LT3758/LT3758A High Input Voltage, Boost, Flyback, SEPIC and Inverting Controller Description Features n n n n n n n n n n Wide Input Voltage Range: 5.5V to 100V Positive or Negative Output Voltage Programming with a Single Feedback Pin Current Mode Control Provides Excellent Transient Response Programmable Operating Frequency (100kHz to 1MHz) with One External Resistor Synchronizable to an External Clock Low Shutdown Current < 1µA Internal 7.
LT3758/LT3758A Absolute Maximum Ratings (Note 1) VIN, SHDN/UVLO (Note 7).......................................100V INTVCC.....................................................VIN + 0.3V, 20V GATE......................................................... INTVCC + 0.3V SYNC...........................................................................8V VC, SS..........................................................................3V RT............................................................................
LT3758/LT3758A Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN VIN Operating Range TYP 5.5 MAX UNITS 100 V VIN Shutdown IQ SHDN/UVLO = 0V SHDN/UVLO = 1.15V 0.1 1 6 µA µA VIN Operating IQ VC = 0.3V, RT = 41.2k 1.75 2.2 mA VIN Operating IQ with Internal LDO Disabled VC = 0.3V, RT = 41.
LT3758/LT3758A Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN INTVCC Current in Shutdown SHDN/UVLO = 0V, INTVCC = 8V TYP MAX 16 INTVCC Voltage to Bypass Internal LDO UNITS µA 7.5 V 1.27 V 0.
LT3758/LT3758A Typical Performance Characteristics –790 VIN = 100V 1600 VIN = 24V 1595 VIN = 8V 1590 VIN = INTVCC = 5.5V SHDN/UVLO = 1.33V 1585 1580 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) REGULATED FEEDBACK VOLTAGE (mV) REGULATED FEEDBACK VOLTAGE (V) 1605 Positive Feedback Voltage vs Temperature, VIN TA = 25°C, unless otherwise noted. Negative Feedback Voltage vs Temperature, VIN VIN = INTVCC = 5.5V SHDN/UVLO = 1.
LT3758/LT3758A Typical Performance Characteristics Switching Frequency vs Temperature SENSE Current Limit Threshold vs Temperature 120 RT = 41.2K 320 115 SENSE THRESHOLD (mV) SWITCHING FREQUENCY (kHz) 330 TA = 25°C, unless otherwise noted. 310 300 290 110 105 280 270 –75 –50 –25 100 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 25 50 75 100 125 150 TEMPERATURE (°C) 3758 G08 3758 G07 SENSE Current Limit Threshold vs Duty Cycle SHDN/UVLO Threshold vs Temperature 1.
LT3758/LT3758A Typical Performance Characteristics INTVCC Minimum Output Current vs VIN INTVCC vs Temperature 45 TJ = 150°C 40 INTVCC CURRENT (mA) 7.2 7.1 30 25 INTVCC = 4.7V 20 15 10 0 1 10 VIN (V) 1000 DROPOUT VOLTAGE (mV) INTVCC VOLTAGE (V) 7.20 7.
LT3758/LT3758A Pin Functions VC (Pin 1): Error Amplifier Compensation Pin. Used to stabilize the voltage loop with an external RC network. FBX (Pin 2): Positive and Negative Feedback Pin. Receives the feedback voltage from the external resistor divider across the output. Also modulates the switching frequency during start-up and fault conditions when FBX is close to GND. SS (Pin 3): Soft-Start Pin. This pin modulates compensation pin voltage (VC) clamp.
LT3758/LT3758A Block Diagram CDC L1 D1 VOUT • VIN R4 + R3 CIN L2 • 9 2.5V IS3 IS2 10µA VC 1 CC2 A10 IS1 2µA 2.5V CC1 1.72V –0.88V – + – + A12 G6 VC 2 8 + – 5V UP 4.5V DOWN G5 R S GATE G2 O A6 VISENSE RAMP 1.28V RAMP GENERATOR – +A3 G1 + + – SS M1 – + + A5 – 110mV SENSE 6 GND RSENSE 100kHz-1MHz OSCILLATOR A4 Q1 FREQ PROG RT SYNC 5 CSS 7 11 D3 3 CVCC PWM COMPARATOR 1.2V D2 INTVCC DRIVER – +A7 FREQUENCY FOLDBACK R5 8k CURRENT LIMIT 7.
