LT3791 60V 4-Switch Synchronous Buck-Boost LED Driver Controller Description Features n n n n n n n n n n n n 4-Switch Single Inductor Architecture Allows VIN Above, Below or Equal to VOUT Synchronous Switching: Up to 98.5% Efficiency Wide VIN Range: 4.
LT3791 Absolute Maximum Ratings (Note 1) Pin Configuration Supply Voltages Input Supply (VIN)......................................................60V SW1, SW2.......................................................–1V to 60V OPENLED, SHORTLED................................................15V EN/UVLO, IVINP, IVINN, ISP, ISN...............................60V INTVCC, (BST1-SW1), (BST2-SW2)..............................6V TEST2, SYNC, RT, CTRL, OVLO, PWM........................
LT3791 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, VEN/UVLO = 12V unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX 1.16 1.2 1.
LT3791 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VIN = 12V, VEN/UVLO = 12V unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX 1.078 1.1 1.122 5 10 15 mV 380 400 450 mV 1.1 2.0 kΩ SHORTLED Pin Output Impedance 1.1 2.0 kΩ SS Latch-Off Threshold 1.75 V SS Reset Threshold 0.
LT3791 Typical Performance Characteristics INTVCC Dropout Voltage vs Current, Temperature VIN-VINTVCC (V) 90 5.20 TA = 150°C TA = 25°C TA = –50°C 2.0 INTVCC Current Limit vs Temperature INTVCC Voltage vs Temperature 80 5.15 INTVCC CURRENT LIMIT (mA) 2.5 TA = 25°C, unless otherwise noted. 5.10 INTVCC (V) 1.5 1.0 5.05 VIN = 60V 5.00 VIN = 12V 4.95 4.90 0.5 4.
LT3791 Typical Performance Characteristics 120 V(ISP-ISN) Threshold vs VFB TA = 25°C, unless otherwise noted. ISMON Voltage vs Temperature 1.04 VIN = 12V V(ISP-ISN) = 100mV 1.03 100 ISMON Voltage vs V(ISP-ISN) 1.0 0.9 0.8 60 40 0.7 1.01 VISMON (V) 80 VISMON (V) V(ISP-ISN) (mV) 1.02 1.00 0.99 1.19 1.18 1.20 1.21 VFB (V) 1.22 0.1 0.96 –50 –25 1.23 0 V(IVINP-IVINN) Threshold vs VIVINP IVINMON Voltage vs Temperature 54 51.5 1.03 51.0 1.02 50.5 1.
LT3791 Typical Performance Characteristics OPENLED Threshold vs Temperature OVLO Threshold vs Temperature 1.125 FALLING 1.100 1.075 1.050 1.025 1.000 –50 –25 0 16 3.2 14 3.1 12 RISING 3.0 2.9 FALLING 2.7 4 2.6 2 0 Supply Current vs Input Voltage 7 IQ (mA) 1.5 1.0 TA = 150°C TA = 25°C TA = –50°C 0 0 10 20 30 VIN (V) 40 50 60 VEN/ULO = 1V 1.28 6 5 4 3 2 TG1, TG2 MINIMUM ON-TIME (ns) SWITCHING FREQUENCY (kHz) 600 RT = 59.
LT3791 Typical Performance Characteristics BG1, BG2 Driver On-Resistance vs Temperature 4.5 3.8 4.0 RISING 3.7 3.6 3.5 3.4 FALLING 3.3 3.2 4.0 3.5 PULL-UP 3.5 3.0 2.5 2.0 PULL-DOWN 1.5 1.0 0 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 1.0 14 1.6 12 1.4 25 50 75 100 125 150 TEMPERATURE (°C) V(SNSP-SNSN) Buck Threshold vs VC PULL-UP 40 BG2 1.0 VC (V) 60 V(SNSP-SNSN) = 0V 1.2 PULL-DOWN 0 3791 G30 V(SNSP-SNSN) (mV) PWMOUT RESISTANCE (Ω) 1.
LT3791 Pin Functions CTRL (Pin 1): Current Sense Threshold Adjustment Pin for Analog Dimming. Regulating threshold V(ISP-ISN) is 1/10th of (VCTRL – 200mV). CTRL linear range is from 200mV to 1.1V. For VCTRL > 1.3V, the current sense threshold is constant at the full-scale value of 100mV. For 1.1V < VCTRL < 1.3V, the dependence of the current sense threshold upon VCTRL transitions from a linear function to a constant value, reaching 98% of full scale by VCTRL = 1.2V.
