Datasheet LT1248 Datasheet (Linear Technology)

FEATURES

LT1248

Power Factor Controller

U

DESCRIPTIO

■

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

The LT®1248 provides active power factor correction for universal off-line power systems. By using fixed high frequency PWM current averaging, without the need for slope compensation, the LT1248 achieves far lower line current distortion with a smaller magnetic element than systems that use either peak-current detection or zero current switching approaches in both continuous and discontinuous modes of operation.

The LT1248 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 LT1248 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 LT1248 provides many protection features including peak current limiting and overvoltage protection, and can be operated at frequencies as high as 300kHz.

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

BLOCK DIAGRA

+

V

CC

2.6V/

2.2V

7.5V

7.9V

12µA

–

+

–

EA

+

–

5V

16V TO 10V

EN/SYNC

10

V

SENSE

11

I

AC

6

OVP

8

SS 13

W

–

CA

+

CA

PK

OUT

32

–

+

0.7V

–

SYNC

C

SET

LIM

GND

1

–

+

R R

Q

S

RUN

OSC

1 6V

1214

R

SET

V

CC

15

16

GTDR

1248 BD

M

OUTISENSE

4

5

7µA

I

2

M

I

A

B

2

32k

+

M1

–

I

A

I

B

ONE SHOT

V

REF

9

7.5V

V

REF

IM =

200ns

RUN

I

200µA

VA

OUT

7

2.2V

–

+

1

LT1248

A

W

O

LUTEXI TIS

S

A

WUW

ARB

U G

PACKAGE

/

O

RDER I FOR ATIO

WU

U

(Note 1)

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

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

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

IAC, R V

SENSE

I

SENSE

SET

, PK

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

LIM

, EN/SYNC, OVP Input Voltage................... V

, M

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

OUT

MAX

Operating Junction Temperature Range

LT1248C................................................ 0°C to 100°C

LT1248I ........................................... –40°C to 125°C

GND

PK CA I

SENSE

M

VA

OVP

LIM

OUT

I

OUT

AC

Thermal Resistance (Junction-to-Ambient)

N Package .................................................. 100°C/W

S Package................................................... 120°C/W

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

16-LEAD NARROW PLASTIC SO

T T

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

Consult factory for Military grade parts.

LECTRICAL C CHARA TERIST

E

ture range, otherwise specifications are at T C

= 1nF to GND, I

SET

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 (Inactive) EN/SYNC = 0V, VCC ≤ V Supply Current, On 11.5V ≤ VCC ≤ V VCC Turn-On Threshold (Undervoltage Lockout) ● 15.5 16.5 17.5 V VCC Turn-Off Threshold ● 9.5 10.5 11.5 V EN/SYNC Threshold, Rising ● 2.2 2.6 2.85 V EN/SYNC Threshold Hysteresis 0.40 V EN/SYNC Input Current EN/SYNC = 0V ● –5 –1 5 µA

Voltage Amplifier

Voltage Amp Offset Voltage VA Input Bias Current V Voltage Gain 70 100 dB Voltage Amp Unity-Gain Bandwidth 3 MHz Voltage Amp Output High (Internally Clamped) ● 11.3 13.3 V Voltage Amp Output Low ● 1.1 2 V Voltage Amp Short-Circuit Current VA SS Current SS = 2.5V ● 51230 µA

Current Amplifier

Current Amp Offset Voltage ● ±1 ±4mV I

Bias Current ● – 25 –250 nA

SENSE

Current Amp Voltage Gain 80 110 dB Current Amp Unity-Gain Bandwidth 3 MHz Current Amp Output High ● 7.2 8.5 V Current Amp Output Low ● 1.1 2 V

= 100µA, I

AC

SENSE

= 0V, CA

ICS

= 25°C. Maximum operating voltage (V

A

OUT

The ● denotes specifications which apply over the full operating tempera-

= 3.5V, VA

3V ≤ EN/SYNC ≤ 7V –50 – 25 50 µA

OUT

SENSE

OUT

= 5V, OVP = 7.5V, no load on any outputs, unless otherwise noted.

