Linear Technology LT8302 Mode d’emploi

LT8302

8302fb

For more information www.linear.com/LT8302

Typical applicaTion

FeaTures DescripTion

42VIN Micropower No-Opto

Isolated Flyback Converter

with 65V/3.6A Switch

The LT

8302 is a monolithic micropower isolated flyback

converter.Bysamplingtheisolatedoutputvoltagedirectly

from the primary-side flyback waveform, the part requires

nothirdwindingoropto-isolatorforregulation.Theoutput

voltage is programmed with two external resistors and a

thirdoptionaltemperaturecompensationresistor.Bound-

arymodeoperationprovidesasmallmagneticsolutionwith

excellentloadregulation.LowrippleBurstModeoperation

maintainshighefficiencyatlightloadwhileminimizingthe

output voltage ripple. A 3.6A, 65V DMOS power switch

is integrated along with all the high voltage circuitry and

controllogicintoathermallyenhanced8-leadSOpackage.

The LT8302 operates from an input voltage range of 2.8V

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

componentcount, and high efficiency applicationsolution

for isolated power delivery.

2.8V to 32VIN/5VOUT Isolated Flyback Converter

applicaTions

n2.8V to 42V Input Voltage Range

n3.6A, 65V Internal DMOS Power Switch

nLow Quiescent Current:

106µA in Sleep Mode

380µA in Active Mode

nQuasi-Resonant Boundary Mode Operation at

Heavy Load

nLow Ripple Burst Mode

Operation at Light Load

nMinimum Load < 0.5% (Typ) of Full Output

nNo Transformer Third Winding or Opto-Isolator

Required for Output Voltage Regulation

nAccurate EN/UVLO Threshold and Hysteresis

nInternal Compensation and Soft-Start

nTemperature Compensation for Output Diode

nOutput Short-Circuit Protection

nThermally Enhanced 8-Lead SO Package

nIsolated Automotive, Industrial, Medical Power

Supplies

nIsolated Auxiliary/Housekeeping Power Supplies

L, LT, LTC , LT M, Linear Technology, the Linear logo and Burst Mode are registered trademarks

of Linear Technology Corporation. All other trademarks are the property of their respective

owners. Protected by U.S. Patents, including 5438499, 7463497, 7471522.

Efficiency vs Load Current

VIN

LT8302

9µH

VIN

2.8V TO 32V 3:1

1µH

RFB

RREF

EN/UVLO

470pF

10µF

1µF

220µF

10mA TO 1.1A (VIN = 5V)

10mA TO 2.0A (VIN = 12V)

10mA TO 2.9A (VIN = 24V)

VOUT–

39Ω

154k

•

115k 10k

8302 TA01a

GND

INTVCC

VOUT+

LOAD CURRENT (A)

EFFICIENCY (%)

1.5 2.5

8302 TA01b

0.5 1.0 2.0 3.0

VIN = 5V

VIN = 12V

VIN = 24V

FRONT PAGE APPLICATION

LT8302

8302fb

For more information www.linear.com/LT8302

pin conFiguraTionabsoluTe MaxiMuM raTings

SW (Note 2) ..............................................................65V

VIN ............................................................................42V

EN/UVLO....................................................................VIN

RFB........................................................VIN – 0.5V to VIN

Current Into RFB....................................................200µA

INTVCC, RREF, TC.........................................................4V

Operating Junction Temperature Range (Notes 3, 4)

LT8302E, LT8302I.............................. –40°C to 125°C

LT8302H............................................–40°C to 150°C

LT8302MP ......................................... –55°C to 150°C

Storage Temperature Range .................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec)...................300°C

(Note 1)

TOP VIEW

RREF

RFB

EN/UVLO

INTVCC

VIN

GND

S8E PACKAGE

8-LEAD PLASTIC SO

GND

θJA = 33°C/W

EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB

orDer inForMaTion

LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE

LT8302ES8E#PBF LT8302ES8E#TRPBF 8302 8-Lead Plastic SO –40°C to 125°C

LT8302IS8E#PBF LT8302IS8E#TRPBF 8302 8-Lead Plastic SO –40°C to 125°C

LT8302HS8E#PBF LT8302HS8E#TRPBF 8302 8-Lead Plastic SO –40°C to 150°C

LT8302MPS8E#PBF LT8302MPS8E#TRPBF 8302 8-Lead Plastic SO –55°C to 150°C

Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/. Some packages are available in 500 unit reels through

designated sales channels with #TRMPBF suffix.

