LT1249IS8#TRPBF [Linear]
LT1249 - Power Factor Controller; Package: SO; Pins: 8; Temperature Range: -40°C to 85°C;型号: | LT1249IS8#TRPBF |
厂家: | Linear |
描述: | LT1249 - Power Factor Controller; Package: SO; Pins: 8; Temperature Range: -40°C to 85°C 稳压器 开关式稳压器或控制器 电源电路 开关式控制器 光电二极管 |
文件: | 总12页 (文件大小:146K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LT1249
Power Factor Controller
U
FEATURES
DESCRIPTIO
The 8-pin LT®1249 provides active power factor correc-
tion for universal offline power systems with very few
externalparts.ByusingfixedhighfrequencyPWMcurrent
averaging without the need for slope compensation, the
LT1249 achieves far lower line current distortion, with a
smallermagneticelementthansystemsthatuseeitherpeak
current detection or zero current switching approach, in
both continuous and discontinuous modes of operation.
■
Standard 8-Pin Packages
■
High Power Factor Over Wide Load Range
with Line Current Averaging
■
International Operation Without Switches
■
Instantaneous Overvoltage Protection
■
Minimal Line Current Dead Zone
■
Typical 250µA Start-Up Supply Current
■
Rejects Line Switching Noise
■
Synchronization Capability
The LT1249 uses a multiplier containing a square gain
function from the voltage amplifier to reduce the AC gain
at light output load and thus maintains low line current
distortion and high system stability. The LT1249 also
provides filtering capability to reject line switching noise
which can cause instability when fed into the multiplier.
Line current dead zone is minimized with low bias voltage
at the current input to the multiplier.
■
Low Quiescent Current: 9mA
■
Fast 1.5A Peak Current Gate Driver
U
APPLICATIO S
■
Universal Power Factor Corrected Power Supplies
Preregulators up to 1500W
■
The LT1249 provides many protection features including
peak current limiting and overvoltage protection. The
switching frequency is internally set at 100kHz.
While the LT1249 simplifies PFC design with minimal
parts count, the LT1248 provides flexibilities in switching
frequency, overvoltage and current limit.
, LTC and LT are registered trademarks of Linear Technology Corporation.
W
BLOCK DIAGRA
CA
M
GND
1
V
VA
OUT
OUT
3
CC
7
OUT
2
5
7.5V
REF
V
R
4k
MOUT
+
–
7.5V
V
SENSE
6
+
–
V
RUN
EA
CC
I
I
A
250µA MAX
16V/10V
MULTIPLIER
2 I
I
M
I
AC
32k
1V
+
I
A
B
I
=
B
M
4
200µA2
–
+
CA
–
R
S
Q
GTDR
8
15µA
+
g
= 1/3k
RUN
m
+
0.7V
M1
–
OSC
+
–
SYNC
4k
16V
44µA
22µA
20µA
35pF
1249 BD
1
LT1249
W W U W
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ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
Supply Voltage ....................................................... 27V
GTDR Current Continuous ..................................... 0.5A
GTDR Output Energy (Per Cycle) ............................. 5µJ
ORDER PART
NUMBER
TOP VIEW
GND
1
2
3
4
GTDR
8
7
6
5
LT1249CN8
LT1249IN8
LT1249CS8
LT1249IS8
IAC Input Current ................................................. 20mA
CA
V
OUT
OUT
CC
VSENSE Input Voltage ............................................ VMAX
MOUT Input Current.............................................. ±5mA
Operating Junction Temperature Range
M
V
SENSE
I
VA
AC
OUT
N8 PACKAGE
8-LEAD PDIP
LT1249C................................................ 0°C to 100°C
LT1249I ........................................... –40°C to 125°C
Thermal Resistance (Junction-to-Ambient)
N8 Package ................................................ 100°C/W
S8 Package................................................. 120°C/W
Storage Temperature Range ..................–65°C to 150°C
Lead Temperature (Soldering, 10 sec)................. 300°C
S8 PART
MARKING
S8 PACKAGE
8-LEAD PLASTIC SO
TJMAX = 125°C, θJA = 100°C/W (N8)
JMAX = 125°C, θJA = 120°C/W (S8)
1249
1249I
T
Consult factory for Military grade parts.
The ● denotes specifications which apply over the operating temperature
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are at TA = 25°C. Maximum operating voltage (VMAX) = 25V, VCC = 18V, IAC = 100µA, CAOUT = 3.5V,
VAOUT = 5V, no load on any outputs, unless otherwise noted.
