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LT8302

Rev E

For more information www.analog.com

TYPICAL APPLICATION

FEATURES DESCRIPTION

42V

Micropower No-Opto

Isolated Flyback Converter

with 65V/3.6A Switch

The LT

8302 is a monolithic micropower isolated fly-

back converter. By sampling the isolated output voltage

directly from the primary-side flyback waveform, the part

requires no third winding or opto-isolator for regulation.

The output voltage is programmed with two external

resistors and a third optional temperature compensation

resistor. Boundary mode operation provides a small mag-

netic solution with excellent load regulation. Low ripple

Burst Mode operation maintains high efficiency at light

load while minimizing the output voltage ripple. A 3.6A,

65V DMOS power switch is integrated along with all the

high voltage circuitry and control logic into a thermally

enhanced 8-lead SO package.

The LT8302 operates from an input voltage range of 3V

to 42V and delivers up to 18W of isolated output power.

The high level of integration and the use of boundary

and low ripple burst modes result in a simple to use, low

component count, and high efficiency application solution

for isolated power delivery.

3V to 32V

/5V

OUT

Isolated Flyback Converter

APPLICATIONS

3V to 42V Input Voltage Range

3.6A, 65V Internal DMOS Power Switch

Low Quiescent Current:

106µA in Sleep Mode

380µA in Active Mode

Quasi-Resonant Boundary Mode Operation at

Heavy Load

Low Ripple Burst Mode

Operation at Light Load

Minimum Load < 0.5% (Typ) of Full Output

No Transformer Third Winding or Opto-Isolator

Required for Output Voltage Regulation

Accurate EN/UVLO Threshold and Hysteresis

Internal Compensation and Soft-Start

Temperature Compensation for Output Diode

Output Short-Circuit Protection

Thermally Enhanced 8-Lead SO Package

AEC-Q100 Qualified for Automotive Applications

Isolated Automotive, Industrial, Medical Power

Supplies

Isolated Auxiliary/Housekeeping Power Supplies

Efficiency vs Load Current

LT8302

9µH

3V TO 32V

3:1

1µH

REF

EN/UVLO

470pF

10µF

1µF

220µF

10mA TO 1.1A (V

= 5V)

10mA TO 2.0A (V

= 12V)

10mA TO 2.9A (V

= 24V)

OUT

–

39Ω

154k

•

115k 10k

8302 TA01a

GND

INTV

OUT

LOAD CURRENT (A)

EFFICIENCY (%)

1.5 2.5

8302 TA01b

0.5 1.0

2.0 3.0

= 5V

= 12V

= 24V

All registered trademarks and trademarks are the property of their respective owners. Protected

by U.S. patents, including 5438499, 7463497, 7471522.

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Summary of content (26 pages)

