Datasheet LT1249 Datasheet (Linear Technology)

FEATURES

LT1249

Power Factor Controller

U

DESCRIPTIO

■

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

■

Low Quiescent Current: 9mA

■

Fast 1.5A Peak Current Gate Driver

U

APPLICATIO S

■

Universal Power Factor Corrected Power Supplies

■

Preregulators up to 1500W

, LTC and LT are registered trademarks of Linear Technology Corporation.

The 8-pin LT®1249 provides active power factor correc­tion for universal offline power systems with very few
external parts. By using fixed high frequency PWM current
averaging without the need for slope compensation, the
LT1249 achieves far lower line current distortion, with a
smaller magnetic element than systems that use either peak
current detection or zero current switching approach, in
both continuous and discontinuous modes of operation.

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.

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.

BLOCK DIAGRA

VA

OUT

5

+

7.5V

V

SENSE

6

I

AC

4

EA

–

W

44µA
22µA

MOUT

250µA MAX

+

CA

–

= 1/3k

g

m

CA

OUT

16V/10V

+

–

GND

1

7.5V
V

REF

+

V

CC

–

–

+

+

0.7V

SYNC

RUN

OSC

R
S

20µA

35pF

RUN

Q

16V

M

OUT

32

R
4k

I

A

MULTIPLIER

32k

I

IM =

B

1V

200µA

+

M1

–

I

M

2

I

I

A

B

2

15µA

4k

V

CC

7

GTDR

8

1249 BD

1

LT1249

WW

W

U

ABSOLUTE MAXIMUM RATINGS

Supply Voltage ....................................................... 27V

GTDR Current Continuous ..................................... 0.5A

GTDR Output Energy (Per Cycle) ............................. 5µJ

IAC Input Current ................................................. 20mA

V
M
Operating Junction Temperature Range

Thermal Resistance (Junction-to-Ambient)

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

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

ELECTRICAL CHARACTERISTICS

range, otherwise specifications are at T
VA

Input Voltage ............................................ V

SENSE

Input Current.............................................. ±5mA

OUT

LT1249C................................................ 0°C to 100°C

LT1249I ........................................... –40°C to 125°C

N8 Package ................................................ 100°C/W

S8 Package................................................. 120°C/W

The ● denotes specifications which apply over the operating temperature

= 25°C. Maximum operating voltage (V

= 5V, no load on any outputs, unless otherwise noted.

OUT

A

MAX

U

W

PACKAGE/ORDER INFORMATION

ORDER PART

TOP VIEW

GND

1

CA

2

OUT

M

3

OUT

I

4

AC

N8 PACKAGE
8-LEAD PDIP

S8 PACKAGE

8-LEAD PLASTIC SO

T

= 125°C, θJA = 100°C/W (N8)

JMAX

T

= 125°C, θJA = 120°C/W (S8)

JMAX

Consult factory for Military grade parts.

) = 25V, VCC = 18V, I

MAX

GTDR

8

V

7

CC

V

6

SENSE

VA

5

OUT

= 100µA, CA

AC

NUMBER

LT1249CN8
LT1249IN8
LT1249CS8
LT1249IS8

S8 PART

MARKING

1249
1249I

= 3.5V,

OUT

U

PARAMETER CONDITIONS MIN TYP MAX UNITS
Overall

Supply Current (VCC in Undervoltage Lockout) VCC = Lockout Voltage – 0.2V ● 0.25 0.45 mA
Supply Current, On 11.5V ≤ VCC ≤ V
VCC Turn-On Threshold ● 15.5 16.5 17.5 V
VCC Turn-Off Threshold ● 9.5 10.5 11.5 V

Voltage Amplifier

V

Bias Current V

SENSE

Voltage Amp Gain 70 100 dB
Voltage Amp Unity-Gain Bandwidth 1.5 MHz
Voltage Amp Output High 0 ≤ Source Current ≤ 50µA ● 10 12 V
Voltage Amp Output Low 0 ≤ Sink Current ≤ 5µA ● 0.1 0.4 V
Voltage Amp Source Current ● 130 260 450 µA
Voltage Amp Sink Current Threshold Linear Operation, 2V < VA
Voltage Amp Sink Current Hysteresis 2V < VA