PARAMETER
Overall
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Current (V in Undervoltage Lockout)
Supply Current, On
V
= Lockout Voltage – 0.2V
●
●
●
●
0.25
8
16.5
10.5
0.45
12
17.5
11.5
mA
mA
V
CC
CC
11.5V ≤ V ≤ V
, CA
= 1V
CC
MAX
OUT
V
V
Turn-On Threshold
Turn-Off Threshold
15.5
9.5
CC
V
CC
Voltage Amplifier
Bias Current
Voltage Amp Gain
Voltage Amp Unity-Gain Bandwidth
Voltage Amp Output High
Voltage Amp Output Low
Voltage Amp Source Current
Voltage Amp Sink Current Threshold
Voltage Amp Sink Current Hysteresis
Current Amplifier
V
V
= 0V to 7V
SENSE
●
–25
100
1.5
12
0.1
260
44
–250
nA
dB
MHz
V
SENSE
70
10
0 ≤ Source Current ≤ 50µA
0 ≤ Sink Current ≤ 5µA
●
●
●
●
●
0.4
450
57
V
130
33
14
µA
µA
µA
Linear Operation, 2V < VA
2V < VA
< 10V
OUT
< 10V
22.5
30
OUT
Current Amp Offset Voltage
Current Amp Transconductance
Current Amp Voltage Gain
Current Amp Source Current
Current Amp Sink Current
Current Amp Output High
Current Amp Output Low
●
●
±2
320
1000
145
95
±15
550
mV
µmho
V/V
µA
∆I
2.5V ≤ V
= ±40µA
150
500
100
67
CAOUT
≤ 7.5V
CAOUT
V
V
= 1V, I = 0µV
220
125
MOUT
MOUT
M
= –0.3V, I = 0µA
µA
V
V
M
7.4
8.1
1.2
2
2
LT1249
The ● denotes specifications which apply over the operating temperature
ELECTRICAL CHARACTERISTICS
range, otherwise specifications are at TA = 25°C. Maximum operating voltage (VMAX) = 25V, VCC = 18V, IAC = 100µA, CAOUT = 3.5V,
VAOUT = 5V, no load on any outputs, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Reference
Reference Output Voltage
Reference Output Voltage Worst Case
Reference Output Voltage Line Regulation
Multiplier
T = 25°C, Measured at V
All Line, Temperature
Pin
7.39
7.32
–20
7.5
7.5
5
7.6
7.68
20
V
V
mV
A
SENSE
●
●
V
< V < V
LOCKOUT
CC
MAX
Multiplier Output Current
Multiplier Output Current Offset
Multiplier Max Output Current (I
Multiplier Max Output Voltage (I
I
R
I
I
= 100µA, VA
= 5V
OUT
35
µA
µA
µA
V
V
kΩ
AC
= 1M from I to GND
●
●
●
–0.05
–250
–1.1
0.035
32
–0.5
–150
–0.96
AC
AC
)
= 450µA, VA
= 450µA, VA
= 7V (Note 2)
= 7V (Note 2)
– 375
–1.25
M(MAX)
AC
AC
OUT
OUT
• R
)
M(MAX)
MOUT
–2
Multiplier Gain Constant (Note 3)
Input Resistance
I
I
from 50µA to 1mA
15
50
AC
AC
Oscillator
Oscillator Frequency
●
●
●
75
1.3
127
100
1.8
125
2.3
160
kHz
V
kHz
Control Pin (CA ) Threshold
Duty Cycle = 0
Synchronizing Pulse Low ≤ 0.35V on CA
OUT
Synchronization Frequency Range
Gate Driver
OUT
Max GTDR Output Voltage
GTDR Output High
GTDR Output Low (Device Unpowered)
GTDR Output Low (Device Active)
Peak GTDR Current
0mA Load, 18V < V < V
–200mA Load, 11.5V ≤ V ≤ 15V
(Note 4)
●
●
●
●
12
– 3.0
CC
15
17.5
V
V
V
V
A
CC
MAX
V
CC
V
= 0V, 50mA Load (Sinking)
0.9
0.5
2
25
96
1.5
1
CC
200mA Load (Sinking)
10nF from GTDR to GND
1nF from GTDR to GND
GTDR Rise and Fall Time
GTDR Max Duty Cycle
ns
%
90
Note 3: Multiplier Gain Constant: K =
I
M
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
I
(VA
– 1.5)2
AC
OUT
Note 4: Maximum GTDR output voltage is internally clamped for higher
voltages.
