Linear Technology LT8302 Instruction Manual

LT8302
1
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
SW
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
TC
VOUT+
5V
LOAD CURRENT (A)
0
EFFICIENCY (%)
80
85
90
1.5 2.5
8302 TA01b
75
70
0.5 1.0 2.0 3.0
65
60
VIN = 5V
VIN = 12V
VIN = 24V
FRONT PAGE APPLICATION

LT8302
2
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)
1
2
3
4
8
7
6
5
TOP VIEW
TC
RREF
RFB
SW
EN/UVLO
INTVCC
VIN
GND
S8E PACKAGE
8-LEAD PLASTIC SO
9
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
3
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
53
106
380
2 µA
µA
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
0
2.5
0
0.1
2.7
0.1
µA
µA
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
4
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)
0
IQ(µA)
6
8
10
40
8302 G07
4
2
010 20 30 50
TJ= 150°C
TJ= 25°C
TJ= –55°C
VIN (V)
0
80
IQ(µA)
90
100
110
120
130
140
10 20 30 40
8302 G08
50
TJ= 150°C
TJ= –55°C
TJ= 25°C
VIN (V)
0
IQ(µA)
380
400
420
40
8302 G09
360
340
320 10 20 30 50
TJ= 150°C
TJ= –55°C
TJ= 25°C
LOAD CURRENT (A)
0 0.5
0
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)
0
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
5
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)
3
4
5
25 75 150
8302 G11
2
1
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)
5
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)
0
20
150
8302 G15
–20
–40 050 100
–25 25 75 125
40
–10
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)
0
VRREF (V)
1.002
1.006
1.010
40
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
6
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
80
40
0–25 0 50 100 125
TEMPERATURE (°C)
–50
ISW (A)
3
4
5
25 75 150
8302 G20
2
1
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)
12
16
20
25 75 150
8302 G22
8
4
0–25 0 50 100 125
TEMPERATURE (°C)
–50
0
TIME (ns)
100
200
300
400
–25 0 25 50
8302 G23
75 100 125 150
TEMPERATURE (°C)
–50
0
TIME (ns)
100
200
300
400
–25 0 25 50
8302 G24
75 100 125 150

LT8302
7
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
8
8302fb
For more information www.linear.com/LT8302
block DiagraM
–
+
–
+
3
2
8
DRIVER
INTVCC
VIN
T1
N:1
A2 RSENSE
A3
TC
8302 BD
RREF
RREF
REN2
REN1
RTC
RFB
–
+
gm
1.214V
1V
M4
OSCILLATOR
LDO
BOUNDARY
DETECTOR
START-UP,
REFERENCE,
CONTROL
PTAT
VOLTAGE
R M1
GND
4, EXPOSED PAD PIN 9
Q
S
M2
VIN
VIN
CIN
6
RFB
5
SW
L1A L1B COUT
DOUT
VOUT+
VOUT–
M3
25µA
INTVCC
1EN/UVLO
CINTVCC
2.5µA
1:4
7
–
+
A1
•
•

LT8302
9
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
10
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.