LT3758/LT3758A Applications Information Main Control Loop The LT3758 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block Diagram in Figure 1. The start of each oscillator cycle sets the SR latch (SR1) and turns on the external power MOSFET switch M1 through driver G2. The switch current flows through the external current sensing resistor RSENSE and generates a voltage proportional to the switch current.
LT3758/LT3758A Applications Information An internal, low dropout (LDO) voltage regulator produces the 7.2V INTVCC supply which powers the gate driver, as shown in Figure 1. The LT3758 contains an undervoltage lockout comparator A8 and an overvoltage lockout comparator A9 for the INTVCC supply. The INTVCC undervoltage (UV) threshold is 4.5V (typical), with 0.5V hysteresis, to ensure that the MOSFETs have sufficient gate drive voltage before turning on.
LT3758/LT3758A Applications Information As illustrated in Figure 2, a trade-off between the operating frequency and the size of the power MOSFET may be needed in order to maintain a reliable IC junction temperature. Prior to lowering the operating frequency, however, be sure to check with power MOSFET manufacturers for their most recent low QG, low RDS(ON) devices. Power MOSFET manufacturing technologies are continually improving, with newer and better performance devices being introduced almost yearly.
LT3758/LT3758A Applications Information Duty Cycle Consideration Soft-Start Switching duty cycle is a key variable defining converter operation. As such, its limits must be considered. Minimum on-time is the smallest time duration that the LT3758 is capable of turning on the power MOSFET. This time is generally about 220ns (typical) (see Minimum On-Time in the Electrical Characteristics table).
LT3758/LT3758A Applications Information FBX Frequency Foldback When VOUT is very low during start-up or a GND fault on the output, the switching regulator must operate at low duty cycles to maintain the power switch current within the current limit range, since the inductor current decay rate is very low during switch off time. The minimum ontime limitation may prevent the switcher from attaining a sufficiently low duty cycle at the programmed switching frequency.
LT3758/LT3758A Applications Information Due to the current limit function of the SENSE pin, RSENSE should be selected to guarantee that the peak current sense voltage VSENSE(PEAK) during steady state normal operation is lower than the SENSE current limit threshold (see the Electrical Characteristics table). Given a 20% margin, VSENSE(PEAK) is set to be 80mV. Then, the maximum switch ripple current percentage can be calculated using the following equation: c= ∆VSENSE 80mV − 0.
LT3758/LT3758A Applications Information The constant c in the preceding equation represents the percentage peak-to-peak ripple current in the inductor, relative to IL(MAX). The inductor ripple current has a direct effect on the choice of the inductor value. Choosing smaller values of ∆IL requires large inductances and reduces the current loop gain (the converter will approach voltage mode).
LT3758/LT3758A Applications Information The RθJA to be used in this equation normally includes the RθJC for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. TJ must not exceed the diode maximum junction temperature rating.
LT3758/LT3758A Applications Information The flyback converter can be designed to operate either in continuous or discontinuous mode. Compared to continuous mode, discontinuous mode has the advantage of smaller transformer inductances and easy loop compensation, and the disadvantage of higher peak-to-average current and lower efficiency. SUGGESTED RCD SNUBBER VIN + CIN – VSN + CSN D NP:NS RSN LS LP ID GATE SENSE + COUT – DSN LT3758 + ISW M + VDS – RSENSE GND 3758 F07 Figure 7.
LT3758/LT3758A Applications Information Flyback Converter: Transformer Design for Discontinuous Mode Operation The transformer design for discontinuous mode of operation is chosen as presented here. According to Figure 8, the minimum D3 (D3MIN) occurs when the the converter has the minimum VIN and the maximum output power (POUT). Choose D3MIN to be equal to or higher than 10% to guarantee the converter is always in discontinuous mode operation.
LT3758/LT3758A Applications Information LLK is the leakage inductance of the primary winding, which is usually specified in the transformer characteristics. LLK can be obtained by measuring the primary inductance with the secondary windings shorted. The snubber capacitor value (CCN) can be determined using the following equation: CCN = VSN ∆VSN • RCN • f where ∆VSN is the voltage ripple across CCN. A reasonable ∆VSN is 5% to 10% of VSN.