LT3791 Pin Functions NC (Pin 23): No Connect Pin. Leave this pin floating. TG2 (Pin 24): Top Gate Drive. Drives the top N-channel MOSFET with a voltage equal to INTVCC superimposed on the switch node voltage SW2. ISP (Pin 25): Connection Point for the Positive Terminal of the Output Current Feedback Resistor. ISN (Pin 26): Connection Point for the Negative Terminal of the Output Current Feedback Resistor. SNSP (Pin 27): The Positive Input to the Current Sense Comparator.
LT3791 Block Diagram 26 – + A = 10 7 8 9 11 IVINP + A2 10 ISN ISP A1 A = 10 A = 20 12 6 13 VREF VIN IVINN INTVCC – 25 A = 24 REGS SHDN_INT ISMON ISMON_INT TSD IVINMON_INT BST1 + IVINMON A13 A3 EN/UVLO A4 3µA 1.2V 14 SW1 – – 15 TG1 SHDN_INT SHDN_INT SS_RESET SS LATCH PWM + 16 BUCK LOGIC INTVCC A14 BG1 18 PGND OSC 35 34 33 A15 INTVCC RT BOOST LOGIC SYNC CLKOUT SW2 + SLOPE_COMP_BUCK A16 + SNSP – SNSN A10 0.
LT3791 Operation The LT3791 is a current mode controller that provides an output voltage above, equal to or below the input voltage. The LTC proprietary topology and control architecture uses a current sensing resistor in buck or boost operation. The sensed inductor current is controlled by the voltage on the VC pin, which is the output of the feedback amplifiers A11 and A12.
LT3791 Operation Buck Region (VIN > VOUT) where D(BUCK-BOOST) is the duty cycle of the buck-boost switch range: Switch M4 is always on and switch M3 is always off during this mode. At the start of every cycle, synchronous switch M2 is turned on first. Inductor current is sensed when synchronous switch M2 is turned on.
LT3791 Operation Boost Region (VIN < VOUT) Low Current Operation Switch M1 is always on and synchronous switch M2 is always off in boost operation. Every cycle switch M3 is turned on first. Inductor current is sensed when synchronous switch M3 is turned on. After the sensed inductor current exceeds the reference voltage which is proportional to VC, switch M3 turns off and synchronous switch M4 is turned on for the remainder of the cycle.
LT3791 Applications Information The Typical Application on the front page is a basic LT3791 application circuit. External component selection is driven by the load requirement, and begins with the selection of RSENSE and the inductor value. Next, the power MOSFETs are selected. Finally, CIN and COUT are selected. This circuit can operate up to an input voltage of 60V.
LT3791 Applications Information where ΔIL is peak-to-peak inductor ripple current. In buck operation, the maximum average load current is: 200 180 ∆IL/ISENSE(MAX) (%) 160 47.5mV ∆IL IOUT(MAX _ BUCK) = + 2 RSENSE 140 120 100 BOOST ∆IL/ ISENSE(MAX) LIMIT The maximum current sensing RSENSE value for the boost operation is: 80 60 40 BUCK ∆IL/ ISENSE(MAX) LIMIT 20 0 50 55 60 65 70 75 80 85 90 95 100 BG1, BG2 DUTY CYCLE (%) 3791 F07 Figure 7.
LT3791 Applications Information The formula has a maximum at VIN = 2VOUT. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to derate the capacitor. In boost operation, the discontinuous current shifts from the input to the output, so COUT must be capable of reducing the output voltage ripple.
LT3791 Applications Information Programming LED Current The LED current is programmed by placing an appropriate value current sense resistor, RLED, in series with the LED string. The voltage drop across RLED is (Kelvin) sensed by the ISP and ISN pins. The CTRL pin should be tied to a voltage higher than 1.2V to get the full-scale 100mV (typical) threshold across the sense resistor. The CTRL pin can also be used to dim the LED current, although relative accuracy decreases with the decreasing sense threshold.
LT3791 Applications Information For loop stability a lowpass RC filter is needed. For most applications, a 50Ω resistor and 470nF capacitor is sufficient. Table 3 RIN (mΩ) ILIMIT (A) 20 2.5 15 3.3 12 4.2 10 5.0 6 8.3 5 10.0 4 12.5 3 16.7 2 25 The output overvoltage threshold can be set by selecting the values of R5 and R6 (see Figure 9) according to the following equation: VOUT(OVP) = 1.25 • R5+R6 R6 Make sure the expected VFB during normal operation does not exceed 1.
