LT3573 Isolated Flyback Converter without an Opto-Coupler FEATURES n n n n n n n n n n DESCRIPTION 3V to 40V Input Voltage Range 1.
LT3573 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION SW.............................................................................60V VIN, SHDN/UVLO, RFB, BIAS......................................40V SS, VC, TC, RREF , RILIM................................................5V Maximum Junction Temperature........................... 125°C Operating Junction Temperature Range (Note 2) LT3573E............................................. –40°C to 125°C Storage Temperature Range...................
LT3573 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V, unless otherwise noted. PARAMETER CONDITIONS Error Amplifier Voltage Gain VIN = 3V MIN 150 V/V Error Amplifier Transconductance DI = 10µA, VIN = 3V 150 µmhos Minimum Switching Frequency VC = 0.35V 40 kHz TC Current into RREF RTC = 20.1k 27.5 µA BIAS Pin Voltage IBIAS = 30mA 2.
LT3573 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit 400 1.8 350 1.6 25°C 125°C 250 –50°C 200 150 100 0.8 RILIM = 10k 0.6 0.25 0.50 0.75 1.00 SWITCH CURRENT (A) 1.25 1.50 0 –50 –25 0 25 50 75 1.0 0.8 0.6 0.2 100 125 TEMPERATURE (°C) 0 0 10 20 30 40 50 RILIM RESISTANCE (k) 3573 G05 3573 G04 3573 G06 SS Pin Current SHDN/UVLO Falling Threshold 1.28 12 10 1.26 SS PIN CURRENT (µA) SHDN/UVLO VOLTAGE (V) 1.2 0.4 MINIMUM CURRENT LIMIT 0.2 0 1.6 1.2 1.
LT3573 PIN FUNCTIONS GND: Ground. TEST: This pin is used for testing purposes only and must be connected to ground for the part to operate properly. SW: Collector Node of the Output Switch. This pin has large currents flowing through it. Keep the traces to the switching components as short as possible to minimize electromagnetic radiation and voltage spikes. VIN : Input Voltage.
LT3573 BLOCK DIAGRAM D1 T1 VIN C1 L1A VOUT + L1B C2 R3 VOUT – N:1 TC CURRENT Q3 TC R6 RFB VIN SW FLYBACK ERROR AMP Q2 I2 20µA 1.23V –g m + – + ONE SHOT CURRENT COMPARATOR A2 – A1 + S S BIAS R DRIVER BIAS MASTER LATCH SHDN/UVLO R2 1.22V + A5 – 2.5µA INTERNAL REFERENCE AND REGULATORS I1 7µA Q1 Q C5 R1 – VIN RREF R4 + V1 120mV + – A4 RSENSE 0.02Ω GND OSCILLATOR VC Q4 R7 SS RILIM C3 C4 3573 BD R5 3573fd 6 For more information www.linear.
LT3573 OPERATION The LT3573 is a current mode switching regulator IC designed specifically for the isolated flyback topology. The special problem normally encountered in such circuits is that information relating to the output voltage 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 optoisolators or extra transformer windings.
LT3573 APPLICATIONS INFORMATION ERROR AMPLIFIER—PSEUDO DC THEORY In the Block Diagram, the RREF (R4) and RFB (R3) resistors can be found. They are external resistors used to program the output voltage. The LT3573 operates much the same way as traditional current mode switchers, the major difference being a different type of error amplifier which derives its feedback information from the flyback pulse.
LT3573 APPLICATIONS INFORMATION ERROR AMPLIFIER—DYNAMIC THEORY Selecting RFB and RREF Resistor Values Due to the sampling nature of the feedback loop, there are several timing signals and other constraints that are required for proper LT3573 operation. The expression for VOUT, developed in the Operation section, can be rearranged to yield the following expression for RFB: Minimum Current Limit The LT3573 obtains output voltage information from the SW pin when the secondary winding conducts current.
LT3573 APPLICATIONS INFORMATION relatively constant maximum output current regardless of input voltage. This is due to the continuous nonswitching behavior of the two currents. A flyback converter has both discontinuous input and output currents which makes it similar to a nonisolated buck-boost. The duty cycle will affect the input and output currents, making it hard to predict output power.
LT3573 APPLICATIONS INFORMATION TRANSFORMER DESIGN CONSIDERATIONS Transformer specification and design is perhaps the most critical part of successfully applying the LT3573. In addition to the usual list of caveats dealing with high frequency isolated power supply transformer design, the following information should be carefully considered. Linear Technology has worked with several leading magnetic component manufacturers to produce pre-designed flyback transformers for use with the LT3573.