PAGE 1
LT8302 42VIN Micropower No-Opto Isolated Flyback Converter with 65V/3.6A Switch FEATURES DESCRIPTION 3V to 42V Input Voltage Range n 3.6A, 65V Internal DMOS Power Switch n Low Quiescent Current: 106µA in Sleep Mode 380µA in Active Mode n Quasi-Resonant Boundary Mode Operation at Heavy Load n Low Ripple Burst Mode® Operation at Light Load n Minimum Load < 0.
PAGE 2
LT8302 ABSOLUTE MAXIMUM RATINGS (Note 1) SW (Note 2)...............................................................65V VIN.............................................................................42V EN/UVLO.....................................................................VIN RFB.........................................................VIN – 0.5V to VIN Current Into RFB.....................................................200µA INTVCC, RREF, TC..........................................................
PAGE 3
LT8302 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 5V, VEN/UVLO = VIN, CINTVCC = 1µF to GND, unless otherwise noted. SYMBOL PARAMETER VIN VIN Voltage Range IQ VIN Quiescent Current CONDITIONS MIN l TYP 3 VEN/UVLO = 0.2V VEN/UVLO = 1.1V Sleep Mode (Switch Off) Active Mode (Switch On) 0.5 53 106 380 EN/UVLO Shutdown Threshold For Lowest Off IQ l 0.2 0.
PAGE 4
LT8302 TYPICAL PERFORMANCE CHARACTERISTICS Output Load and Line Regulation 5.3 5.00 4.95 VIN = 5V VIN = 12V VIN = 24V 4.80 0.5 0 1.0 2.0 1.5 LOAD CURRENT (A) 2.5 5.1 RTC = 115k 5.0 RTC = OPEN 4.9 4.
PAGE 5
LT8302 TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Enable Threshold TA = 25°C, unless otherwise noted. EN/UVLO Hysteresis Current 1.240 INTVCC Voltage vs Temperature 5 RISING IHYST (µA) VEN/UVLO (V) 1.225 1.220 FALLING 1.215 3.05 4 1.230 1.210 3.00 3 VINTVCC (V) 1.235 3.10 2 1 0 IINTVCC = 10mA 2.85 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 2.95 2.90 1.205 1.200 –50 –25 IINTVCC = 0mA 0 2.
PAGE 6
LT8302 TYPICAL PERFORMANCE CHARACTERISTICS Switch Current Limit 5 160 4 120 3 80 40 MAXIMUM CURRENT LIMIT 0 MINIMUM CURRENT LIMIT 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 Minimum Switching Frequency 25 50 75 100 125 150 TEMPERATURE (°C) 25 50 75 100 125 150 TEMPERATURE (°C) Minimum Switch-Off Time 400 400 300 300 200 100 0 0 8302 G21 TIME (ns) TIME (ns) FREQUENCY (kHz) 0 –50 –25 0 –50 –25 Minimum Switch-On Time 16 4 200 8302 G20 20 8 300 100 25 50 75 100 125
PAGE 7
LT8302 PIN FUNCTIONS EN/UVLO (Pin 1): Enable/Undervoltage Lockout. The EN/UVLO pin is used to enable the LT8302. Pull the pin below 0.3V to shut down the LT8302. This pin has an accurate 1.214V threshold and can be used to program a VIN undervoltage lockout (UVLO) threshold using a resistor divider from VIN to ground. A 2.5µA current hysteresis allows the programming of VIN UVLO hysteresis. If neither function is used, tie this pin directly to VIN. INTVCC (Pin 2): Internal 3V Linear Regulator Output.
PAGE 8
LT8302 BLOCK DIAGRAM T1 N:1 VIN CIN L1A RFB 3 2 6 1:4 M3 M2 REN2 OSCILLATOR – 1.214V + 1V – A1 VOUT– START-UP, REFERENCE, CONTROL BOUNDARY DETECTOR + 1 EN/UVLO COUT SW 25µA REN1 L1B VIN LDO CINTVCC • VOUT+ 5 RFB VIN INTVCC • DOUT INTVCC – gm + S A3 R Q M1 DRIVER 2.5µA PTAT VOLTAGE M4 + A2 RSENSE – RREF 7 TC GND 4, EXPOSED PAD PIN 9 8 RTC 8302 BD RREF Rev E 8 Downloaded from Arrow.com. For more information www.analog.
PAGE 9
LT8302 OPERATION The LT8302 is a current mode switching regulator IC designed specially for the isolated flyback topology. The key problem in isolated topologies is how to communicate the output voltage information from the isolated secondary side of the transformer to the primary side for regulation. Historically, opto-isolators or extra transformer windings communicate this information across the isolation boundary.