Current Amplifier

Current Amp Offset Voltage ● ±2 ±15 mV
Current Amp Transconductance ∆I
Current Amp Voltage Gain 2.5V ≤ V
Current Amp Source Current V
Current Amp Sink Current V
Current Amp Output High 7.4 8.1 V
Current Amp Output Low 1.2 2 V

= 0V to 7V ● –25 –250 nA

SENSE

OUT

= ±40µA ● 150 320 550 µmho

CAOUT

CAOUT

= 1V, IM = 0µV 100 145 220 µA

MOUT

= –0.3V, IM = 0µA 67 95 125 µA

MOUT

, CA

MAX

< 10V ● 14 22.5 30 µA

≤ 7.5V 500 1000 V/V

= 1V ● 812 mA

OUT

< 10V ● 33 44 57 µA

OUT

2

LT1249

ELECTRICAL CHARACTERISTICS

range, otherwise specifications are at T
VA

= 5V, no load on any outputs, unless otherwise noted.

OUT

= 25°C. Maximum operating voltage (V

A

The ● denotes specifications which apply over the operating temperature

) = 25V, VCC = 18V, I

MAX

= 100µA, CA

AC

OUT

= 3.5V,

PARAMETER CONDITIONS MIN TYP MAX UNITS
Reference

Reference Output Voltage TA = 25°C, Measured at V

Pin 7.39 7.5 7.6 V

SENSE

Reference Output Voltage Worst Case All Line, Temperature ● 7.32 7.5 7.68 V
Reference Output Voltage Line Regulation V

LOCKOUT

< VCC < V

MAX

● –20 5 20 mV

Multiplier

Multiplier Output Current IAC = 100µA, VA

= 5V 35 µA

OUT

Multiplier Output Current Offset RAC = 1M from IAC to GND ● –0.05 –0.5 µA
Multiplier Max Output Current (I
Multiplier Max Output Voltage (I

)I

M(MAX)

• R

M(MAX)

)IAC = 450µA, VA

MOUT

= 450µA, VA

AC

= 7V (Note 2) ● – 375 –250 –150 µA

OUT

= 7V (Note 2) ● –1.25 –1.1 –0.96 V

OUT

Multiplier Gain Constant (Note 3) 0.035 V
IAC Input Resistance IAC from 50µA to 1mA 15 32 50 kΩ

Oscillator

Oscillator Frequency ● 75 100 125 kHz
Control Pin (CA
Synchronization Frequency Range Synchronizing Pulse Low ≤ 0.35V on CA

) Threshold Duty Cycle = 0 ● 1.3 1.8 2.3 V

OUT

OUT

● 127 160 kHz

Gate Driver

Max GTDR Output Voltage 0mA Load, 18V < V
GTDR Output High – 200mA Load, 11.5V ≤ VCC ≤ 15V ● V

CC

< V

(Note 4) ● 12 15 17.5 V

MAX

– 3.0 V

CC

GTDR Output Low (Device Unpowered) VCC = 0V, 50mA Load (Sinking) ● 0.9 1.5 V
GTDR Output Low (Device Active) 200mA Load (Sinking) ● 0.5 1 V
Peak GTDR Current 10nF from GTDR to GND 2 A
GTDR Rise and Fall Time 1nF from GTDR to GND 25 ns
GTDR Max Duty Cycle 90 96 %

–2

Note 1: Absolute Maximum Ratings are those values beyond which the life

Note 3: Multiplier Gain Constant: K =

of a device may be impaired.
Note 2: Current amplifier is in linear mode with 0V input common mode.

W

U

Note 4: Maximum GTDR output voltage is internally clamped for higher

voltages.