LT8302

8302fb

For more information www.linear.com/LT8302

elecTrical characTerisTics

The ldenotes 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 CONDITIONS MIN TYP MAX UNIT

VIN VIN Voltage Range l2.8 42 V

IQVIN Quiescent Current VEN/UVLO = 0.3V

VEN/UVLO = 1.1V

Sleep Mode (Switch Off)

Active Mode (Switch On)

0.5

106

380

2 µA

µA

EN/UVLO Shutdown Threshold For Lowest Off IQl0.3 0.75 V

EN/UVLO Enable Threshold Falling l1.178 1.214 1.250 V

EN/UVLO Enable Hysteresis 14 mV

IHYS EN/UVLO Hysteresis Current VEN/UVLO = 0.3V

VEN/UVLO = 1.1V

VEN/UVLO = 1.3V

–0.1

2.3

–0.1

2.5

0.1

2.7

0.1

µA

VINTVCC INTVCC Regulation Voltage IINTVCC = 0mA to 10mA 2.85 3 3.1 V

IINTVCC INTVCC Current Limit VINTVCC = 2.8V 10 13 16 mA

INTVCC UVLO Threshold Falling 2.39 2.47 2.55 V

INTVCC UVLO Hysteresis 105 mV

(RFB – VIN) Voltage IRFB = 75µA to 125µA –50 50 mV

RREF Regulation Voltage l0.98 1.00 1.02 V

RREF Regulation Voltage Line Regulation 2.8V ≤ VIN ≤ 42V –0.01 0 0.01 %/V

VTC TC Pin Voltage 1.00 V

ITC TC Pin Current VTC = 1.2V

VTC = 0.8V 12 15

–200 18 µA

µA

fMIN Minimum Switching Frequency 11.3 12 12.7 kHz

tON(MIN) Minimum Switch-On Time 160 ns

tOFF(MAX) Maximum Switch-Off Time Backup Timer 170 µs

ISW(MAX) Maximum Switch Current Limit 3.6 4.5 5.4 A

ISW(MIN) Minimum Switch Current Limit 0.78 0.87 0.96 A

RDS(ON) Switch On-Resistance ISW = 1.5A 80 mΩ

ILKG Switch Leakage Current VSW = 65V 0.1 0.5 µA

tSS Soft-Start Timer 11 ms

Note 1: Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

Note 2: The SW pin is rated to 65V for transients. Depending on the

leakage inductance voltage spike, operating waveforms of the SW pin

should be derated to keep the flyback voltage spike below 65V as shown

in Figure 5.

Note 3: The LT8302E is guaranteed to meet performance specifications

from 0°C to 125°C junction temperature. Specifications over the –40°C

to 125°C operating junction temperature range are assured by design,

characterization and correlation with statistical process controls. The

LT8302I is guaranteed over the full –40°C to 125°C operating junction

temperature range. The LT8302H is guaranteed over the full –40°C to

150°C operating junction temperature range. The LT8302MP is guaranteed

over the full –55°C to 150°C operating junction temperature range. High

junction temperatures degrade operating lifetimes. Operating lifetime is

derated at junction temperature greater than 125°C.

Note 4: The LT8302 includes overtemperature protection that is intended

to protect the device during momentary overload conditions. Junction

temperature will exceed 150°C when overtemperature protection is active.

Continuous operation above the specified maximum operating junction

temperature may impair device reliability.

LT8302

8302fb

For more information www.linear.com/LT8302

Typical perForMance characTerisTics

Boundary Mode Waveforms Discontinuous Mode Waveforms Burst Mode Waveforms

VIN Shutdown Current

VIN Quiescent Current,

Sleep Mode

VIN Quiescent Current,

Active Mode

Output Load and Line Regulation Output Temperature Variation

Switching Frequency

vs Load Current

TA= 25°C, unless otherwise noted.