OUT

MAX

, CA

MAX

OUT

= 3.5V ● –8 8 mV

= 0V to 7V ● – 25 –250 nA

= 0V ● 51430 mA

TOP VIEW

1 2 3 4 5 6 7 8

N PACKAGE

16-LEAD PDIP

S PACKAGE

= 125°C, θJA = 100°C/W (N)

JMAX

= 125°C, θJA = 120°C/W (S)

JMAX

) = 25V, VCC = 18V, R

MAX

= 1V ● 8.5 12.0 mA

GTDR

16

V

15

CC

C

14

SET

SS

13

R

12

SET

V

11

SENSE

EN/SYNC

10

V

9

REF

● 0.5 1.5 mA

ORDER PART

NUMBER

LT1248CN LT1248IN LT1248CS LT1248IS

= 15k to GND,

SET

2

LT1248

LECTRICAL C CHARA TERIST

E

ture range, otherwise specifications are at T C

= 1nF to GND, I

SET

= 100µA, I

AC

SENSE

A

= 0V, CA

ICS

= 25°C. Maximum operating voltage (V

OUT

The ● denotes specifications which apply over the full operating tempera-

= 3.5V, VA

) = 25V, VCC = 18V, R

= 5V, OVP = 7.5V, no load on any outputs, unless otherwise noted.

OUT

MAX

= 15k to GND,

SET

PARAMETER CONDITIONS MIN TYP MAX UNITS Current Amplifier

Current Amp Short-Circuit Current CA Input Range, I

SENSE

, M

(Linear Operation) ● –0.3 1 V

OUT

= 0V ● 51430 mA

OUT

Reference

Reference Output Voltage I V

Load Regulation –5mA < I

REF

V

Line Regulation 11.5V < VCC < V

REF

V

Short-Circuit Current V

REF

V

Worst Case Load, Line, Temperature ● 7.32 7.5 7.68 V

REF

= 0mA, TA = 25°C 7.39 7.50 7.60 V

REF

< 0mA 5 mV

REF

MAX

= 0V ● 12 28 50 mA

REF

● –20 5 20 mV

Current Limit

PK

Offset Voltage ● –15 15 mV

LIM

PK

Input Current PK

LIM

PK

to GTDR Propagation Delay PK

LIM

= –0.1V ● – 50 –100 µA

LIM

Falling from 50mV to –50mV 400 ns

LIM

Multiplier

Multiplier Output Current IAC = 100µA, R

= 15k 35 µA

SET

Multiplier Output Current Offset RAC = 1M from IAC to GND ● –0.05 –0.5 µA Multiplier Maximum Output Current IAC = 450µA, R

= 15k, VA

SET

OUT

= 7V, M

= 0V ● –286 –260 – 235 µA

OUT

Multiplier Gain Constant (Note 2) 0.035 V IAC Input Resistance IAC from 50µA to 1mA 15 32 50 kΩ

Oscillator

Oscillator Frequency R

C

Ramp Peak-to-Peak Amplitude 4.35 4.7 5.0 V

SET

C

Ramp Valley Voltage 1.25 1.4 1.55 V

SET

Synchronization Pulse Threshold on EN/SYNC Pin Synchronization Frequency Range

SET

R

SET

= 15k, C = 15k, C

= 1000pF ● 85 100 115 kHz

SET

= 1500pF ● 58 68 78 kHz

SET

Pulse Low = 3.5V, High = 7V, Width > 200ns R

SET

= 15k, C

= 1000pF

SET

4.5 5.6 6.5 V

● 1.2 1.6 f

NOM

Overvoltage Comparator

Comparator Trip Voltage Ratio (V

TRIP/VREF

) ● 1.04 1.05 1.06

Hysteresis 0.35 V OVP Bias Current OVP = 7.5V ● –50 – 250 nA OVP Propagation Delay 100 ns

Gate Driver

Max GTDR Output Voltage 0mA Load, 18V < V

CC

GTDR Output High –200mA Load, 11.5V ≤ VCC ≤ 15V ● V

● 12 15 17.5 V

– 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

10mA Load

● 0.2 0.4 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 of a device may be impaired

I

Note 2: Multiplier Gain Constant: K =

IAC (VA

M

OUT

– 2)

2

3

LT1248

LPER

UW

R

F

O

ATYPICA

CCHARA TERIST

E

C

ICS

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)

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

0 25 50 100

–25

JUNCTION TEMPERATURE (°C)

75

1M 10M

1148 G01

125

1248 G03

0

–20

–40

–60

–80

–100

–120

PHASE (DEG)

Current Amplifier Open-Loop Gain and Phase

100

80

GAIN

60

40

GAIN (dB)