V

CC

TYPICAL PERFORMANCE CHARACTERISTICS

Voltage Amplifier Open-Loop
Gain and Phase

100

80

60

40

GAIN (dB)

20

0

–20

10

100

GAIN

PHASE

1k 10k 100k

FREQUENCY (Hz)

1M 10M

1249 G01

0

–20

–40

–60

–80

–100

–120

PHASE (DEG)

400

350

300

250

200

150

100

TRANSCONDUCTANCE (µmho)

50

Transconductance of
Current Amplifier

θ

g

m

0

1k

100k10k

FREQUENCY (Hz)

IAC (VA

1M

I

OUT

M

– 1.5)

1249 G02

10M

2

20

0

–20

–40

–60

–80

–100

–120

–140

PHASE (DEG)

3

LT1249

TEMPERATURE (°C)

–75

FREQUENCY (kHz)

75

1249 G10

–25 25 125

140

130

120

110

100

90

80

70

–50 0 50 100

W

U

TYPICAL PERFORMANCE CHARACTERISTICS

Reference Voltage vs
Temperature

7.536

7.524

7.512

7.500

7.488

7.476

7.464

REFERENCE VOLTAGE (V)

7.452

7.440

7.428
–50 150

–75

–25

JUNCTION TEMPERATURE (°C)

0 25 50 100

75

125

1249 G03

Multiplier Current

300

150

(µA)

M

I

0

0

VA

500

= 5V

OUT

VA

= 4.5V

OUT

VA

= 4V

OUT

VA

= 3.5V

OUT

VA

= 3V

OUT

VA

= 2.5V

OUT

VA

= 2V

OUT

VA

= 6.5V

OUT

VA

= 6V

OUT

VA

= 5.5V

OUT

250

IAC (µA)

1249 G04

SUPPLY CURRENT (mA)

400

300

200

TIME (ns)

100

Supply Current vs Supply Voltage

10

TJ = –55°C

9

TJ = 25°C

8
7

TJ = 125°C

6
5
4
3
2
1
0

1412 16 20 24 28

10

SUPPLY VOLTAGE (V)

22

18

26

GTDR Rise and Fall Time

FALL TIME

RISE TIME

NOTE: GTDR SLEWS

0

0

10

BETWEEN 1V AND 16V

20 30 40

LOAD CAPACITANCE (nF)

1249 G05

1249 G08

GTDR Source Current

18.5
VCC = 18V

18.0

17.5

17.0

16.5

16.0

15.5

15.0

GTDR VOLTAGE (V)

14.5

14.0

13.5

13.0

30

0

TJ = 125°C

TJ = 25°C

TJ = –55°C

–120 –180 –240

–60

SOURCE CURRENT (mA)

–300

1249 G06

GTDR Sink Current

1.1

1.0

0.9

0.8

0.7

0.6

0.5
TA = –55°C

0.4

GTDR VOLTAGE (V)

0.3

0.2

0.1

0

TA = 125°C

0

TA = 25°C

60

120 180 240

SINK CURRENT (mA)

300

1249 G07

Start-Up Supply Current vs
Supply Voltage Switching Frequency

550
500
450
400
350
300
250
200
150

SUPPLY CURRENT (µA)

100

50

50

0

0

4

2610

SUPPLY VOLTAGE (V)

81216

–55°C

25°C

125°C

14 18

20

1249 G09

4

W

TEMPERATURE (°C)

–50

TRANSCONDUCTANCE (µmho)

100

1249 G13

050

400

350

300

250

200

150

100

50

0

–25 25 75 125

U

TYPICAL PERFORMANCE CHARACTERISTICS

LT1249

Synchronization Threshold
at CA

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

SYNCHRONIZATION THRESHOLD (V)

0.1

OUT

0

–25 25 125–50 0 50

TEMPERATURE (°C)

75

100

Voltage Amp Sink Current Limits
(Threshold)

60

50

40

30

CURRENT (µA)

20

10

0

–75

NOTE: THESE SINK CURRENT THRESHOLDS ARE
FOR OVERVOLTAGE PROTECTION FUNCTION.