LT3758/LT3758A Applications Information Flyback Converter: Input Capacitor Selection The input capacitor in a flyback converter is subject to a large RMS current due to the discontinuous primary current. To prevent large voltage transients, use a low ESR input capacitor sized for the maximum RMS current.
LT3758/LT3758A Applications Information Accepting larger values of ∆IL allows the use of low inductances, but results in higher input current ripple and greater core losses. It is recommended that c falls in the range of 0.2 to 0.6. ISW where cL1 = ∆IL1 IL1(MAX) c 2L2 IL2(RMS) =IL2(MAX) • 1+ 12 ∆ISW = χ • ISW(MAX) where ISW(MAX) t DTS TS cL2 = ∆IL2 IL2 (MAX) 3758 F09 Figure 9.
LT3758/LT3758A Applications Information MOSFET, its junction temperature can be obtained using the following equation: TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA) TJ must not exceed the MOSFET maximum junction temperature rating. It is recommended to measure the MOSFET temperature in steady state to ensure that absolute maximum ratings are not exceeded. SEPIC Converter: Output Diode Selection To maximize efficiency, a fast switching diode with a low forward drop and low reverse leakage is desirable.
LT3758/LT3758A Applications Information Inverting Converter: Inductor, Sense Resistor, Power MOSFET, Output Diode and Input Capacitor Selections The selections of the inductor, sense resistor, power MOSFET, output diode and input capacitor of an inverting converter are similar to those of the SEPIC converter. Please refer to the corresponding SEPIC converter sections.
LT3758/LT3758A Applications Information Check the stress on the power MOSFET by measuring its drain-to-source voltage directly across the device terminals (reference the ground of a single scope probe directly to the source pad on the PC board). Beware of inductive ringing, which can exceed the maximum specified voltage rating of the MOSFET. If this ringing cannot be avoided, and exceeds the maximum rating of the device, either choose a higher voltage device or specify an avalancherated power MOSFET.
LT3758/LT3758A Applications Information Recommended Component Manufacturers Some of the recommended component manufacturers are listed in Table 2. Table 2. Recommended Component Manufacturers VENDOR COMPONENTS WEB ADDRESS Capacitors avx.com Inductors, Transformers bhelectronics.com Coilcraft Inductors coilcraft.com Cooper Bussmann Inductors bussmann.com Diodes diodes.com MOSFETs fairchildsemi.com Diodes generalsemiconductor.
LT3758/LT3758A Typical Applications 10V to 40V Input, 48V Output Boost Converter VIN 10V TO 40V CIN 4.7µF 50V X7R ×2 R3 200k L1 18.7µH VIN D1 SHDN/UVLO R4 32.4k SYNC LT3758 SENSE RT SS RT 41.2k 300kHz CC2 100pF + FBX GND INTVCC VC CSS 0.68µF R2 464k M1 GATE RC 10k CC1 10nF CVCC 4.7µF 10V X5R VOUT 48V 1A RS 0.012Ω COUT1 100µF 63V R1 15.8k COUT2 4.
LT3758/LT3758A Typical Applications 12V Output Nonisolated Flyback Power Supply VIN 36V TO 72V D1 CIN 2.2µF 100V X7R 0.022µF 100V 1M 6.2k VIN DSN SHDN/UVLO 44.2k SYNC LT3758 10k 100pF 4,5,6 (PARALLEL) SW GATE FBX GND INTVCC VC 0.47µF T1 1,2,3 (SERIES) 105k 1% M1 SENSE RT SS 63.4k 200kHz VOUT 12V 1.2A 10nF 1N4148 CVCC 4.7µF 10V X5R 5.1Ω 15.8k 1% 0.
LT3758/LT3758A Typical Applications VFD (Vacuum Fluorescent Display) Flyback Power Supply D1 VIN 9V TO 16V COUT2 2.2µF 100V X7R 4 CIN 22µF 25V D2 178k 22Ω SHDN/UVLO 220pF 32.4k SYNC LT3758 47pF 95.3k GATE VOUT2 64V 40mA COUT1 1µF 100V X7R 6 FBX GND INTVCC VC 0.47µF 5 M1 SW SENSE RT SS 63.4k 200kHz T1 1, 2, 3 VIN VOUT 96V 80mA 10k 10nF CVCC 4.7µF 10V X5R 0.019Ω 0.5W 1.