Note 2: Current amplifier is in linear mode with 0V input common mode.
V
CC
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TYPICAL PERFORMANCE CHARACTERISTICS
Voltage Amplifier Open-Loop
Transconductance of
Current Amplifier
Gain and Phase
100
80
60
40
20
0
0
400
350
300
250
200
150
100
50
20
θ
0
g
m
–20
–40
–60
–80
–100
–120
–20
–40
–60
–80
–100
–120
–140
GAIN
PHASE
0
–20
10
1k
10k 100k
1M
10M
1k
10k
100k
1M
10M
100
FREQUENCY (Hz)
FREQUENCY (Hz)
1249 G01
1249 G02
3
LT1249
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TYPICAL PERFORMANCE CHARACTERISTICS
Reference Voltage vs
Temperature
Multiplier Current
7.536
7.524
7.512
7.500
7.488
7.476
7.464
7.452
7.440
7.428
300
150
0
VA
= 5V
VA
= 6.5V
OUT
OUT
VA
= 6V
OUT
VA
= 5.5V
VA
VA
= 4.5V
= 4V
OUT
OUT
OUT
VA
VA
= 3.5V
= 3V
OUT
OUT
VA
VA
= 2.5V
= 2V
OUT
OUT
125
150
0
250
(µA)
500
–75 –50
0
25 50
100
75
–25
JUNCTION TEMPERATURE (°C)
I
AC
1249 G04
1249 G03
Supply Current vs Supply Voltage
GTDR Source Current
GTDR Sink Current
18.5
18.0
17.5
17.0
16.5
16.0
15.5
15.0
14.5
14.0
13.5
13.0
10
9
8
7
6
5
4
3
2
1
0
1.1
T
T
= –55°C
= 25°C
V
CC
= 18V
J
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
J
T
= 125°C
J
T = 125°C
J
T
= –55°C
A
T = 25°C
J
T = –55°C
J
T
= 25°C
A
T
= 125°C
A
0
–120
–180
–240
–300
–60
10 12 14 16 18 20 22 24 26 28 30
0
120
180
240
300
60
SOURCE CURRENT (mA)
SUPPLY VOLTAGE (V)
SINK CURRENT (mA)
1249 G06
1249 G07
1249 G05
Start-Up Supply Current vs
Supply Voltage
GTDR Rise and Fall Time
Switching Frequency
400
300
200
100
0
550
500
450
400
350
300
250
200
150
100
50
140
130
120
110
100
90
FALL TIME
RISE TIME
–55°C
25°C
125°C
80
NOTE: GTDR SLEWS
BETWEEN 1V AND 16V
70
0
0
20
30
40
50
10
0
8
12 14 16 18 20
–50 –25
0
25 50
100 125
2
4
6
10
–75
75
LOAD CAPACITANCE (nF)
SUPPLY VOLTAGE (V)
TEMPERATURE (°C)
1249 G08
1249 G09
1249 G10
4
LT1249
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TYPICAL PERFORMANCE CHARACTERISTICS
Synchronization Threshold
at CAOUT
Transconductance of Current
Amplifier Over Temperature
MOUT Pin Characteristics
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.2
1.0
400
350
300
250
200
150
100
50
125°C
25°C
0.8
–50°C
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
0
–50
–50 –25
0
25
50
125
75
–2.4
0
2.4
–1.2
M
–25
0
25
50
75
125
100
1.2
100
TEMPERATURE (°C)
VOLTAGE (V)
TEMPERATURE (°C)
OUT
1249 G12
1249 G11
1249 G13
Voltage Amp Sink Current Limits
(Threshold)
Maximum Multiplier Output
Voltage (IM(MAX) • RMOUT
)
Maximum Duty Cycle
100
99
98
97
96
95
94
93
92
91
90
60
50
40
30
20
10
0
–1.30
–1.25
–1.20
–1.15
–1.10
–1.05
–1.00
–0.95
–0.90
UP THRESHOLD
DOWN THRESHOLD
75
–75 –50 –25
0
25 50
100 125
–50 –25
0
25
50
125
75
75
–75 –50 –25
0
25 50
100 125
100
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
NOTE: THESE SINK CURRENT THRESHOLDS ARE
FOR OVERVOLTAGE PROTECTION FUNCTION.