UP THRESHOLD

DOWN THRESHOLD

–25–50 250 50 10075125

TEMPERATURE (°C)

1249 G11

1249 G14

M

Pin Characteristics

OUT

1.2

1.0

0.8

0.6

0.4

0.2
0

CURRENT (mA)

–0.2

OUT

M

–0.4
–0.6
–0.8
–1.0

–2.4

125°C
25°C
–50°C

–1.2

M

OUT

0

VOLTAGE (V)

1.2

Maximum Multiplier Output
Voltage (I

–1.30

–1.25

–1.20

(V)

–1.15

MOUT

–1.10

× R

–1.05

M(MAX)

I

–1.00

–0.95

–0.90

–75 –25–50 250 50 10075125

• R

M(MAX)

TEMPERATURE (°C)

MOUT

)

2.4

1249 G12

1249 G15

Transconductance of Current
Amplifier Over Temperature

Maximum Duty Cycle

100

99
98
97
96
95
94

DUTY CYCLE (%)

93
92
91
90

–25 25 125–50 0 50

TEMPERATURE (°C)

75

100

1249 G16

UUU

PIN FUNCTIONS

GND (Pin 1): Ground.

CA

that senses and forces the line current to follow the
reference signal that comes from the multiplier by com­manding the pulse width modulator. When CA
the modulator has zero duty cycle.

M

through the 4k resistor R
across R
and it is limited to 1.1V. The noninverting input of the
current amplifier is also tied to R

(Pin 2): This is the output of the current amplifier

OUT

OUT

(Pin 3): The multiplier current goes out of this pin

OUT

. The voltage developed

MOUT

is the reference voltage of the current loop

MOUT

. In operation, M

MOUT

is low,

OUT

is normally at negative potential and only AC signals
appear at the noninverting input of the current amplifier.

I

(Pin 4): This is the AC line voltage sensing input to the

AC

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.

VA

(Pin 5): This is the output of the voltage error

OUT

amplifier. The output is clamped at 12V. When the output
goes below 1.5V, the multiplier output current is zero.

5

LT1249

UUU

PIN FUNCTIONS

V

(Pin 6): This is the inverting input to the voltage

SENSE

amplifier.

VCC (Pin 7): This is the supply of the chip. The LT1249 has
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

U

WUU

APPLICATIONS INFORMATION

Error Amplifier

The error amplifier has a 100dB DC gain and 1.5MHz unity­gain frequency. It is internally clamped at 12V. The nonin­verting input is tied to the 7.5V reference.

Current Amplifier

The multiplier output current IM flows out of the M
through the 4k resistor R

and develops the reference

MOUT

signal to the current loop that is controlled by the current
amplifier. Current gain is the ratio of R

to line current

MOUT

sense resistor. The current amplifier is a transconductance
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
CA

may need to swing 5V over one line cycle at high line

OUT

condition, 11mV will be present at the inputs of the current
amplifier if gain is rolled off to 450 at 120Hz (1nF in series
with 10k at CA

). At light load, when (IM)(R

OUT

MOUT

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.

pin

OUT

) can be

capacitor in parallel with a low ESR electrolytic capacitor,
56µF or higher is required in close proximity to IC GND.

GTDR (Pin 8): The MOSFET gate driver is a 1.5A fast totem
pole output. It is clamped at 15V. Capacitive loads like
MOSFET gates may cause overshoot. A gate series resis­tor of at least 5Ω will prevent the overshoot.

Multiplier

The multiplier is a current multiplier with high noise
immunity in a high power switching environment. The
current gain is:

IM = (IAC)(I
IEA = (VA

2

)/(200µA)2, and

EA

– 1.5V)/25k

OUT

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
high impedance lines could get into the multiplier from the
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

500

VA

VA

VA

VA
VA

VA

OUT

OUT

OUT

OUT

OUT
OUT

OUT

OUT

= 5V

= 4.5V

= 4V

= 3.5V

= 3V
= 2.5V

= 2V

VA

= 6.5V

OUT

VA

= 6V

OUT

VA

= 5.5V

OUT

150

(µA)

M

I

0

0

Figure 1. Multiplier Current IM vs IAC and VA

250

IAC (µA)

1249 G04

6

LT1249

U

WUU

APPLICATIONS INFORMATION

Line Current Limiting

Maximum voltage across R

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

IRV

()()( )

M MAX MOUT LINE MIN

R

S

where P

() ()

=

OUT(MAX)

1 414

KP

(. )

OUT 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
current reaches limit, output voltage V
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
V

drop and clipping of the line current, which degrades

OUT

power factor.

