LT3759 Wide Input Voltage Range Boost/SEPIC/Inverting Controller DESCRIPTION FEATURES n n n n n n n n n n Wide VIN Range: 1.
LT3759 ABSOLUTE MAXIMUM RATINGS (Note 1) VIN.............................................................................42V EN/UVLO (Note 2)......................................................42V DRIVE........................................................................42V PGOOD.......................................................................42V INTVCC.........................................................................8V GATE.................................................................
LT3759 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 12V, EN/UVLO = 12V, INTVCC = 4.75V, unless otherwise noted. PARAMETER CONDITIONS VIN Operating Voltage VIN Shutdown IQ MIN l TYP 1.6 EN/UVLO < 0.4V EN/UVLO = 1.15V VIN Operating IQ MAX UNITS 42 V 0.1 1 6 µA µA 350 450 µA DRIVE Shutdown Quiescent Current EN/UVLO < 0.4V EN/UVLO = 1.15V 0.1 0.
LT3759 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VIN = 12V, EN/UVLO = 12V, INTVCC = 4.75V, unless otherwise noted. PARAMETER CONDITIONS VIN LDO Dropout Voltage (VIN – VINTVcc) VIN = 3V, DRIVE = 0V, IINTVCC = 20mA MIN l INTVCC Undervoltage Lockout Threshold Falling INTVCC Current in Shutdown EN/UVLO = 0V TYP MAX UNITS 190 400 mV 1.3 1.
LT3759 TYPICAL PERFORMANCE CHARACTERISTICS FBX Positive Regulation Voltage vs Temperature TA = 25°C, unless otherwise noted. FBX Negative Regulation Voltage vs Temperature Quiescent Current vs Temperature 2.2 –0.78 1.62 1.61 1.60 1.59 IQ (DRIVE) 1.8 –0.79 1.6 1.4 IQ (mA) FBX REGULATION VOLTAGE (V) FBX REGULATION VOLTAGE (V) 2.0 –0.80 VIN = 12V DRIVE = 6V 1.2 1.0 0.8 0.6 –0.81 IQ (VIN) 0.4 0.2 1.58 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) –0.
LT3759 TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Threshold vs Temperature GATE Minimum On- and Off-Times vs Temperature 1.27 EN/UVLO RISING 1.21 EN/UVLO FALLING 1.19 2.2 180 MINIMUM OFF TIME 170 160 150 130 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 25 50 75 100 125 150 TEMPERATURE (°C) 3759 G12 3759 G11 INTVCC vs Temperature 6.0 INTVCC Load Regulation 5.0 INTVCC Line Regulation 5.5 DRIVE LDO INTVCC VOLTAGE (V) 4.6 4.4 4.2 4.0 VIN LDO 3.8 3.6 –75 –50 –25 DRIVE LDO 5.
LT3759 PIN FUNCTIONS DRIVE: DRIVE LDO Supply Pin. This pin can be connected to either VIN or a quasi-regulated voltage supply such as a DC converter output. This pin must be bypassed with a minimum of 1µF capacitor placed close to the pin. Tie this pin to VIN if not used. EN/UVLO: Shutdown and Undervoltage Detect Pin. An accurate 1.22V (nominal) falling threshold with externally programmable hysteresis detects when power is okay to enable switching.
LT3759 BLOCK DIAGRAM L1 CDC D1 VOUT • VIN R4 + R3 CIN R2 L2 • FBX + R1 12 1 CC2 – BG BG_LOW Q3 UVLO G4 RC 1.22V INTERNAL BIAS GENERATOR INTERNAL BIAS A8 CC1 1.72V –0.86V – + A11 – + A12 1.6V FBX + BANDGAP REFERENCE IS2 10µA VC 2 6 FBX PGOOD –0.8V 1.52V –0.76V G6 VC Q2 – + A14 VIN LDO DRIVE LDO R G5 S DRIVER Q VISENSE RAMP RAMP GENERATOR + 2.7V 1.25V – +A3 1.
LT3759 APPLICATIONS INFORMATION Main Control Loop The LT3759 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.
LT3759 APPLICATIONS INFORMATION regulates the INTVCC to 3.75V. VIN LDO is turned off when the INTVCC voltage is greater than 3.75V (typical). Both LDO’s can be turned off if the INTVCC pin is driven by a supply of 4.75V or higher but less than 8V (the INTVCC maximum voltage rating is 8V). A table of the LDO supply and output voltage combination is shown in Table 1. Table 1. LDO’s Supply and Output Voltage Combination (Assuming That the LDO Dropout Voltage is 0.
LT3759 APPLICATIONS INFORMATION operation improves efficiency by reducing gate drive current and MOSFET and diode switching losses. However, lower frequency operation requires a physically larger loop inductor. Switching frequency also has implications for loop compensation. The LT3759 uses a constant-frequency architecture that can be programmed over a 100kHz to 1MHz range with a single external resistor from the RT pin to ground, as shown in Figure 1.