TEMPERATURE (°C)

–50

OUTPUT VOLTAGE (V)

5.1

5.2

5.3

25 75

8302 G02

5.0

4.9

–25 0 50 100 150125

4.8

4.7

FRONT PAGE APPLICATION

VIN = 12V

IOUT = 1A

RTC = 115k

RTC = OPEN

VSW

20V/DIV

VOUT

50mV/DIV

2µs/DIV

FRONT PAGE APPLICATION

VIN = 12V

IOUT = 2A

8302 G04

VSW

20V/DIV

VOUT

50mV/DIV

2µs/DIV

FRONT PAGE APPLICATION

VIN = 12V

IOUT = 0.5A

8302 G05

VSW

20V/DIV

VOUT

50mV/DIV

20µs/DIV

FRONT PAGE APPLICATION

VIN = 12V

IOUT = 10mA

8302 G06

VIN (V)

IQ(µA)

8302 G07

010 20 30 50

TJ= 150°C

TJ= 25°C

TJ= –55°C

VIN (V)

IQ(µA)

100

110

120

130

140

10 20 30 40

8302 G08

TJ= 150°C

TJ= –55°C

TJ= 25°C

VIN (V)

IQ(µA)

380

400

420

8302 G09

360

340

320 10 20 30 50

TJ= 150°C

TJ= –55°C

TJ= 25°C

LOAD CURRENT (A)

0 0.5

FREQUENCY (kHz)

200

500

1.0 2.0 2.5

8302 G03

100

400

300

1.5 3.0

VIN = 5V

VIN = 12V

VIN = 24V

FRONT PAGE APPLICATION

LOAD CURRENT (A)

OUTPUT VOLTAGE (V)

5.15

1.5

8302 G01

5.00

4.90

0.5 1.0 2.0

4.85

4.80

5.20

5.10

5.05

4.95

2.5 3.0

VIN = 5V

VIN = 12V

VIN = 24V

LT8302

8302fb

For more information www.linear.com/LT8302

Typical perForMance characTerisTics

INTVCC Voltage vs VIN INTVCC UVLO Threshold (RFB-VIN) Voltage

RREF Regulation Voltage RREF Line Regulation TC Pin Voltage

EN/UVLO Enable Threshold EN/UVLO Hysteresis Current INTVCC Voltage vs Temperature

TA= 25°C, unless otherwise noted.

TEMPERATURE (°C)

–50

VEN/UVLO (V)

1.220

1.230

150

8302 G10

1.210

1.200 050 100

–25 25 75 125

1.240

1.215

1.225

1.205

1.235 RISING

FALLING

TEMPERATURE (°C)

–50

IHYST (µA)

25 75 150

8302 G11

0–25 0 50 100 125

TEMPERATURE (°C)

–50

2.80

VINTVCC (V)

2.85

2.90

2.95

3.00

0 50 100 150

8302 G12

3.05

3.10

–25 25 75 125

IINTVCC = 0mA

IINTVCC = 10mA

VIN (V)

VINTVCC (V)

2.95

3.00

3.05

35 4020 25 30

8302 G13

2.90

2.85

10 15 45

2.80

3.10

IINTVCC = 0mA

IINTVCC = 10mA

TEMPERATURE (°C)

–50

2.2

VINTVCC (V)

2.3

2.4

2.5

2.6

0 50 100 150

8302 G14

2.7

2.8

–25 25 75 125

FALLING

RISING

TEMPERATURE (°C)

–50

VOLTAGE (mV)

150

8302 G15

–20

–40 050 100

–25 25 75 125

–10

–30

30 IRFB = 125µA

IRFB = 100µA

IRFB = 75µA

TEMPERATURE (°C)

–50

0.990

VRREF (V)

0.992

0.996

0.998

1.000

1.010

1.004

050 75

8302 G16

0.994

1.006

1.008

1.002

–25 25 100 125 150

VIN (V)

VRREF (V)

1.002

1.006

1.010

8302 G17

0.998

0.994

1.000

1.004

1.008

0.996

0.992

0.990 10 20 30 50

TEMPERATURE (°C)

–50

VTC (V)

1.1

1.3

150

8302 G18

0.9

0.7 050 100

–25 25 75 125

1.5

1.0

1.2

0.8

1.4

LT8302

8302fb

For more information www.linear.com/LT8302

Minimum Switching Frequency Minimum Switch-On Time Minimum Switch-Off Time

RDS(ON) Switch Current Limit Maximum Switching Frequency

Typical perForMance characTerisTics

TA= 25°C, unless otherwise noted.