1249 G16
1249 G15
1249 G14
U
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PIN FUNCTIONS
is normally at negative potential and only AC signals
appear at the noninverting input of the current amplifier.
GND (Pin 1): Ground.
CAOUT (Pin 2): This is the output of the current amplifier
that senses and forces the line current to follow the
reference signal that comes from the multiplier by com-
manding the pulse width modulator. When CAOUT is low,
the modulator has zero duty cycle.
IAC (Pin 4): This is the AC line voltage sensing input to the
multiplier. It is a current input that is biased at 2V to
minimize the crossover dead zone caused by low line
voltage. A 32k resistor is in series with the current input,
so that a small external capacitor can be used to filter out
the switching noise from the high impedance lines.
MOUT (Pin 3): The multiplier current goes out of this pin
through the 4k resistor RMOUT. The voltage developed
across RMOUT is the reference voltage of the current loop
and it is limited to 1.1V. The noninverting input of the
current amplifier is also tied to RMOUT. In operation, MOUT
VAOUT (Pin 5): This is the output of the voltage error
amplifier. The output is clamped at 12V. When the output
goes below 1.5V, the multiplier output current is zero.
5
LT1249
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PIN FUNCTIONS
VSENSE (Pin 6): This is the inverting input to the voltage
amplifier.
capacitor in parallel with a low ESR electrolytic capacitor,
56µF or higher is required in close proximity to IC GND.
V
CC (Pin 7): This is the supply of the chip. The LT1249 has
GTDR(Pin8):TheMOSFETgatedriverisa1.5Afasttotem
pole output. It is clamped at 15V. Capacitive loads like
MOSFET gates may cause overshoot. A gate series resis-
tor of at least 5Ω will prevent the overshoot.
a very fast gate driver required to fast charge high power
MOSFET gate capacitance. High current spikes occur
during charging. For good supply bypass, a 0.1µF ceramic
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APPLICATIONS INFORMATION
Multiplier
Error Amplifier
The multiplier is a current multiplier with high noise
immunity in a high power switching environment. The
current gain is:
Theerroramplifierhasa100dBDCgainand1.5MHzunity-
gain frequency. It is internally clamped at 12V. The nonin-
verting input is tied to the 7.5V reference.
IM = (IAC)(IEA2)/(200µA)2, and
IEA = (VAOUT – 1.5V)/25k
Current Amplifier
The multiplier output current IM flows out of the MOUT pin
through the 4k resistor RMOUT and develops the reference
signal to the current loop that is controlled by the current
amplifier. Current gain is the ratio of RMOUT to line current
senseresistor.Thecurrentamplifierisatransconductance
amplifier. Typical gm is 320µmho and gain is 60dB with no
load. The inverting input is internally tied to GND. The
noninverting input is tied to the multiplier output. The
output is internally clamped at 8V. Output resistance is
about 4M; DC loading should be avoided because it will
lower the gain and introduce offset voltage at the inputs
which becomes a false reference signal to the current loop
and can distort line current. Note that in the current
averaging operation, high gain at twice the line frequency
is necessary to minimize line current distortion. Because
CAOUT mayneedtoswing5Voveronelinecycleathighline
condition, 11mVwillbepresentattheinputsofthecurrent
amplifier if gain is rolled off to 450 at 120Hz (1nF in series
with10katCAOUT). Atlightload, when(IM)(RMOUT)canbe
less than 100mV, lower gain will distort the current loop
reference signal and line current. If signal gain at the
100kHz switching frequency is too high, the system
behaves more like a current mode system and can cause
subharmonic oscillation. Therefore, the current amplifier
should be compensated to have a gain of less than 15 at
100kHz and more than 300 at 120Hz.
With a square function, because of the lower gain at light
power load, system stability is maintained and line current
distortion caused by the AC ripple fed back to the error
amplifier is minimized. Note that switching ripple on the
highimpedancelinescouldgetintothemultiplierfromthe
IAC pin and cause instability. The LT1249 provides an
internal 25k resistor in series with the low impedance
multiplier current input so that only a capacitor from the
IAC pin to GND is needed to filter out the noise. Maximum
multiplier output current is limited to 250µA. Figure 1
shows the multiplier transfer curves.