Synchronization

The LT1249 can be externally synchronized in a frequency
range of 127kHz to 160kHz. Figure 2 shows the synchro­nizing circuit. Synchronizing occurs when CA
pulled below 0.5V with an external transistor and a Schottky
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 CA
turned off. Positive slewing on CA
than the oscillator ramp rate of 0.5V/µs.

The width of the synchronizing pulse should be under
60ns. The synchronizing pulses introduce an offset volt­age on the current amplifier inputs, according to:



V

C

+

C

g

m

∆V

OS

ts fs I

()()

=





ts = pulse width
fs = pulse frequency
IC = CA
VC = CA

source current (≈ 150µA)

OUT

operating voltage (1.8V to 6.8V)

OUT

R2 = resistor for the midfrequency “zero” in the current loop
gm = current amplifier transconductance (≈ 320µmho)

is internally limited to

MOUT

()

will drop to keep

OUT

after the transistor is

OUT

should be faster

OUT



−

.05



R

2



OUT

pin is

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.

CA

OUT

R2
10k

1nF

Figure 2. Synchronizing the LT1249

1N5712

2N2369

V

CC

R1
10k

80pF

2k

5V
0V

1249 F02

Overvoltage Protection

In Figure 3, R1 and R2 set the regulator output DC level:
V
V

OUT
OUT

= V
is 382V.

[(R1 + R2)/R2]. With R1 = 1M and R2 = 20k,

REF

Because of the slow loop response necessary for power
factor correction, output overshoot can occur with sudden
load removal or reduction. To protect the power compo­nents and output load, the LT1249 voltage error amplifier
senses the output voltage and quickly shuts off the current
switch when overvoltage occurs. When overshoot occurs
on V
because amplifier feedback keeps V

, the overcurrent from R1 will go through VA

OUT

locked at 7.5V.

SENSE

OUT

When this overcurrent reaches 44µA amplifier sinking
limit, the amplifier loses feedback and its output snaps low
to turn the multiplier off.

Overvoltage trip level: ∆V

V

OUT

R1
1M

R2
20k

0.047µF

C1

0.47µF

V

SENSE

–

+

V

REF

7.5V

Figure 3. Overvoltage Protection

= (44µA)(R1)

OUT

EA

MULTIPLIER

LT1249

R3

330k

VA

OUT

44µA
22µA

1249 F03

7

LT1249

U

WUU

APPLICATIONS INFORMATION

The Figure 3 circuit therefore has 382V on V
overvoltage level = (V
hysteresis, V

then has to drop 22V to 404V before

OUT

+ 44V), or 426V. With a 22µA

OUT

feedback recovers and the switch turns back on.
M

is a high impedance current output. In the current

OUT

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
load power is less than this offset input power, V
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
current if the current amplifier has offset. To regulate V
under this condition, the amplifier M1 (see Block Dia­gram), becomes active in the current loop when VA
goes down to 1V. The M1 can put out up to 15µA to the 4k
resistor at the inverting input to cancel the current ampli­fier negative VOS and keep V

error to within 2V.

OUT

OUT

OUT

, and an

would

OUT

OUT

LINE

MAIN INDUCTOR

N

P

N

S

D2

ALL CAPACITORS ARE RATED 35V

Figure 4. Power Supply for LT1249

C2

1000pF

450V

D2

+

D1

C3
390µF
35V

R1
90k
1W

+

C1
2µF

+

+

C2
2µF

D3

+

MAIN INDUCTOR

R1
90k
1W

LINE

18V

C4
56µF
35V

D3D1

C3
390µF

V

CC

+

C4
56µF

1249 F04

V

CC

1249 F05

Undervoltage Lockout

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 V

and the

REF

GTDR pins remain low to keep the power MOSFET off.