TEMPERATURE (°C)

–50

RESISTANCE (mΩ)

120

160

200

25 75 150

8302 G19

0–25 0 50 100 125

TEMPERATURE (°C)

–50

ISW (A)

25 75 150

8302 G20

0–25 0 50 100 125

MAXIMUM CURRENT LIMIT

MINIMUM CURRENT LIMIT

TEMPERATURE (°C)

–50

FREQUENCY (kHz)

300

400

500

25 75 150

8302 G21

200

100

0–25 0 50 100 125

TEMPERATURE (°C)

–50

FREQUENCY (kHz)

25 75 150

8302 G22

0–25 0 50 100 125

TEMPERATURE (°C)

–50

TIME (ns)

100

200

300

400

–25 0 25 50

8302 G23

75 100 125 150

TEMPERATURE (°C)

–50

TIME (ns)

100

200

300

400

–25 0 25 50

8302 G24

75 100 125 150

LT8302

8302fb

For more information www.linear.com/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 ac-

curate 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

allowsthe programming of VIN UVLO hysteresis.If neither

function is used, tie this pin directly to VIN.

INTVCC (Pin 2): Internal 3V Linear Regulator Output. The

INTVCC pin is supplied from VIN and powers the internal

control circuitry and gate driver. Do not overdrive the

INTVCC pinwithanyexternalsupply,suchasathirdwinding

supply. Locally bypass this pin to ground with a minimum

1µF ceramic capacitor.

VIN (Pin 3): Input Supply. The VIN pin supplies current to

the internal circuitry and serves as a reference voltage for

the feedback circuitry connected to the RFB pin. Locally

bypass this pin to ground with a capacitor.

GND (Pin 4, Exposed Pad Pin 9): Ground. The exposed

pad provides both electrical contact to ground and good

thermal contact to the printed circuit board. Solder the

exposed pad directly to the ground plane.

SW (Pin 5): Drain of the Internal DMOS Power Switch.

Minimize trace area at this pin to reduce EMI and voltage

spikes.

RFB (Pin 6): Input Pin for External Feedback Resistor.

Connectaresistorfromthispintothetransformer primary

SW pin. The ratio of the RFB resistor to the RREF resistor,

timestheinternalvoltagereference,determinestheoutput

voltage (plus the effect of any non-unity transformer turns

ratio). Minimize trace area at this pin.

RREF (Pin 7): Input Pin for External Ground Referred Ref-

erence Resistor. The resistor at this pin should be in the

range of 10k, but for convenience in selecting a resistor

divider ratio, the value may range from 9.09k to 11.0k.

TC(Pin8): OutputVoltageTemperatureCompensation.The

voltage at this pin is proportional to absolute temperature

(PTAT) with temperature coefficient equal to 3.35mV/°K,

i.e., equal to 1V at room temperature 25°C. The TC pin

voltage can be used to estimate the LT8302 junction tem-

perature. Connect a resistor from this pin to the RREF pin

to compensate the output diode temperature coefficient.

LT8302

8302fb

For more information www.linear.com/LT8302

block DiagraM

–

DRIVER

INTVCC

VIN

N:1

A2 RSENSE

8302 BD

RREF

REN2

REN1

RTC

RFB

–

1.214V

OSCILLATOR

LDO

BOUNDARY

DETECTOR

START-UP,

REFERENCE,

CONTROL

PTAT

VOLTAGE

R M1

GND

4, EXPOSED PAD PIN 9

VIN

CIN

RFB

L1A L1B COUT

DOUT

VOUT+

VOUT–

25µA

INTVCC

1EN/UVLO

CINTVCC

2.5µA

1:4

–

•

LT8302

8302fb

For more information www.linear.com/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

theoutputvoltageinformationfromtheisolatedsecondary

side of the transformer to the primary side for regulation.