300
VA
= 5V
VA
= 6.5V
OUT
OUT
VA
= 6V
OUT
VA
= 5.5V
VA
VA
= 4.5V
= 4V
OUT
OUT
OUT
150
VA
VA
= 3.5V
= 3V
OUT
OUT
VA
VA
500
= 2.5V
= 2V
OUT
OUT
0
0
250
(µA)
I
AC
1249 G04
Figure 1. Multiplier Current IM vs IAC and VAOUT
6
LT1249
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APPLICATIONS INFORMATION
Line Current Limiting
With ts = 30ns, fs = 130kHz, VC = 3V and R2 = 10k, offset
voltage shift is ≈5mV. Note that this offset voltage will add
slight distortion to line current at light load.
Maximum voltage across RMOUT is internally limited to
1.1V. Therefore, line current limit is 1.1V divided by the
sense resistor RS. With a 0.2Ω sense resistor RS line
current limit is 5.5A. As a general rule, RS is chosen
according
CA
OUT
V
CC
R1
10k
1N5712
80pF
(I
)(R
)(V
)
M(MAX)
MOUT LINE(MIN)
5V
0V
R2
10k
2N2369
R =
S
K(1.414)P
OUT(MAX)
2k
1nF
1249 F02
where POUT(MAX) is the maximum power output and K is
usually between 1.1 and 1.3 depending on efficiency and
resistor tolerance. When the output is overloaded and line
currentreacheslimit,outputvoltageVOUT willdroptokeep
line current constant. System stability is still maintained
by the current loop which is controlled by the current
amplifier. Further load current increase results in further
VOUT drop and clipping of the line current, which degrades
power factor.
Figure 2. Synchronizing the LT1249
Overvoltage Protection
In Figure 3, R1 and R2 set the regulator output DC level:
VOUT = VREF[(R1 + R2)/R2]. With R1 = 1M and R2 = 20k,
VOUT is 382V.
Because of the slow loop response necessary for power
factorcorrection,outputovershootcanoccurwithsudden
load removal or reduction. To protect the power compo-
nents and output load, the LT1249 voltage error amplifier
sensestheoutputvoltageandquicklyshutsoffthecurrent
switch when overvoltage occurs. When overshoot occurs
on VOUT, the overcurrent from R1 will go through VAOUT
because amplifier feedback keeps VSENSE locked at 7.5V.
When this overcurrent reaches 44µA amplifier sinking
limit, theamplifierlosesfeedbackanditsoutputsnapslow
to turn the multiplier off.
Synchronization
The LT1249 can be externally synchronized in a frequency
range of 127kHz to 160kHz. Figure 2 shows the synchro-
nizing circuit. Synchronizing occurs when CAOUT pin is
pulledbelow0.5VwithanexternaltransistorandaSchottky
diode. The Schottky diode and the 10k pull-up resistor are
necessary for the required fast slewing back up to the
normal operating voltage on CAOUT after the transistor is
turned off. Positive slewing on CAOUT should be faster
than the oscillator ramp rate of 0.5V/µs.
Overvoltage trip level: ∆VOUT = (44µA)(R1)
The width of the synchronizing pulse should be under
60ns. The synchronizing pulses introduce an offset volt-
age on the current amplifier inputs, according to:
0.047µF
V
OUT
C1
0.47µF
R3
330k
V − 0.5
C
(ts)(fs) I +
C
R2
∆V =
R1
OS
1M
g
V
SENSE
m
VA
OUT
–
+
EA
ts = pulse width
fs = pulse frequency
IC = CAOUT source current (≈ 150µA)
VC = CAOUT operating voltage (1.8V to 6.8V)
R2 = resistorfor the midfrequency “zero” in the current loop
gm = current amplifier transconductance (≈ 320µmho)
R2
20k
44µA
22µA
MULTIPLIER
LT1249
V
REF
7.5V
1249 F03
Figure 3. Overvoltage Protection
7
LT1249
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APPLICATIONS INFORMATION
LINE
MAIN INDUCTOR
The Figure 3 circuit therefore has 382V on VOUT, and an
overvoltage level = (VOUT + 44V), or 426V. With a 22µA
hysteresis, VOUT then has to drop 22V to 404V before
feedback recovers and the switch turns back on.
N
N
P
S
R1
90k
1W
D1
D3
V
CC
+
+
C1
M
OUT is a high impedance current output. In the current
2µF
+
+
C3
390µF
C4
56µF
loop, offset line current is determined by multiplier offset
current and input offset voltage of the current amplifier.