LT3759 APPLICATIONS INFORMATION Besides start-up, soft-start can also be triggered by INTVCC undervoltage lockout and/or thermal lockout, which causes the LT3759 to stop switching immediately. The SS pin will be discharged by Q3. When all faults are cleared and the SS pin has been discharged below 0.2V, a 10µA current source IS2 starts charging the SS pin, initiating a soft-start operation. The soft-start interval is set by the soft-start capacitor selection according to the equation: TSS = CSS • 1.
LT3759 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 40mV. Then, the maximum switch ripple current percentage can be calculated using the following equation: c= DVSENSE 40mV - 0.
LT3759 APPLICATIONS INFORMATION The selection of switching frequency is the starting point. The maximum frequency that can be used is based on the maximum duty cycle. The conversion ratio as a function of duty cycle is: VOUT 1 = VIN 1−D The peak and RMS inductor current are: in continuous conduction mode (CCM). The equations that follow assume CCM operation.
LT3759 APPLICATIONS INFORMATION to choose a MOSFET whose BVDSS is higher than VOUT by a safety margin (a 10V safety margin is usually sufficient). It is recommended that the peak repetitive reverse voltage rating VRRM is higher than VOUT by a safety margin (a 10V safety margin is usually sufficient).
LT3759 APPLICATIONS INFORMATION change, depending on the requirements of the application, and the following equations can easily be modified. For a 1% contribution to the total ripple voltage, the ESR of the output capacitor can be determined using the following equation: ESRCOUT ≤ 0.01• VOUT ID(PEAK) For the bulk C component, which also contributes 1% to the total ripple: COUT ≥ IO(MAX) 0.01• VOUT • f The output capacitor in a boost regulator experiences high RMS ripple currents, as shown in Figure 5.
LT3759 APPLICATIONS INFORMATION Flyback Converter: Switch Duty Cycle and Turns Ratio The flyback converter conversion ratio in the continuous mode operation is: VOUT NS D = • VIN NP 1−D where NS/NP is the second to primary turns ratio. Figure 7 shows the waveforms of the flyback converter in discontinuous mode operation. During each switching period TS, three subintervals occur: DTS, D2TS, D3TS. During DTS, M is on, and D is reverse-biased. During D2TS, M is off, and LS is conducting current.
LT3759 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 7, the minimum D3 (D3MIN) occurs when the converter has the minimum VIN and the maximum output power (POUT).
LT3759 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: VSN CCN = ΔVSN • RCN • fOSC where ∆VSN is the voltage ripple across CCN. A reasonable ∆VSN is 5% to 10% of VSN.
LT3759 APPLICATIONS INFORMATION The RMS ripple current rating of the output capacitors in discontinuous operation can be determined using the following equation: IRMS(COUT),DISCONTINUOUS 4 −(3 • D2) ≥ IO(MAX) • 3 • D2 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.
LT3759 APPLICATIONS INFORMATION The inductor ripple currents ΔIL1 and ΔIL2 are identical: DIL1 = DIL2 = 0.5 • DISW ISW L= ∆ISW = χ • ISW(MAX) = ISW(MAX) TS 3759 F08 Figure 8. The Switch Current Waveform of a SEPIC Converter The inductor ripple current has a direct effect on the choice of the inductor value. Choosing smaller values of ΔIL requires large inductances and reduces the current loop gain (the converter will approach voltage mode).
LT3759 APPLICATIONS INFORMATION For maximum efficiency, RDS(ON) and CRSS should be minimized.
LT3759 APPLICATIONS INFORMATION Inverting Converter: Switch Duty Cycle and Frequency For an inverting converter operating in CCM, the duty cycle of the main switch can be calculated based on the negative output voltage (VOUT) and the input voltage (VIN).
LT3759 APPLICATIONS INFORMATION • In flyback configuration, the high di/dt primary loop contains the input capacitor, the primary winding, the power MOSFET and sensing resistor. The high di/dt secondary loop contains the output capacitor, the secondary winding and the output diode. ringing, which can exceed the maximum specified voltage rating of the MOSFET.
LT3759 TYPICAL APPLICATIONS 1.8V to 3.3V Input, 5V/2A Output Boost Converter VIN 1.8V TO 3.3V CIN 47µF 6.3V X5R R3 59k VIN SYNC LT3759 SS VC CC2 100pF CSS 0.1µF D1 GATE M1 SENSE RT RT 27.4k 300kHz L1 2.2µH PGOOD EN/UVLO R4 124k R5 10k DRIVE VOUT 5V 2A R2 34k 1% FBX GND INTVCC RC 7.5k CC1 22nF CVCC 4.7µF 10V X5R RS 5mΩ 0.5W R1 15.8k 1% COUT2 100µF 6.3V X5R ×3 3759 TA02a M1: VISHAY SiA414DJ L1: TOKO FDA1055-2R2M D1: VISHAY 6CWQ06FN Load Step Response at VIN = 2.