–20

20

0

10

100

PHASE

1k 10k 100k

FREQUENCY (Hz)

Multiplier Current

300

VA

= 7V

OUT

VA

= 6.5V

OUT

VA

= 6V

OUT

150

(µA)

M

I

0

250

IAC (µA)

1M 10M

1148 G02

500

1248 G04

0

–20

–40

–60

–80

–100

–120

VA

VA VA

VA

OUT

OUT OUT

OUT

PHASE (DEG)

= 5.5V

= 5V

= 4.5V

= 4V

= 3.5V = 3V

= 2.5V

Supply Current vs Supply Voltage GTDR Source Current

11 10

TJ = –55°C

9 8 7 6 5 4 3

SUPPLY CURRENT (mA)

2 1 0

10

SUPPLY VOLTAGE (V)

TJ = 25°CTJ = 125°C

21

32

1248 G05

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

0

–60

SOURCE CURRENT (mA)

4

TJ = 125°C

TJ = 25°C

TJ = –55°C

–120 –180 –240

1248 G06

–300

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

1248 G07

LPER

GTDR Rise and Fall Time

400

300

200

TIME (ns)

100

0

FALL TIME

20 30 40

10

LOAD CAPACITANCE (nF)

R

F

O

RISE TIME

NOTE: GTDR SLEWS BETWEEN 1V AND 16V

ATYPICA

1248 G08

UW

CCHARA TERIST

E

C

Start-Up Supply Current vs Supply Voltage

550 500 450 400 350 300 250 200 150

SUPPLY CURRENT (µA)

100

50

0

2610

81216

4

SUPPLY VOLTAGE (V)

ICS

–55°C

25°C

125°C

14 18

1248 G09

LT1248

Frequency vs R

500 450 400 350 300 250 200

FREQUENCY (kHz)

150 100

50

0

20

600 1000 1800

200

C

SET

and C

SET

1400

CAPACITANCE (pF)

SET

R R R R

SET SET SET SET

= 10k = 15k = 20k = 30k

2200

1248 G10

GTDR Maximum Duty Cycle vs R

SET

1.00

0.99

0.98

0.97

0.96

0.95

0.94

0.93

MAXIMUM DUTY CYCLE

0.92

0.91

0.90 200

Synchronization and Shutdown Thresholds at EN/SYNC Pin

–44 –40

SHUTDOWN

–36

THRESHOLD

–32 –28 –24 –20 –16 –12

EN/SYNC CURRENT (µA)

–8 –4

0

123

EN/SYNC VOLTAGE (V)

SYNCHRONIZATION THRESHOLD

TJ = –55°C TJ = 25°C

TJ = 125°C

4

5

6789

and C

SET

R R R R

600 1000 1800

C

SET

1400

CAPACITANCE (pF)

SS CURRENT (µA)

10

1248 G13

–22 –20 –18 –16 –14 –12 –10

SET SET SET SET

–8 –6 –4 –2

= 10k = 15k = 20k = 30k

0

Shutdown Mode Supply Current and Reference Voltage

1248 G12

–1.2

M

1.1

1.0

0.9

REFERENCE VOLTAGE (V)

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1 0

32

0

VOLTAGE (V)

OUT

1.2

2.4

1248 G15

1.1

EN/SYNC ≤ 1.8V

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

SUPPLY CURRENT (mA)

0.2

0.1 0

2200

1248 G11

TJ = –55°C

TJ = 25°C

TJ = 125°C

0

4

SS VOLTAGE (V)

0

8

1248 G14

SUPPLY CURRENT –55°C ≤ TJ ≤ 25°C

TJ = 125°C

REFERENCE VOLTAGE

≤ 125°C

T

J

16

SUPPLY VOLTAGE (V)

M

OUT

1.5

1.0

0.5 0

–0.5 –1.0 –1.5

CURRENT (mA)

–2.0

OUT

M

–2.5 –3.0 –3.5 –4.0

–2.4

Pin CharacteristicsSS Pin Characteristics

TJ = 125°C

= 25°C

T

J

T

= –55°C

J

5

LT1248

LPER

R

SET

120 100

80 60 40

(mV)

20

REF

0

– V

SET

–20

VR

–40 –60 –80

–100

0

R

F

O

Voltage vs Current

–0.2

ATYPICA

–0.4

R

CURRENT (mA)

SET

–0.6

UW

CCHARA TERIST

E

C

TJ = 125°C

= 25°C

T

J

= –55°C

T

J

–0.8

–1.0

1248 G16

ICS

–360 –300 –240 –180 –120

CURRENT (µA)

LIM

PK

–60

60 120 180 240 300

PK

Pin Characteristics

LIM

0

–0.4

–0.8

PK

LIM

0

VOLTAGE (V)

TJ = 125°C T

= 25°C

J

= –55°C

T

J

0.4

0.8

1248 G17

U

UU

PI FU CTIO S

Pin 1 (GND).