A negative 4mV current amplifier VOS translates into
20mA line current and 5W input power for 250V line if
0.2Ω sense resistor is used. Under no load or when the
loadpowerislessthanthisoffsetinputpower, VOUT would
slowly charge up to an overvoltage state because the
overvoltage comparator can only reduce multiplier output
current to zero. This does not guarantee zero output
currentifthecurrentamplifierhasoffset.Toregulate VOUT
under this condition, the amplifier M1 (see Block Dia-
gram), becomes active in the current loop when VAOUT
goes down to 1V. The M1 can put out up to 15µA to the 4k
resistor at the inverting input to cancel the current ampli-
fier negative VOS and keep VOUT error to within 2V.
D2
C2
2µF
1249 F04
ALL CAPACITORS ARE RATED 35V
Figure 4. Power Supply for LT1249
C2
1000pF
450V
MAIN INDUCTOR
LINE
R1
90k
1W
D2
D1
D3
V
CC
+
C3
390µF
35V
C4
+
18V
56µF
35V
1249 F05
Undervoltage Lockout
Figure 5. Power Supply for LT1249
The LT1249 turns on when VCC is higher than 16V and
remains on until VCC falls below 10V, whereupon the chip
enters the lockout state. In the lockout state, the LT1249
only draws 250µA, the oscillator is off, the VREF and the
GTDR pins remain low to keep the power MOSFET off.
auxiliary winding determines VCC according to: VOUT/(VCC
– 2V) = NP/NS. For 382V VOUT and 18V VCC, NP/NS ≈ 19.
In Figure 5 a new technique for supply voltage eliminates
the need for an extra inductor winding. It uses capacitor
charge transfer to generate a constant current source
which feeds a Zener diode. Current to the Zener is equal to
(VOUT – VZ)(C)(f), where VZ is Zener voltage and f is
switching frequency. For VOUT = 382V, VZ = 18V, C =
1000pF and f = 100kHz, Zener current will be 36mA. This
is enough to operate the LT1249, including the FET gate
drive.
Start-Up and Supply Voltage
The LT1249 draws only 250µA before the chip starts at
16V on VCC. To trickle start, a 90k resistor from the power
linetoVCC suppliesthetricklecurrentandC4holdstheVCC
up while switching starts (see Figure 4). Then the auxiliary
winding takes over and supplies the operating current.
Note that D3 and the large value C3, in both Figures 4 and
5, are only necessary for systems that have sudden large
load variation down to minimum load and/or very light
load conditions. Under these conditions, the loop may
exhibitastart/restartmodebecauseswitchingremainsoff
long enough for C4 to discharge below 10V. The C3 will
hold VCC up until switching resumes. For less severe load
variations, D3 is replaced with a short and C3 is omitted.
The turns ratio between the primary winding and the
Output Capacitor
The peak-to-peak 120Hz output ripple is determined by:
V
P-P = (2)(ILOADDC)(Z)
where ILOADDC: DC load current
Z: capacitor impedance at 120Hz
For 180µF at 300W load, ILOADDC = 300W/385V = 0.78A,
8
LT1249
U
W U U
APPLICATIONS INFORMATION
VP-P = (2)(0.78A)(7.4Ω) = 11.5V. If less ripple is desired,
The 120Hz ripple current rating at 105°C ambient is 0.95A
forthe180µFKMH400Vcapacitor.Theexpectedlifeofthe
output capacitor may be calculated from the thermal
stress analysis:
higher capacitance should be used.
The selection of the output capacitor should also be based
on the operating ripple current through the capacitor.
(105°C+∆T )–(T
+∆T )
O
The ripple current can be divided into three major compo-
nents. The first is at 120Hz whose RMS value is related to
the DC load current as follows:
K
AMB
L = (L )(2)
10
O
where
L = expected life time
I1RMS ≈ (0.71)(ILOADDC)
The second component contains the PF switching fre-
quency ripple current and its harmonics. Analysis of this
rippleiscomplicatedbecauseitismodulatedwitha120Hz
signal.However,computernumericalintegrationandFou-
rier analysis approximate the RMS value reasonably close
to the bench measurements. The RMS value is about
0.82A at a typical condition of 120VAC, 200W load. This
ripple is line voltage dependent, and the worst case is at
low line.