PAGE 10
LT8302 OPERATION Boundary conduction mode returns the secondary current to zero every cycle, so parasitic resistive voltage drops do not cause load regulation errors. Boundary conduction mode also allows the use of smaller transformers compared to continuous conduction mode and does not exhibit subharmonic oscillation. Discontinuous Conduction Mode Operation As the load gets lighter, boundary conduction mode increases the switching frequency and decreases the switch peak current at the same ratio.
PAGE 11
LT8302 APPLICATIONS INFORMATION Output Voltage The RFB and RREF resistors as depicted in the Block Diagram are external resistors used to program the output voltage. The LT8302 operates similar to traditional current mode switchers, except in the use of a unique flyback pulse sense circuit and a sample-and-hold error amplifier, which sample and therefore regulate the isolated output voltage from the flyback pulse.
PAGE 12
LT8302 APPLICATIONS INFORMATION To cancel the output diode temperature coefficient, the following two equations should be satisfied: VOUT = VREF • ( RFB 1 • – VF (TO) RREF NPS RFB(NEW) = R 1 VTC / T) • FB • = – ( VF / T ) R TC NPS Selecting Actual RREF, RFB, RTC Resistor Values The LT8302 uses a unique sampling scheme to regulate the isolated output voltage.
PAGE 13
LT8302 APPLICATIONS INFORMATION Output Power A flyback converter has a complicated relationship between the input and output currents compared to a buck or a boost converter. A boost converter has a relatively constant maximum input current regardless of input voltage and a buck converter has a relatively constant maximum output current regardless of input voltage. This is due to the continuous non-switching behavior of the two currents.
PAGE 14
LT8302 APPLICATIONS INFORMATION The equations below calculate output power: POUT = η • VIN • D • ISW(MAX) • 0.5 η = Efficiency = ~85% D = Duty Cycle = ( VOUT + VF ) •NPS ( VOUT + VF ) •NPS + VIN ISW(MAX) = Maximum switch current limit = 3.6A (MIN) LPRI ≥ Primary Inductance Requirement The LT8302 obtains output voltage information from the reflected output voltage on the SW pin. The conduction of secondary current reflects the output voltage on the primary SW pin.
PAGE 15
LT8302 APPLICATIONS INFORMATION Turns Ratio Note that when choosing 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, the use of simple ratios of small integers, e.g., 3:1, 2:1, 1:1, etc., provides more freedom in settling total turns and mutual inductance. Typically, choose the transformer turns ratio to maximize available output power. For low output voltages (3.
PAGE 16
LT8302 APPLICATIONS INFORMATION <65V VLEAKAGE <50V VSW tOFF > 350ns tSP < 250ns TIME 8302 F05 Figure 5. Maximum Voltages for SW Pin Flyback Waveform In addition to the voltage spikes, the leakage inductance also causes the SW pin ringing for a while after the power switch turns off. To prevent the voltage ringing falsely trigger boundary mode detector, the LT8302 internally blanks the boundary mode detector for approximately 250ns.
PAGE 17
LT8302 APPLICATIONS INFORMATION Undervoltage Lockout (UVLO) A resistive divider from VIN to the EN/UVLO pin implements undervoltage lockout (UVLO). The EN/UVLO enable falling threshold is set at 1.214V with 14mV hysteresis. In addition, the EN/UVLO pin sinks 2.5µA when the voltage on the pin is below 1.214V. This current provides user programmable hysteresis based on the value of R1. The programmable UVLO thresholds are: 1.228V • (R1+R2) + 2.5µA •R1 R2 1.
PAGE 18