LT3791 Applications Information SHORTLED Pin The LT3791 provides an open-drain status pin, SHORTLED, which pulls low when the FB pin is below 400mV. The only time the FB pin will be below 400mV is during start-up or if the LEDs are shorted. During start-up the LT3791 ignores the voltage on the FB pin until the soft-start capacitor reaches 1.75V.
LT3791 Applications Information Power MOSFET Selections and Efficiency Considerations The LT3791 requires four external N-channel power MOSFETs, two for the top switches (switch M1 and M4, shown in Figure 1) and two for the bottom switches (switch M2 and M3 shown in Figure 1). Important parameters for the power MOSFETs are the breakdown voltage, VBR(DSS), threshold voltage, VGS(TH), on-resistance, RDS(ON), reverse transfer capacitance, CRSS, and maximum current, IDS(MAX).
LT3791 Applications Information From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following formula: TJ = TA + P • RTH(JA) The RTH(JA) to be used in the equation normally includes the RTH(JC) for the device plus the thermal resistance from the case to the ambient temperature (RTH(JC)). This value of TJ can then be compared to the original, assumed value used in the iterative calculation process. ρT NORMALIZED ON-RESISTANCE (Ω) 2.0 1.5 1.0 0.
LT3791 Applications Information Top Gate (TG) MOSFET Driver Supply (C1, D1, C2, D2) The external bootstrap capacitors C1 and C2 connected to the BST1 and BST2 pins supply the gate drive voltage for the topside MOSFET switches M1 and M4. When the top MOSFET switch M1 turns on, the switch node SW1 rises to VIN and the BST1 pin rises to approximately VIN + INTVCC. When the bottom MOSFET switch M2 turns on, the switch node SW1 drops low and the bootstrap capacitor C1 is charged through D1 from INTVCC.
LT3791 Applications Information PC Board Layout Checklist n The basic PC board layout requires a dedicated ground plane layer. Also, for high current, a multilayer board provides heat sinking for power components. n n n n n n n n The PGND ground plane layer should not have any traces and it should be as close as possible to the layer with power MOSFETs. Place CIN, switch M1, switch M2 and D1 in one compact area. Place COUT, switch M3, switch M4 and D2 in one compact area.
LT3791 Typical Applications 98% Efficient 50W (25V 2A) Buck-Boost LED Driver VIN 4.7V TO 58V RIN 0.003Ω VIN C3 R7 1µF 50Ω IVINN TG1 R3 1M R9 200k R4 54.9k 100Hz SYNC TWO SIGNALS 300kHz C8 0.1µF R11 1M R10 200k LT3791 L1 10µH M2 M3 R6 44.2k COUT 4.7µF 50V ×4 RLED 0.05Ω RSENSE 0.
LT3791 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 38-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1772 Rev C) Exposed Pad Variation AA 4.75 REF 38 9.60 – 9.80* (.378 – .386) 4.75 REF (.187) 20 6.60 ±0.10 4.50 REF 2.74 REF SEE NOTE 4 6.40 2.74 REF (.252) (.108) BSC 0.315 ±0.05 1.05 ±0.10 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .
LT3791 Revision History REV DATE DESCRIPTION A 08/12 Clarified Features and Description PAGE NUMBER Clarified graph labels/titles Clarified Pin Functions 1 8 9,10 Clarified buck-boost function 13 Clarified programming output for overvoltage or open led/overvoltage threshold 19 Clarified Typical Application 25, 28 3791fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use.
LT3791 Typical Application 98.5% Efficient 100W (33.3V 3A) Buck-Boost LED Driver RIN 0.003Ω VIN 15V TO 58V R7 50Ω R2 15.8k D1 IVINN BST1 TG1 IVINP BG1 LT3791 R10 200k M2 CVCC 4.7µF D2 C1 0.1µF L1 10µH C2 0.1µF M4 M3 COUT 4.7µF 50V ×5 R5 1M R6 34.2k SNSP RLED 0.033Ω RSENSE 0.004Ω SHORTLED OPENLED 3A, 100W LED POWER SNSN PGND PWM IVINMON ISMON CLKOUT BG2 SW2 TG2 FB VREF CTRL C8 0.1µF M1 SWI EN/UVLO OVLO INTVCC R9 200k INTVCC BST2 C7 470nF R1 499k R3 28k VIN C3 1µF CIN 2.