Historically, opto-isolators or extra transformer windings

communicatethisinformation acrosstheisolationbound-

ary. Opto-isolator circuits waste output power, and the

extra components increase the cost and physical size of

the power supply. Opto-isolators can also cause system

issuesduetolimiteddynamicresponse,nonlinearity,unit-

to-unitvariationandagingoverlifetime.Circuitsemploying

extra transformer windings also exhibit deficiencies, as

using an extra winding adds to the transformer’s physical

size and cost, and dynamic response is often mediocre.

The LT8302 samples the isolated output voltage through

the primary-side flyback pulse waveform. In this manner,

neither opto-isolator nor extra transformer winding is re-

quired for regulation. Since the LT8302 operates in either

boundary conduction mode or discontinuous conduction

mode, the output voltage is always sampled on the SW

pin when the secondary current is zero. This method im-

proves load regulation without the need of external load

compensation components.

The LT8302 is a simple to use micropower isolated fly-

back converter housed in a thermally enhanced 8-lead

SO package. The output voltage is programmed with two

external resistors. An optional TC resistor provides easy

output diode temperature compensation. By integrating

the loop compensation and soft-start inside, the part

reduces the number of external components. As shown

in the Block Diagram, many of the blocks are similar to

those found in traditional switching regulators including

reference, regulators, oscillator, logic, current amplifier,

current comparator, driver, and power switch. The novel

sections include a flyback pulse sense circuit, a sample-

and-hold error amplifier, and a boundary mode detector,

as well as the additional logic for boundary conduction

mode, discontinuous conduction mode, and low ripple

Burst Mode operation.

Quasi-Resonant Boundary Mode Operation

TheLT8302featuresquasi-resonantboundaryconduction

mode operation at heavy load, where the chip turns on the

primary power switch when the secondary current is zero

and the SW rings to its valley. Boundary conduction mode

is a variable frequency, variable peak-current switching

scheme. The power switch turns on and the transformer

primarycurrentincreasesuntilaninternallycontrolledpeak

current limit. After the power switch turns off, the voltage

on the SW pin rises to the output voltage multiplied by

the primary-to-secondary transformer turns ratio plus the

input voltage. When the secondary current through the

output diode falls to zero, the SW pin voltage collapses

and rings around VIN. A boundary mode detector senses

this event and turns the power switch back on at its valley.

LT8302

8302fb

For more information www.linear.com/LT8302

operaTion

Boundaryconductionmodereturnsthesecondarycurrent

to zero every cycle, so parasitic resistive voltage drops

do not cause load regulation errors. Boundary conduc-

tion 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 in-

creases the switching frequency and decreases the switch

peakcurrentatthesameratio.Runningatahigherswitching

frequency up to several MHz increases switching and gate

charge losses. To avoid this scenario, the LT8302 has an

additional internal oscillator, which clamps the maximum

switching frequency to be less than 380kHz. Once the

switching frequency hits the internal frequency clamp,

the part starts to delay the switch turn-on and operates

in discontinuous conduction mode.

Low Ripple Burst Mode Operation

Unlike traditional flyback converters, the LT8302 has to

turn on and off at least for a minimum amount of time

andwithaminimumfrequencytoallowaccuratesampling

of the output voltage. The inherent minimum switch cur-

rent limit and minimum switch-off time are necessary to

guarantee the correct operation of specific applications.

As the load gets very light, the LT8302 starts to fold back

theswitchingfrequencywhilekeepingtheminimumswitch

current limit. So the load current is able to decrease while

stillallowing minimumswitch-offtimeforthesample-and-

holderroramplifier.Meanwhile,thepartswitchesbetween

sleep mode and active mode, thereby reducing the effec-

tive quiescent current to improve light load efficiency. In

this condition, the LT8302 runs in low ripple Burst Mode

operation.Thetypical12kHzminimumswitchingfrequency

determines how often the output voltage is sampled and

also the minimum load requirement.

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