LT3573 APPLICATIONS INFORMATION Turns Ratio Leakage Inductance Note that when using an RFB /RREF resistor ratio to set output voltage, the user has relative freedom in selecting a transformer turns ratio to suit a given application. In contrast, simpler ratios of small integers, e.g., 1:1, 2:1, 3:2, etc., can be employed to provide more freedom in setting total turns and mutual inductance.
LT3573 APPLICATIONS INFORMATION LS – + VSW < 60V C R < 55V < 50V D t OFF > 350ns tSP 3573 F04 Figure 4. RCD Clamp 3573 F05 TIME Figure 5. Maximum Voltages for SW Pin Flyback Waveform 10V/DIV 10V/DIV 100ns/DIV 3573 F06 Figure 6. Good Snubber Diode Limits SW Pin Voltage 100ns/DIV 3573 F07 Figure 7. Bad Snubber Diode Does Not Limit SW Pin Voltage 3573fd For more information www.linear.
LT3573 APPLICATIONS INFORMATION Secondary Leakage Inductance In addition to the previously described effects of leakage inductance in general, leakage inductance on the secondary in particular exhibits an additional phenomenon. It forms an inductive divider on the transformer secondary that effectively reduces the size of the primary-referred flyback pulse used for feedback. This will increase the output voltage target by a similar percentage.
LT3573 APPLICATIONS INFORMATION Minimum Load Requirement The LT3573 obtains output voltage information through the transformer while the secondary winding is conducting current. During this time, the output voltage (multiplied times the turns ratio) is presented to the primary side of the transformer. The LT3573 uses this reflected signal to regulate the output voltage. This means that the LT3573 must turn on every so often to sample the output voltage, which delivers a small amount of energy to the output.
LT3573 APPLICATIONS INFORMATION The transformer turns ratio is selected such that the converter has adequate current capability and a switch stress below 50V. Table 6 shows the switch voltage stress and output current capability at different transformer turns ratio. Table 6. Switch Voltage Stress and Output Current Capability vs Turns-Ratio N VSW(MAX) AT VIN(MAX) (V) IOUT(MAX) AT VIN(MIN) (A) DUTY CYCLE (%) 1:1 33.5 0.39 16~22 2:1 39 0.65 28~35 3:1 44.5 0.825 37~45 4:1 50 0.
LT3573 APPLICATIONS INFORMATION Zener clamps. Among them, RCD is widely used. Figure 9 shows the RCD snubber in a flyback converter. RTC resistor for temperature compensation of the output voltage. RREF is selected as 6.04k. A typical switch node waveform is shown in Figure 10. A small capacitor in parallel with RREF filters out the noise during the voltage spike, however, the capacitor should limit to 10pF. A large capacitor causes distortion on voltage sensing.
LT3573 TYPICAL APPLICATIONS Low Input Voltage 5V Isolated Flyback Converter VIN 5V D1 3:1 C1 10µF R1 200k C6 0.22µF VIN R8 T1 2k 24µH 2.6µH SHDN/UVLO R2 90.9k LT3573 RFB RREF R3 80.6k VOUT+ 5V, 350mA C5 47µF VOUT– D2 R4 6.04k TC SW RILIM SS VC R6 28.7k R5 10k GND TEST BIAS R7 57.6k C3 1000pF C2 10nF VIN 3573 TA02 T1: PULSE PA2454NL OR WÜRTH ELEKTRONIK 750310471 D1: B340A D2: PMEG6010 C5: MURATA, GRM32ER71A476K ±12V Isolated Flyback Converter VIN 5V D1 2:1:1 C1 10µF R1 200k C6 0.
LT3573 TYPICAL APPLICATIONS 5V Isolated Flyback Converter VIN 12V TO 24V (*30V) 3:1:1 C1 10µF R1 499k C6 0.22µF VIN R2 71.5k R8 T1 2k 24µH D1 2.6µH VOUT + 5V, 700mA C5 47µF SHDN/UVLO LT3573 R3 80.6k RFB RREF VOUT – D3 R4 6.04k TC RILIM SW SS VC R6 28.7k R5 10k GND TEST BIAS D2 R7 45.3k C2 10nF C4 4.7µF C3 1000pF L1C 2.