LT8302 APPLICATIONS INFORMATION Design Example Step 2: Determine the primary inductance. Use the following design example as a guide to designing applications for the LT8302. The design example involves designing a 5V output with a 1.5A load current and an input range from 8V to 32V. Primary inductance for the transformer must be set above a minimum value to satisfy the minimum switch-off and switch-on time requirements: VIN(MIN) = 8V, VIN(NOM) = 12V, VIN(MAX) = 32V, VOUT = 5V, IOUT = 1.
PAGE 19
LT8302 APPLICATIONS INFORMATION Example: D= and cost of a larger capacitor. Use the following equation to calculate the output capacitance: (5V + 0.3V ) • 3 = 0.57 (5V + 0.3V ) • 3 + 12V COUT = 5V • 1.5A • 2 0.8 • 12V • 0.57 fSW = 277kHz ISW = Example: The transformer also needs to be rated for the correct saturation current level across line and load conditions. A saturation current rating larger than 7A is necessary to work with the LT8302.
PAGE 20
LT8302 APPLICATIONS INFORMATION Step 6: Select the RREF and RFB resistors. Example: Use the following equation to calculate the starting values for RREF and RFB: RFB = ( ) RREF • NPS • VOUT + VF ( TO) VREF RTC = RREF = 10k VIN(HYS) = 2.5µA • R1 For 1% standard values, a 158k resistor is chosen. Step 7: Adjust RFB resistor based on output voltage. Build and power up the application with application components and measure the regulated output voltage.
PAGE 21
LT8302 TYPICAL APPLICATIONS 8V to 32VIN/12VOUT Isolated Flyback Converter VIN 8V TO 32V Z1 C1 10µF R1 806k D1 VIN EN/UVLO R2 232k SW RFB RREF INTVCC C2 1µF D2 • C4 47µF 9µH R6 OPEN TC D1: DIODES DFLS1100 D2: DIODES PDS360 T1: SUMIDA 12387-TO79 Z1: CENTRAL CMZ5934B R5 10k 8302 TA02a Load and Line Regulation 95 12.4 90 12.2 OUTPUT VOLTAGE (V) EFFICIENCY (%) Efficiency vs Load Current 85 80 75 70 65 200 400 600 800 1000 LOAD CURRENT (mA) 12.0 11.8 11.6 11.
PAGE 22
LT8302 TYPICAL APPLICATIONS 8V to 36VIN/±12VOUT Isolated Flyback Converter T1 D2 1:1:1 VIN 8V TO 36V Z1 C1 10µF R1 806k R2 232k C2 1µF D1 VIN EN/UVLO SW RFB RREF INTVCC TC • R4 121k LT8302 GND C3 470pF R3 9µH 39Ω • C4 22µF D3 • R6 OPEN 9µH R5 10k 9µH C5 22µF VOUT1+ 12V 5mA TO 0.4A (VIN = 12V) 5mA TO 0.55A (VIN = 24V) VOUT2– VOUT2+ 12V 5mA TO 0.4A (VIN = 12V) 5mA TO 0.
PAGE 23
LT8302 TYPICAL APPLICATIONS 8V to 32VIN/5VOUT Isolated Flyback Converter with LT8309 C3 470pF R3 9µH 39Ω • Z1 C1 10µF R1 806k R2 232k C2 1µF D1 VIN EN/UVLO SW RFB RREF INTVCC • R6 OPEN TC 1µH D2 R8 2.1k VCC DRAIN LT8309 GATE INTVCC GND M1 D1: DIODES DFLS1100 D2: CENTRAL CMMSH1-60 M1: INFINEON BSC059N04LS T1: WURTH 750311564 Z1: CENTRAL CMZ5934B 95 90 C4 220µF R7 5Ω C4 10µF R5 10k Efficiency vs Load Current VOUT+ 5V/2.0A (VIN = 12V) 5V/2.
PAGE 24
LT8302 PACKAGE DESCRIPTION S8E Package 8-Lead Plastic SOIC (Narrow .150 Inch) Exposed Pad (Reference LTC DWG # 05-08-1857 Rev C) .050 (1.27) BSC .189 – .197 (4.801 – 5.004) NOTE 3 .045 ±.005 (1.143 ±0.127) .005 (0.13) MAX 7 5 6 8 .089 .160 ±.005 (2.26) (4.06 ±0.127) REF .245 (6.22) MIN .150 – .157 .080 – .099 (2.032 – 2.530) (3.810 – 3.988) NOTE 3 .228 – .244 (5.791 – 6.197) 1 .030 ±.005 (0.76 ±0.127) TYP .118 (2.99) REF 3 2 .118 – .139 (2.997 – 3.550) 4 RECOMMENDED SOLDER PAD LAYOUT .010 – .
PAGE 25
LT8302 REVISION HISTORY REV DATE DESCRIPTION A 11/14 Modified IQ and IHYS conditions. PAGE NUMBER 3 Modified LPRI equation. 14 Modified schematic. 23 Updated Related Parts. 26 B 11/15 Revised package drawing. 24 C 9/16 Reduced EN/UVLO shutdown threshold. 3 Increased IINTVCC max current limit. 3 D 5/19 Changed ISW(MIN) current limit range. 3 Corrected ILOAD(MIN) equation. 20 Changed VIN minimum from 2.8V to 3V.
PAGE 26
LT8302 TYPICAL APPLICATION 4V to 42VIN/48VOUT Boost Converter L1 22µH VIN 4V TO 42V VIN SW RFB EN/UVLO C1 10µF LT8302 R3 1M C3 10µF R4 Z1 464k RREF INTVCC C2 1µF VOUT 48V/1.4A (VIN = 42V) 48V/0.8A (VIN = 24V) 48V/0.4A (VIN = 12V) 48V/0.