Pin 2 (PK

comparator is GND. To set current limit, a resistor divider can be connected from V

Pin 3 (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.

Pin 4 (I

amplifier. This pin is clamped at –0.6V by an ESD protection diode.

Pin 5 (M

current output and the noninverting input of the current amplifier. This pin is clamped at –0.6V and 2V.

Pin 6 (IAC): 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. At the pin, a 32k resistor is in series with the current input, so that a lowpass RC can be used to filter out the switching noise from the high impedance lines.

): The threshold of the peak current limit

LIM

to current sense resistor.

REF

): This is the output of the current amplifier

OUT

): This is the inverting input of the current

SENSE

): This is the multiplier high impedance

OUT

is low,

Pin 7 (VA

): This is the output of the voltage error

OUT

amplifier. The output is clamped at 13.5V. When the output goes below 2.5V, the multiplier output current is zero.

Pin 8 (OVP): This is the input to the overvoltage comparator. The threshold is 1.05 times the reference voltage. When the comparator trips, the multiplier is quickly inhibited and outputs no current. Figure 4 in the Applications Information section shows how to set overvoltage threshold with only one additional resistor.

Pin 9 (V

or EN/SYNC goes low, V

): This is the 7.5V reference. When either V

REF

will stay at 0V. V

REF

CC

biases most of the internal circuity and can source up to 5mA externally.

Pin 10 (EN/SYNC): This pin has two functions. When it goes below 2.6V, the chip goes into shutdown mode and draws little current. Pulses at this pin that go below the 5V threshold will synchronize the chip. The synchronizing pulses should have an on-time of at least 200ns for the LT1248 resetting circuit to work.

Pin 11 (V

): This is the inverting input to the voltage

SENSE

amplifier.

6

LT1248

U

UU

PI FU CTIO S

Pin 12 (R

oscillator charging current and the maximum multiplier output current which is used to limit the maximum line current.

I

M(MAX)

Pin 13 (SS): Soft-Start. When either VCC or EN/SYNC goes low, the SS pin will stay at 0V. With a capacitor from the pin to GND, the 12µA charging current slowly brings up the SS to 8V; below 7.5V SS is the reference input to the voltage amplifier. At supply dropout or EN/SYNC low, the soft start capacitor will be quickly discharged.

Pin 14 (C

R

, determine oscillator frequency. The oscillator ramp

SET

is 5V, and the frequency = 1.5/(R

PPLICATI

A

): A resistor from R

SET

= 3.75V/R

): The capacitor from this pin to GND, and

SET

U

O

S

I FOR ATIO

to GND sets the

SET

• C

SET

).

SET

WU

U

Pin 15 (VCC): This is the supply for the chip. The LT1248 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 capacitor in parallel with a low ESR electrolytic capacitor, 56µF or higher is required in close proximity to IC GND.

Pin 16 (GTDR): The MOSFET gate driver is a 1.5A fast totem pole output. It is clamped at 15V, but capacitive loads like MOSFET gates may cause overshoot. A gate series resistor of at least 5Ω will prevent the overshoot.

Error Amplifier

The error amplifier has a 100dB DC gain and 3MHz unitygain frequency. The output is internally clamped at 13.5V. The noninverting input is tied to the 7.5V V

through a

REF

diode and can be pulled down from the SS (soft-start) pin.

Current Amplifier

The current amplifier has a 110dB DC gain, 3MHz unitygain frequency, and a 2V/µs slew rate. It is internally clamped at 8.5V. 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

OUT

to swing 5V over one line cycle at high line condition, 14mV AC will be needed at the inputs of the current amplifier for a gain of 350 at 120Hz. Especially at light load when the current loop reference signal is small, lower gain will distort the reference signal and line current. If signal gain at 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 the switching frequency, but more than 250 at twice the line frequency.