LO = hours of load life at rated ripple current and rated
ambient temperature
∆TK = capacitor internal temperature rise at rated condi-
tion. ∆TK = (I2R)/(KA), where I is the rated current, R is
capacitor ESR, and KA is a volume constant.
TAMB = operating ambient temperature
∆TO = capacitor internal temperature rise at operating
condition
I2RMS = 0.82A at 120VAC, 200W
In our example, LO = 2000 hours and ∆TK = 10°C at rated
0.95A. ∆TO can then be calculated from:
The third component is the switching ripple from the load,
if the load is a switching regulator.
2
2
I
0.77A
0.95A
RMS
∆T =
(∆T ) =
(10°C) = 6.6°C
O
K
I3RMS ≈ ILOADDC
0.95A
For United Chemicon KMH 400V capacitor series, ripple
current multiplier for currents at 100kHz is 1.43. The
equivalent 120Hz ripple current can then be found:
Assuming the operating ambient temperature is 60°C, the
approximate life time is:
(105°C+10°C)–(60°C+6.6°C)
L ≈ (2000)(2)
2
2
10
O
2
)
I2RMS
1.43
I3RMS
1.43
≈ 57,000 Hrs.
IRMS = I
+
+
(
1RMS
For longer life, capacitor with higher ripple current rating
or parallel capacitors should be used.
For a typical system that runs at an average load of 200W
and 385V output:
Protection Against Abnormal Current Surge
Conditions
ILOADDC = 0.52A
I1RMS ≈ (0.71)(0.52A) = 0.37A
I2RMS ≈ 0.82A at 120VAC
I3RMS ≈ ILOADDC = 0.52A
The LT1249 has an upper limit on the allowed voltage
across the current sense resistor. The voltage into the
M
OUT pin connected to this resistor must not exceed –6V
while the chip is running and –12V under any conditions.
The LT1249 gate drive will malfunction if the MOUT pin
voltage exceeds –6V while VCC is powered, destroying the
power FET. The 12V absolute limit is imposed by ESD
clamps on the MOUT pin. Large currents will flow at
2
2
2
)
0.82A
1.43
0.52A
1.43
IRMS
=
0.37A +
+
= 0.77A
(
9
LT1249
U
W U U
APPLICATIONS INFORMATION
voltages above 8V and the 12V limit is only for surge
resistor, the standard LT1249 application will not be
affectedbecausethechipisnotyetpowered.Problemsare
only created if the VCC pin is powered from some external
housekeeping supply that remains powered when bridge
power is switched off.
conditions.
In normal operation, the voltage into MOUT does not
exceed 1.1V, but under surge conditions, the voltage
could temporarily go higher. To date, no field failures due
to surges have been reported for normal LT1249 configu-
rations, but if the possibility exists for extremely large
current surges, please read the following discussion.
A huge line voltage surge, beyond the normal worst-case
limits, can also create a large current surge. The peak of
the line voltage must significantly exceed the storage
capacitorvoltage(typically380V)forthistooccur,sopeak
line voltage would probably have to exceed 450V. Such
excessive surges might occur if a very large mains load
was suddenly removed, with a resulting line “kickback”. If
the surge results in voltage at the MOUT pin greater than
6V, it must also last more than 30µs (three switch cycles)
to cause FET problems.
Offline switching power supplies can create large current
surges because of the high value storage capacitor used.
The surge can be the result of closing the line switch near
the peak of the AC line voltage, or because of a large
transient in the line itself. These surges are well known in
the power supply business, and are normally controlled
with a negative temperature coefficient thermistor in
series with the rectifier bridge. When power is switched
on, the thermistor is cold (high resistance) and surges are
limited. Currentflowinthethermistorcausesittoheatand
resistance drops to the point where overall efficiency loss
in the resistor is acceptable.
External Clamp
The external clamp shown in Figure 6 will protect the
LT1249 MOUT pin against extremely large line current
surges (see above). Protection is provided for all VCC
power methods. The 100Ω resistor and three diodes limit
the peak negative voltage into MOUT to less than 3V.
Current sense gain is attenuated by only 100Ω/4000Ω =
2.5%. Three diodes are used because the peak negative
voltage into MOUT in normal operation could go as high as
–1.1V and the diodes should not conduct more than a few
microamps under this condition.