LT3758/LT3758A Typical Applications 36V to 72V Input, 3.3V Output Isolated Telecom Power Supply PA1277NL 4 5 VIN 36V TO 72V 0.022µF 100V CIN 2.2µF 100V X7R 5.6k 7 3 1M VIN SHDN/UVLO 44.2k SYNC FDC2512 10Ω GATE SENSE INTVCC 4.7µF 25V X5R FBX RT 63.4k 200kHz SS GND UPS840 VOUT- 8 BAS516 2 4.7µF 25V X5R 0.03Ω LT3758 COUT 100µF 6.3V ×3 6 BAV21W BAT54CWTIG 100pF 100k 1 16k 274Ω VC BAS516 47pF LT4430 PS2801-1 0.47µF VOUT+ 3.3V 3A 2200pF 250VAC 0.47µF VIN 1µF GND COMP OC 0.
LT3758/LT3758A Typical Applications 18V to 72V Input, 24V Output SEPIC Converter VIN 18V TO 72V + CIN1 10µF 100V CIN2 2.2µF 100V X7R 232k VIN L1A SHDN/UVLO 20k • CDC 2.2µF 100V X7R, ×2 D1 VOUT 24V 1A LT3758A GATE SYNC M1 L1B • SENSE 0.025Ω RT SS GND VC 41.2k 300kHz 0.47µF 280k 1% 10k 10nF FBX INTVCC 20k 1% CVCC 4.
LT3758/LT3758A Typical Applications 10V to 40V Input, –12V Output Inverting Converter CIN2 4.7µF 50V X7R ×2 CIN1 4.7µF 50V ×2 R1 200k VIN L1A SHDN/UVLO R2 32.4k • CDC 2.2µF 100V X7R, ×2 L1B LT3758A GATE SYNC M1 D1 SENSE 105k 0.015Ω RT SS GND VC 41.2k 300kHz VOUT –12V 2A • + FBX INTVCC 7.5k 0.47µF 10k 6.8nF COUT1 22µF 16V X5R ×4 CVCC 4.
LT3758/LT3758A Package Description DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) R = 0.125 TYP 6 0.40 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ±0.05 0.00 – 0.05 5 1 (DD) DFN REV C 0310 0.25 ± 0.05 0.
LT3758/LT3758A Package Description MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev H) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 ±0.102 (.074 ±.004) 5.23 (.206) MIN 1 0.889 ±0.127 (.035 ±.005) 1.68 ±0.102 (.066 ±.004) 0.05 REF 10 0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ±0.102 (.118 ±.004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE.
LT3758/LT3758A Revision History REV DATE DESCRIPTION PAGE NUMBER A 3/10 Deleted Bullet from Features and Last Line of Description Updated All Sections to Include H-Grade and Military Grade B C D 5/10 5/11 07/12 1 2 to 7 Deleted Vendor Telephone Information from Table 2 in Applications Information Section 26 Revised TA04 and TA04c in Typical Applications 29 Replaced Related Parts List 36 Revised last sentence of SYNC Pin description 8 Updated Block Diagram 9 Revised value in last sent
LT3758/LT3758A Typical Applications 8V to 72V Input, 12V Output SEPIC Converter Efficiency vs Output Current 100 CIN 2.2µF 100V X7R ×2 154k VIN L1A SHDN/UVLO 32.4k LT3758 SYNC GATE M1 Si7456DP SENSE 0.012Ω RT SS VOUT 12V 2A + L1B • 105k 1% COUT1 47µF 20V ×2 0.47µF 10nF VIN = 42V 70 VIN = 72V 60 50 40 30 20 15.8k 1% 10k VIN = 8V 80 FBX GND INTVCC VC 41.2k 300kHz 90 CDC 2.2µF 100V D1 X7R, ×2 MBRS3100T3G • EFFICIENCY (%) VIN 8V TO 72V COUT2 10µF 16V X5R ×4 CVCC 4.