Multiplier

The multiplier is a current multiplier with high noise immunity in a high power switching environment. The current gain is: IM = (IAC • I

2

)/(200µA)2, with IEA = (VA

EA

OUT

– 2V)/25k. With a square function, because of the lower gain at light power load, system stability is maintained and line current distortion caused by the line frequency AC

300

VA

500

= 5.5V

OUT

VA

= 5V

OUT

VA

= 4.5V

OUT

VA

= 4V

OUT

VA

= 3.5V

OUT

VA

= 3V

OUT

VA

= 2.5V

OUT

VA

= 7V

OUT

VA

= 6.5V

OUT

VA

= 6V

OUT

150

(µA)

M

I

0

Figure 1. Multiplier Current IM vs IAC and VA

250

IAC (µA)

1248 G04

7

LT1248

PPLICATI

A

U

O

S

I FOR ATIO

WU

U

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 LT1248 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. The maximum multiplier output current, which limits the system line current, is set by the R to the formula: I

M(MAX)

= 3.75V/R

SET

.

according

SET

Oscillator Frequency and Maximum Line Current Settling

Oscillator frequency is set by R tude is 5V and C

charging current is set by V

SET

Typical discharging time for C

and C

SET

= 1nF is 250ns. R

SET

. Ramp ampli-

SET

REF/RSET

SET

.

should always be determined first to set the maximum multiplier output current for system line current limit. For a 300W preregulator, with R = 250µA. With a 4k resistor R line current sense resistor RS, the line current limit is: (I

SET

REF

= 15k, I

from M

M(MAX)

OUT

= 3.75V/15k

to the 0.2Ω

M

• 4k)/RS. As a general rule, RS is chosen according to: RS = I

M(MAX)

K(1.414)P where P

OUT(MAX)

• R

• V

REF

OUT(MAX)

LINE(MIN)

is the maximum power output and K is usually between 1.1 and 1.3 depending on efficiency and resistor tolerance. With R determined by: C C

= 1.5/(100kHz • 15k) = 1nF. For optional double

SET

= 1.5/(Frequency • R

SET

selected, C

SET

can then be

SET

). For 100kHz,

SET

protection, the LT1248 provides a current limit comparator. When the comparator trips at 0V, the GTDR pin quickly goes low to shut off the MOS switch. A resistor divider from V

to RS (Figure 2) senses the voltage across the

REF

line current sense resistor and the current limit is set by: I

= [(7.5V/R1) + 50µA](R2/RS), where 50µA is I

LINE

PKLIM

.

With I (I

LINE

Always use R PK

LIM

and RS chosen, let R1 = 10k, then R2 =

LINE

• RS)/0.8mA. to set the primary line current limit. The

SET

comparator is only for secondary protection. The

secondary limit should be higher than the primary limit;

6.5A is good (5A for primary limit) for a 300W regulator. When line current reaches the primary limit, V

drops to

OUT

keep the line current constant, and system stability is still maintained by the current loop which is controlled by the current amplifier. When line current reaches the secondary limit, the comparator controls the system and loop hysteresis may occur and can cause audible noise.

Synchronization

The LT1248 can be synchronized to a frequency that is up to 1.6 times the natural frequency. With a 200ns one-shot timer on-chip, the LT1248 provides flexibility on the synchronizing pulse width. Because the EN/SYNC pin also serves the chip shutdown function, the pulses at the pin should not go below 3V and must go below 5V with widths greater than 200ns. The Figure 3 circuit will synchronize the LT1248.

V

REF

30k

1N4148

EN/SYNC

VN2222

200k

1N4685

3.6V SYNC PULSE

AT LEAST 200ns

Figure 3

V

CC

1248 F03

Overvoltage Protection

R2

1.6k

+

R

S

Ω

–

C1 IS TO REJECT NOISE, CURRENT LIMIT DELAY IS ABOUT 2µs.

0.2

I

LINE

8

R1

10k

I

PKLIM

Figure 2

C1 1nF

7.5V

Because of the slow loop response necessary for power

V

REF

factor correction, output overshoot can occur with sudden load removal or reduction. To protect the power compo-

PK

LIM

–

+

nents and output load, the LT1248 provides an overvoltage comparator which senses the output voltage and quickly shuts off the current switch. In Figure 4, because

1248 F02

there is no DC current going through R3, R1 and R2 set the regulator output DC level: V R1 = 1M, R2 = 20k, V

OUT

= V

OUT

is 382V.

