUT(MAX) = 13.5V, Dimming = 3,000:1 True Color PWM, ISD < 1μA, TSSOP20E PackageVIN: 2.8V to 16V, VOUT(MAX) = 36V, Dimming = 1,000:1 True Color PWM, ISD < 10μA, 5mm × 7mm QFN PackageVIN: 2.5V to 25V, VOUT(MAX) = 40V, Dimming = Analog/PWM, ISD < 1μA, QFN, TSSOP20E Packages4.5A, 42V, 2.5MHz High Current LED Driver with 3,000:1 VIN: 2.8V to 36V, VOUT(MAX) = 42V, Dimming = 3,000:1 True Color PWM, DimmingISD < 3μA, TSSOP16E Packages3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current ProtectionDual 1.3A, 2MHz High Current LED DriverTriple Output LED DriverFull-Featured LED Driver with 2.3A Switch Current48V Buck Mode 50mA LED Driver16 Channel Buck LED Driver ModeHigh Current LED ControllerVIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD < 1μA, DFN and TSSOP Packages VIN: 2.5V to 24V, VOUT(MAX) = 36V, Dimming = 1,000:1 True Color PWM, ISD < 1μA, 5mm × 3mm DFN, TSSOP16EVIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 3,000:1 True Color PWM, ISD < 10μA, 4mm × 5mm QFN PackageVIN: 3V to 40V, VOUT(MAX) = 45V, Dimming = 3,000:1 True Color PWM, ISD < 10μA, 4mm × 4mm QFN PackageVIN: 4.5V to 55V, Drives Up to 10 LEDs, 200:1 Dimming, ISO = 15mA, 2mm × 2mm DFN SC70VIN: 4.5V to 45V, Drives Up to 160 LEDs, 5,000:1 Dimming, 5mm × 9mm QFNVIN: 3V to 36V, VOUT(MAX) = Ext FET, Dimming = 3,000:1 True Color PWM, ISD < 20μA, 5mm × 4mm QFN10, TSSOP16E Packages3517fa20
Linear Technology CorporationLT 0308 REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com© LINEAR TECHNOLOGY CORPORATION 2007
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