LT3759 TYPICAL APPLICATIONS 8V to 16V Input, 24V/2A Output Boost Converter VIN 8V TO 16V CIN 22µF 25V X5R R3 200k VIN SYNC LT3759 SS CSS 0.1µF GATE M1 RC 20k CC1 10nF CVCC 4.7µF 10V X5R VOUT 24V 2A R2 226k 1% + FBX GND INTVCC VC CC2 100pF D1 SENSE RT RT 27.4k 300kHz L1 10µH PGOOD EN/UVLO R4 43.2k R5 100k DRIVE RS 5mΩ 0.5W COUT1 33µF 35V ×2 R1 16.
LT3759 TYPICAL APPLICATIONS 1.8V to 5V Input, 3.3V/3A Output SEPIC Converter VIN 1.8V TO 5V CIN 47µF 10V 59k VIN SYNC LT3759 SS 3.01k 22nF CDC 4.7µF 10V, X5R, ×2 D1 VOUT 3.3V 2A, 1.8V ≤ VIN ≤ 3V 3A, 3V < VIN ≤ 5V VSW GATE M1 IL1B L1B 0.004Ω 1W FBX GND INTVCC VC 0.1µF IL1A SENSE RT 27.4k 300kHz L1A PGOOD EN/UVLO 124k 10k DRIVE 16.9k 1% 15.8k 1% 4.7µF 10V X5R COUT 100µF 6.
LT3759 TYPICAL APPLICATIONS 2.5V to 36V Input, 12V/1A Output SEPIC Converter (Automotive 12V Regulator) VIN 2.5V TO 36V CIN 4.7µF 50V ×4 105k VIN SYNC LT3759 IL1A CDC 4.7µF 50V, X5R, ×2 SS GATE FBX GND INTVCC VC 0.1µF 7.5k 22nF VOUT 12V 0.5A, 2.5V ≤ VIN ≤ 8V 2A, 8V < VIN ≤ 36V D1 VSW M1 IL1B SENSE RT 41.2k 200kHz L1A PGOOD EN/UVLO 118k 100k DRIVE 105k 1% L1B 0.005Ω 0.5W + COUT1 47µF 20V ×4 15.8k 1% 4.
LT3759 TYPICAL APPLICATIONS 5V to 15V Input, –5V/3A Output Inverting Converter VIN 5V TO 15V CIN 47µF 16V X5R R2 105k VIN LT3759 SYNC GATE SS 84.5k 5mΩ 0.5W 15.8k CVCC 4.7µF 10V X5R 9.1k 0.1µF VOUT –5V 3A, 5V ≤ VIN ≤ 10V 4A, 10V < VIN ≤ 15V D1 FBX GND INTVCC VC L1B 3.3µH M1 SENSE RT 27.4k 300kHz CDC 4.7µF 25V, X5R, ×2 PGOOD EN/UVLO R1 45.3k L1A 3.3µH 100k DRIVE 10nF M1: VISHAY SILICONIX Si7848BDP L1A, L1B: COILTRONICS DRQ127-3R3 D1: VISHAY 6CWQ03FN COUT 47µF 6.
LT3759 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 12-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1666 Rev F) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 ±0.102 (.112 ±.004) 5.23 (.206) MIN 2.845 ±0.102 (.112 ±.004) 0.889 ±0.127 (.035 ±.005) 6 1 1.651 ±0.102 (.065 ±.004) 1.651 ±0.102 3.20 – 3.45 (.065 ±.004) (.126 – .136) 12 0.65 0.42 ±0.038 (.0256) (.0165 ±.
LT3759 REVISION HISTORY REV DATE DESCRIPTION A 12/11 SS Pull-Up Current MIN and TYP values updated and INTVCC Current in Shutdown TYP value updated in Electrical Characteristics table. PAGE NUMBER 3, 4 Revised Typical Application drawing TA02a 25 B 4/12 Revised Typical Applications Schematic TA01a 1 C 1/13 Added UN/UVLO Rising Spec 4 Corrected M1 part number on Typical Application 25 3759fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
LT3759 TYPICAL APPLICATION 1.8V to 5V Input, –5V/2A Output Inverting Converter VIN 1.8V TO 5V L1A, 3.3µH CIN 47µF 10V X5R 1:1 59k CDC 4.7µF ×2 25V, X5R VIN EN/UVLO 100k 124k LT3759 PGOOD TIE TO GND IF NOT USED GATE M1 D2 SENSE DRIVE D1 1µF 16V X5R RT SS CVCC 4.7µF 10V X5R 9.1k 10nF M1: VISHAY SILICONIX Si74116DY L1A, L1B: COILTRONICS DRQ127-3R3 D1: VISHAY 6CWQ03FN D2: PHILIPS PMEG2005EJ 84.5k FBX GND INTVCC VC 0.1µF VOUT –5V 1A, 1.8V ≤ VIN ≤ 2.5V 2A, 2.5V < VIN ≤ 5V 5mΩ 0.