[(R1 + R2)/R2], with

REF

LT1248

U

O

PPLICATI

A

Note that V

SENSE

S

I FOR ATIO

is the summing node and it stays at 7.5V.

When overshoot occurs on V

, the overcurrent from R1

OUT

WU

U

will go through R2 as well as R3. Amplifier feedback will keep V

locked at 7.5V. The equivalent AC resistance,

SENSE

seen by the comparator input pin OVP, is R2 in parallel with R3, which is 10k. Therefore, with the comparator trip level of 1.05V V

overshoot exceeds 10%. Overvoltage trip level:

OUT

%%V

OUT

M

is a high impedance current output. In the current

OUT

and R3 of 20k, the comparator trips when

REF



5

RR





=



+

23



R

3



loop, offset line current is determined by multiplier offset current and input offset voltage of the current amplifier. A – 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

would slowly

OUT

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

OUT

condition, the amplifier M1 (see Block Diagram), becomes active in the current loop when VA The M1 can put out up to 7µA to the resistor at the I

goes down to 2.2V.

OUT

SENSE

pin to cancel any current amplifier negative VOS and keep V

error to within 2V.

OUT

0.047µF

= 7.5V

REF

C1

0.47µF

–

+

ERROR AMP

–

+

OVERVOLTAGE COMPARATOR

330k

VA

OUT

LT1248

1248 F04

REGULATOR OUTPUT

= 382V

V

OUT

R1 1M

R2 20k

R3

20k

V

SENSE

OVP

V

REF

1.05V

Figure 4

Undervoltage Lockout

The LT1248 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 LT1248 only draws 250µA, the oscillator is off, and the V

REF

and

the GTDR pins remain low to keep the power MOSFET off.

Start-Up and Supply Voltage

The LT1248 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. Then the auxiliary winding takes over and supplies the operating current. Note that D3 and the large value C3, in both Figures 5 and 6, 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 auxiliary winding determines VCC according to:

LINE MAIN INDUCTOR

N

P

C2

1000pF

D2

D1

N

S

D2

Figure 5

+

C3 390µF

Figure 6

+

18V

LINE

D3

R1

90k, 1W

C1 2µF

+

C2 2µF

MAIN INDUCTOR

R1 90k 1W

+

C4 56µF

D3D1

C3 390µF

V

CC

+

C4 56µF

1248 F05

V

CC

1248 F06

9

LT1248

PPLICATI

A

V

/(VCC – 2V) = NP/NS.

OUT

For 382V V

U

O

S

I FOR ATIO

and 18V VCC, Np/Ns ≈ 19.

OUT

WU

U

In Figure 6, 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 switching frequency. For V

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

OUT

= 382V, VZ = 18V, C =

OUT

1000pF, and f = 100kHz, Zener current will be 36mA. This is enough to operate the LT1248, including the FET gate drive. Normally soft-start is not needed because the LT1248 has overcurrent limit and overvoltage protection. If soft-start is used with a 0.01µF capacitor on SS pin, V

ramps up slower during start-up. Then C4 has to

OUT

hold VCC longer, and the circuit may not start. Increasing C4 to 100µF ensures start-up, but start-up time will be extended if the same 90k trickle charge resistor is used.

Output Capacitor

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

V

= (2) (I

P-P

where I

LOAD(DC)

)(Z)

: DC load current.

Z: capacitor impedance at 120Hz.

For 180µF at 300W load, I V

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

P-P

LOAD(DC)

= 300W/385V = 0.78A,

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 components. The first is at 120Hz; it’s RMS value is related to the DC load current as follows:

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

I

≈ I

3RMS

LOAD(DC)

For the United Chemicon KMH 400V capacitor series, ripple current multiplier for currents at 100kHz is 1.43. The equivalent 120Hz ripple current can be then found:

I

RMS

= (I

√

1RMS

)2 + (I

2RMS

/1.43)2 + (I

3RMS

/1.43)

2

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

I

LOAD(DC)

I

1RMS

I

2RMS

I

3RMS

I

RMS

= 0.52A

≈ 0.71 • 0.52A = 0.37A ≈ 0.82A at 120VAC ≈ I

LOAD(DC)

= (0.37A)2+(0.82A/1.43)2+(0.52A/1.43)2 = 0.77A

√

= 0.52A

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:

(105°C+∆TK) – (TA+∆TO)

L = L

O •

2

10

where:

L: expected life time L

: hours of load life at rated ripple current and rated

O

ambient temperature.

