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

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LT1249

Power Factor Controller

Linear Technology 

LT1249

APPLICATIONS INFORMATION

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.

MOUT is a high impedance current output. In the current

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, 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

current if the current amplifier has offset. To regulate 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.

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 VREF and the

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 VCC

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

LINE

MAIN INDUCTOR

NP

R1

NS

90k

1W

D1

D2

D3

+ C1

VCC

2µF + C3 + C4

+ C2

390µF

56µF

2µF

ALL CAPACITORS ARE RATED 35V

1249 F04

Figure 4. Power Supply for LT1249

C2

1000pF

450V

LINE

MAIN INDUCTOR

D2

+

D1

C3

390µF

35V

R1

D3

90k

1W

+ C4

18V

56µF

35V

VCC

1249 F05

Figure 5. Power Supply for LT1249

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.

Output Capacitor

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

VP-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

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