LT1249 Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LT1249

Power Factor Controller

Linear Technology 

LT1249

APPLICATIONS INFORMATION

Line Current Limiting

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

RS

=

(IM(MAX))(RMOUT )(VLINE(MIN))

K(1.414)POUT(MAX)

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

current reaches limit, output voltage VOUT will drop to keep

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.

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

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 CAOUT after the transistor is

turned off. Positive slewing on CAOUT should be faster

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 volt-

age on the current amplifier inputs, according to:

∆VOS

=



(ts)(fs)IC

+

VC − 0.5

R2 

gm

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)

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.

CAOUT

VCC

R1

1N5712 10k

R2

10k

2N2369

1nF

80pF

2k

5V

0V

1249 F02

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

factor correction, output overshoot can occur with sudden

load removal or reduction. To protect the power compo-

nents 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 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, the amplifier loses feedback and its output snaps low

to turn the multiplier off.

Overvoltage trip level: ∆VOUT = (44µA)(R1)

VOUT

0.047µF

C1

0.47µF

R3

330k

R1

1M

VSENSE

R2

20k

VREF

7.5V

–

VAOUT

EA

+

MULTIPLIER

44µA

22µA

LT1249

1249 F03

Figure 3. Overvoltage Protection

7

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