∆T

: Capacitor internal temperature rise at rated condi-

K

tion. ∆T

= (I2R)/(KA). Where I is the rated current,

K

R is capacitor ESR, and KA is a volume constant.

T

: Operating ambient temperature.

A

: Capacitor internal temperature rise at operating

∆T

O

condition.

I

≈ 0.71 • I

1RMS

LOAD(DC)

The second component contains the PF switching frequency ripple current and its harmonics. Analysis of the 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

10

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

0.95A. ∆TO can then be calculated from:

∆TK = (I

RMS

/0.95A)

2

•

∆TK = (0.77A/0.95A)

2

•

10°C = 6.6°C

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

(105°C+10°C) – (60°+6.6°C)

≈ 2000 • 2

L

O

10

≈ 57,000 hours

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

A

LT1248

U

O

PPLICATITYPICAL

300W, 382V Preregulator

1M

90V

TO

270V

4.7nF

50k

20k

T

6A

16V TO 10V

10

EN/SYNC

V

11

I

6

OVP

8

SS

13

2.6V/2.2V

SENSE

AC

12µA

0.047µF

0.47µF

V

CC

7.5V

EMI

FILTER

+

–

+

7.9V

5V

330k

+

750µH*

–

IRF840

R

S

0.2Ω

R

REF

4k

0.1µF

VA

OUT

–

EA

+

–

+

7

2.2V

32k

V

REF

9

7.5V V

REF

RUN

+

M1

–

I

A

IM =

I

B

ONE SHOT

200ns

2

I

A

200µA

4k

100pF

1nF

20k

3

–

+

SYNC

2

RUN

PK

–

+

LIM

R R

S

OSC

M

OUT

7µA

I

B

2

I

SENSE

4

5

CA

OUT

I

–

M

CA

+

0.7V

–

MURH860

1M 1%

20k 1%

V

= 18V**

CC

+

56µF 35V

GND

1

Q

16V

V

CC

15

GTDR

16

10Ω

1N5819

V

OUT

+

180µF

†

0.01µF

1. COILTRONICS CTX02-12236-1 (TYPE 52 CORE)

*

AIR MOVEMENT NEEDED AT POWER LEVEL GREATER THAN 250W.

2. COILTRONICS CTX02-12295 (MAGNETICS Kool Mµ

**

SEE START-UP AND SUPPLY VOLTAGE SECTION FOR V

†

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.

®

77930 CORE)

GENERATOR.

CC

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.

C

1000pF

SET

14

R

SET

12

15k

Kool Mµ is a registered trademark of Magnetics, Inc.

1248 TA01

11

LT1248

PACKAGE DESCRIPTIO

0.300 – 0.325

(7.620 – 8.255)

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)

0.020

(0.508)

MIN

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

U

Dimensions in inches (millimeters) unless otherwise noted.

N Package

16-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

0.045 – 0.065

(1.143 – 1.651)

0.255 ± 0.015*

0.065

(6.477 ± 0.381)

(1.651)

TYP

0.100 (2.54)

BSC

0.018 ± 0.003

(0.457 ± 0.076)

S Package

16-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

15

16

2

1

0.770*

(19.558)

MAX

14

3

12

13

4

11

6

5

910

8

7

N16 1098

0.386 – 0.394*

2

(9.804 – 10.008)

13

14

3

12

11

10

9

0.150 – 0.157** (3.810 – 3.988)

5

4

7

6

8

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

× 45°

0.016 – 0.050

(0.406 – 1.270)

(1.346 – 1.752)

0° – 8° TYP

0.053 – 0.069

0.014 – 0.019

(0.355 – 0.483)

TYP

0.050

(1.270)

BSC

0.004 – 0.010

(0.101 – 0.254)

0.228 – 0.244

(5.791 – 6.197)

16

15

1

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1103 Off-Line Switching Regulator Universal Off-Line Inputs with Outputs to 100W LT1249 PFC in SO-8 Simplified PFC Design with Minimal Part Count LT1508 Power Factor and PWM Controller Voltage Mode PWM, Simplified PFC Design LT1509 Power Factor and PWM Controller Complete Solution for Universal Off-Line Switching Power Supplies

S16 1098

12

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear-tech.com

1248fd LT/GP 0799 2K REV D • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1993