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
line to VCC supplies the trickle current and C4 holds the V

CC

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
exhibit a start/restart mode because switching remains off
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

Figure 5. Power Supply for LT1249

auxiliary winding determines VCC according to: V
– 2V) = NP/NS. For 382V V

and 18V VCC, NP/NS ≈ 19.

OUT

OUT

/(V

CC

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
(V

– VZ)(C)(f), where VZ is Zener voltage and f is

OUT

switching frequency. For V

= 382V, VZ = 18V, C =

OUT

1000pF and f = 100kHz, Zener current will be 36mA. This
is enough to operate the LT1249, including the FET gate
drive.

Output Capacitor

The peak-to-peak 120Hz output ripple is determined by:

V

= (2)(I

P-P

where I

LOAD

DC)(Z)

LOAD

DC: DC load current

Z: capacitor impedance at 120Hz

For 180µF at 300W load, I

DC = 300W/385V = 0.78A,

LOAD

8

LT1249

U

WUU

APPLICATIONS INFORMATION

V

= (2)(0.78A)(7.4Ω) = 11.5V. If less ripple is desired,

P-P

higher capacitance should be used.
The selection of the output capacitor should also be based

on the operating ripple current through the capacitor.
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:

I

≈ (0.71)(I

1RMS

The second component contains the PF switching fre­quency ripple current and its harmonics. Analysis of this
ripple is complicated because it is modulated with a 120Hz
signal. However, computer numerical integration and Fou­rier 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.

I

= 0.82A at 120VAC, 200W

2RMS

The third component is the switching ripple from the load,
if the load is a switching regulator.

I

3RMS

≈ I

LOAD

DC

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:

II

=

RMS RMS

()

1

For a typical system that runs at an average load of 200W
and 385V output:

I

DC = 0.52A

LOAD

I

≈ (0.71)(0.52A) = 0.37A

1RMS

I

≈ 0.82A at 120VAC

2RMS

I

≈ I

3RMS

IA

=

RMS

DC = 0.52A

LOAD

.

()

DC)

LOAD

2



II

+





2



+





2



RMS RMS

2

143 143..

082

.

143

.



3

+









22





AA

+














052

.

143

.

2



077

.

A

=037





The 120Hz ripple current rating at 105°C ambient is 0.95A
for the 180µF KMH 400V capacitor. The expected life of the
output capacitor may be calculated from the thermal
stress analysis:

CTKT

°+ +

()–()

LL

=

()()

O

10510∆∆

2

AMBTO

where
L = expected life time

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.

T

= operating ambient temperature

AMB

∆TO = capacitor internal temperature rise at operating
condition

In our example, LO = 2000 hours and ∆TK = 10°C at rated

0.95A. ∆TO can then be calculated from:

∆∆T

I



=



O



095

22



RMS

.





A

()

T

K

=

077






095

.

A



().

CC

°= °

10 6 6





.

A

Assuming the operating ambient temperature is 60°C, the
approximate life time is:

CC C C

°+ ° °+ °

()–(.)

105 10 60 6 6

L

≈

()()

2000 2

O

≈

57,000 Hrs.

10

For longer life, capacitor with higher ripple current rating
or parallel capacitors should be used.

Protection Against Abnormal Current Surge
Conditions

The LT1249 has an upper limit on the allowed voltage
across the current sense resistor. The voltage into the
M

pin connected to this resistor must not exceed –6V

OUT

while the chip is running

The LT1249 gate drive will malfunction if the M
voltage exceeds – 6V while V

and –12V under any conditions.

pin

OUT

is powered, destroying the

CC

power FET. The 12V absolute limit is imposed by ESD
clamps on the M

pin. Large currents will flow at

OUT

9

LT1249

U

WUU

APPLICATIONS INFORMATION

voltages above 8V and the 12V limit is only for surge
conditions.

In normal operation, the voltage into M
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 configu­rations, but if the possibility exists for extremely large
current surges, please read the following discussion.

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. Current flow in the thermistor causes it to heat and
resistance drops to the point where overall efficiency loss
in the resistor is acceptable.