This basic protection mechanism can be partially defeated
if the power supply is switched off for a few seconds, then
turned back on. The thermistor has not had time to cool
significantly and if the subsequent turn-on catches the AC
line near its peak, the resulting surge is much higher than
normal. Even if this surge current generates a voltage
greater than 6V (but less than 12V) across the sense
THERMISTOR
+
+
STORAGE
CAPACITOR
BRIDGE
SURGE PATH
R
S
–
100Ω
M
OUT
LT1249
Figure 6. Protecting MOUT from Extremely High Current Surges
10
LT1249
U
PACKAGE DESCRIPTION
Dimensions in inches (millimeters) unless otherwise noted.
N8 Package
8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
0.400*
(10.160)
MAX
8
7
6
5
4
0.255 ± 0.015*
(6.477 ± 0.381)
1
2
3
0.130 ± 0.005
0.300 – 0.325
0.045 – 0.065
(3.302 ± 0.127)
(1.143 – 1.651)
(7.620 – 8.255)
0.065
(1.651)
TYP
0.009 – 0.015
(0.229 – 0.381)
0.125
0.020
(0.508)
MIN
(3.175)
MIN
+0.035
0.325
–0.015
0.018 ± 0.003
(0.457 ± 0.076)
0.100
(2.54)
BSC
+0.889
8.255
(
)
N8 1098
–0.381
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
0.189 – 0.197*
(4.801 – 5.004)
7
5
8
6
0.150 – 0.157**
(3.810 – 3.988)
0.228 – 0.244
(5.791 – 6.197)
1
3
4
2
0.010 – 0.020
(0.254 – 0.508)
× 45°
0.053 – 0.069
(1.346 – 1.752)
0.004 – 0.010
(0.101 – 0.254)
0.008 – 0.010
(0.203 – 0.254)
0°– 8° TYP
0.016 – 0.050
(0.406 – 1.270)
0.050
(1.270)
BSC
0.014 – 0.019
(0.355 – 0.483)
TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
SO8 1298
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.
11
LT1249
U
TYPICAL APPLICATION
MURH860
750µH*
+
V
OUT
90V
TO
270V
EMI
FILTER
6A
–
+
IRF840
100pF
180µF
1M
0.047µF
0.47µF
R
S
20k
0.2Ω
1nF
10k
10Ω
330k
†
GND
V
CC
VA
M
OUT
CA
OUT
OUT
5
3
2
1
7
7.5V
REF
R
V
MOUT
+
7.5V
4k
EA
+
–
RUN
V
–
6
4
CC
MAX
I
A
V
250µA
SENSE
MULTIPLIER
2 I
16V/10V
1M
I
M
+
I
A
B
I
B
I
M
=
–
+
200µA2
CA
–
R
S
Q
32k
I
AC
+
GTDR
RUN
OSC
g
m
= 1/3k
15µA
8
0.7V
–
+
1V
M1
–
+
–
SYNC
4.7nF
16V
44µA
22µA
4k
**
1N5819
20µA
35pF
1249 TA01
1. COILTRONICS CTX02-12236 (TYPE 52 CORE)
*
AIR MOVEMENT NEEDED AT POWER LEVEL GREATER THAN 250W.
2. COILTRONICS CTX02-12295 (MAGNETICS Kool Mµ® 77930 CORE)
**
THIS SCHOTTKY DIODE IS TO CLAMP GTDR WHEN MOS SWITCH TURNS OFF.
PARASITIC INDUCTANCE AND GATE CAPACITANCE MAY TURN ON CHIP SUBSTRATE
DIODE AND CAUSE ERRATIC OPERATIONS IF GTDR IS NOT CLAMPED.
† SEE APPLICATIONS INFORMATION SECTION FOR CIRCUITRY TO SUPPLY POWER TO V
CC
.
RELATED PARTS
PART NUMBER
LT1103
DESCRIPTION
COMMENTS
Off-Line Switching Regulator
Universal Off-Line Inputs with Outputs to 100W
Provides All Features in 16-Lead Package
Simplified PFC Design
LT1248
LT1508
LT1509
Full Feature Average Current Mode Power Factor Controller
Power Factor and PWM Controller
Power Factor and PWM Controller
Complete Solution for Universal Off-Line Switching Power Supplies
Kool Mµ is a registered trademark of Magnetics, Inc.
1249fb LT/TP 0799 2K REV B • PRINTED IN USA
LINEAR TECHNOLOGY CORPORATION 1994
12 LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
●
(408)432-1900 FAX:(408)434-0507 www.linear-tech.com
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