LT3573 TYPICAL APPLICATIONS 3.3V Isolated Flyback Converter VIN 12V TO 24V (*30V) 4:1:1 C1 10µF R1 499k C6 0.22µF VIN R8 T1 2k 24µH D1 1.5µH VOUT + 3.3V, 1A C5 47µF SHDN/UVLO R2 71.5k LT3573 R3 76.8k RFB RREF VOUT – D3 R4 6.04k TC RILIM SW SS GND TEST BIAS VC R6 19.1k R5 10k D2 R7 25.5k C3 1500pF C2 10nF C4 4.7µF L1C 1.
LT3573 TYPICAL APPLICATIONS Four Output 12V Isolated Flyback Converter VIN 12V TO 24V D1 2:1:1:1:1 C1 10µF R1 499k C6 0.22µF VIN LT3573 RFB RREF TC VC C2 10nF R3 118k VOUT2+ 12V, 60mA VOUT 2– D3 R4 6.04k VOUT3+ 12V, 60mA C7 47µF VOUT 3– 10.9µH GND TEST BIAS R7 20k C3 0.01µF C6 47µF 10.9µH SW SS R5 10k VOUT 1– D5 RILIM R6 59k C5 47µF 10.9µH D2 SHDN/UVLO R2 71.5k T1 R8 2k 43.6µH VOUT1+ 12V, 60mA D4 VIN VOUT4+ 12V, 60mA C8 47µF 10.
LT3573 TYPICAL APPLICATIONS 5V Isolated Flyback Converter Using Coupling Inductor VIN 5V 1:1 C1 10µF R1 200k R2 90.9k C6 0.22µF VIN SHDN/UVLO LT3573 RFB RREF TC RILIM SS VC R6 26.1k R5 10k C2 10nF 3573 TA09 R8 T1 2k 23.6µH R3 26.1k D2 D1 23.6µH VOUT+ 5V, 0.2A C5 47µF VOUT– R4 6.04k SW GND TEST BIAS R7 56.2k C3 1500pF VIN T1: BH ELECTRONICS, L10-1022 D1: B220A D2: CMD5H-3 3573fd 22 For more information www.linear.
LT3573 TYPICAL APPLICATIONS 300V Isolated Flyback Converter VIN 6V TO 15V 1:10 C1 10µF R1 100k C6 0.22µF VIN SHDN/UVLO R2 36k LT3573 RFB RREF TC RILIM SS VC R6 20.5k R5 10k C2 10nF 3573 TA10 R8 1k C8 100pF R3 150k D2 T1 100µH D1 VOUT+ 300V, 5mA 1M C5, 500V, 0.056µF ×4 VOUT– R4 6.04k SW GND TEST BIAS R7 25k C3 2200pF VIN T1: WÜRTH ELEKTRONIK 750311681 D1: CMMRIF-06 D2: CMMSHI-60 3573fd For more information www.linear.
LT3573 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) 5.23 (.206) MIN 2.845 ± 0.102 (.112 ± .004) 0.889 ± 0.127 (.035 ± .005) 8 1 1.651 ± 0.102 (.065 ± .004) 1.651 ± 0.102 3.20 – 3.45 (.065 ± .004) (.126 – .136) 0.305 ± 0.038 (.0120 ± .0015) TYP 16 0.50 (.0197) BSC 4.039 ± 0.102 (.159 ± .004) (NOTE 3) RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 0.
LT3573 REVISION HISTORY REV DATE DESCRIPTION B 10/09 Replaced Figure 1 (Revision history begins at Rev B) PAGE NUMBER 10 Updated Typical Applications drawings C D 07/10 12/13 18, 21, 22 Added patent numbers and revised Typical Application drawing 1 Revised D1 on Block Diagram 6 Revised Table 5 11 Revised Figure 4 in Applications Information section 13 Revised all drawings in Typical Applications section 18-23 Replaced Related Parts list 26 Added reference to Note 1 2 Changed flybac
LT3573 TYPICAL APPLICATION 9V to 30VIN, +5V/–5VOUT Isolated Flyback Converter T1 3:1:1:1 VIN 9V TO 30V C1 10µF R1 357k SHDN/UVLO R2 51.1k LT3573 RFB C5 47µF COM C6 47µF D2 VOUT – –5V, 350mA R4 6.04k TC SW SS VC C2 10nF VOUT + +5V, 350mA L1B 7µH L1C 7µH R3 80.6k RILIM R6 10k L1A 63µH D4 RREF R5 28.7k R8 2k C6 0.22µF VIN D1 GND TEST BIAS R7 23.7k C3 2700pF D3 C4 4.