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

does not

OUT

resistor, the standard LT1249 application will not be
affected because the chip is not yet powered. Problems are
only created if the VCC pin is powered from some external
housekeeping supply that remains powered when bridge
power is switched off.

A huge line voltage surge,

limits

, can also create a large current surge. The peak of

beyond the normal worst-case

the line voltage must significantly exceed the storage
capacitor voltage (typically 380V) for this to occur, so peak
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 M

pin greater than

OUT

6V, it must also last more than 30µs (three switch cycles)
to cause FET problems.

External Clamp

The external clamp shown in Figure 6 will protect the
LT1249 M
surges (see above). Protection is provided for all V

pin against extremely large line current

OUT

CC

power methods. The 100Ω resistor and three diodes limit
the peak negative voltage into M

to less than 3V.

OUT

Current sense gain is attenuated by only 100Ω/4000Ω =

2.5%. Three diodes are used because the peak negative
voltage into M

in normal operation could go as high as

OUT

–1.1V and the diodes should not conduct more than a few
microamps under this condition.

10

THERMISTOR

+

BRIDGE

–

Figure 6. Protecting M

+

SURGE PATH

R

S

100Ω

M

OUT

LT1249

from Extremely High Current Surges

OUT

STORAGE
CAPACITOR

PACKAGE DESCRIPTION

U

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

876

0.255 ± 0.015*
(6.477 ± 0.381)

5

LT1249

12

0.300 – 0.325

(7.620 – 8.255)

0.065

(1.651)

0.009 – 0.015

(0.229 – 0.381)

+0.035

0.325
–0.015

+0.889

8.255

()

–0.381

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)

TYP

0.045 – 0.065

(1.143 – 1.651)

0.100

(2.54)

BSC

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

8

3

0.189 – 0.197*
(4.801 – 5.004)

7

6

4

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

0.018 ± 0.003

(0.457 ± 0.076)

5

0.020

(0.508)

MIN

N8 1098

0.228 – 0.244

(5.791 – 6.197)

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*

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

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 represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

× 45°

(1.346 – 1.752)

0°– 8° TYP

0.016 – 0.050

(0.406 – 1.270)

0.053 – 0.069

0.014 – 0.019

(0.355 – 0.483)

TYP

0.150 – 0.157**
(3.810 – 3.988)

1

3

2

4

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

BSC

SO8 1298

11

LT1249

TYPICAL APPLICATION

U

90V

TO

270V

1M

4.7nF

100pF

+

–

–

+

SYNC

MURH860

GND

1

OSC

RUN

35pF

R
S

20µA

1M

20k

V

OUT

+

180µF

10Ω

†

V

CC

7

7.5V
V

REF

RUN

Q

GTDR

8

16V

**

1N5819

EMI

FILTER

6A

+

750µH*

–

IRF840

0.047µF

0.47µF

7.5V

6

V

SENSE

4

I

AC

330k

VA

OUT

5

+

EA

–

44µA
22µA

I

A

I

B

1V

32k

MULTIPLIER

IM =

+

M1

–

I

A

200µA

2

I

B

2

15µA

R

S

0.2Ω

1nF

10k

M

OUT

3

R

MOUT

4k

MAX

250µA

I

M

+

CA

–

g

= 1/3k

m

4k

CA

2

16V/10V

0.7V

+

–

OUT

V

CC

+

–

1. COILTRONICS CTX02-12236 (TYPE 52 CORE)

*

AIR MOVEMENT NEEDED AT POWER LEVEL GREATER THAN 250W.

2. COILTRONICS CTX02-12295 (MAGNETICS Kool Mµ

**

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

®

77930 CORE)

.

CC

1249 TA01

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1103 Off-Line Switching Regulator Universal Off-Line Inputs with Outputs to 100W
LT1248 Full Feature Average Current Mode Power Factor Controller Provides All Features in 16-Lead Package
LT1508 Power Factor and PWM Controller Simplified PFC Design
LT1509 Power Factor and PWM Controller Complete Solution for Universal Off-Line Switching Power Supplies

Kool Mµ is a registered trademark of Magnetics, Inc.

12

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

1249fb LT/TP 0799 2K REV B • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1994