8

LT1508

To avoid subharmonic oscillations, the amplified downslope

of the inductor current must be less than the slope of the

oscillator ramp.

(5V)(500

µH)(100k)

(382V)(0.15

Ω)

≤

=

= 4.4

If the current amplifier gain at 100kHz is less than 4.4,

there will be no subharmonic oscillation. The open-loop

gain of the current loop is given by:

(j)(2

3648

(j)(f)

(382V)(0.15

Ω)

(j)(2

πf)(500µH)(5V)

=

==

The current error amp, with R5 = 4k, R6 = 20k, C3 =

0.001

µF and C4 = 300pF, provides zero pole compensa-

tion resulting in 16kHz loop crossover frequency. The

current amp gain at 100kHz is 1.7. The resulting current

loop gain and phase margin is shown in Figure 4.

0.15

Ω

L

4k

R5

4k

D1

470

µF

C4

300pF

C3

0.001

µF

R6

20k

CA

1M

1508 • F02

LT1508

+

–

Figure 2

FREQUENCY (kHz)

0.1

–40

40

60

80

1

10

100

1000

1508 • F04

20

0

–20

–30

30

45

60

15

0

–15

Figure 4

FREQUENCY (Hz)

0.1

–40

40

60

80

1

10

100

1000

1508 • F03

20

0

–20

–30

30

45

60

15

0

–15

Figure 3

Current Amplifier (PFC Section)

The current amplifier has a 110dB DC gain, 3MHz unity-

gain frequency and a 2V/

µs slew rate. It is internally

clamped at 8.5V. Note that in the current averaging opera-

tion, high gain at twice the line frequency is necessary to

to swing 5V over one line cycle at high line condition,

20mV AC will be needed at the inputs of the current

amplifier for a gain of 260 at 120Hz. Especially at light load

when the current loop reference signal is small, lower gain

will distort the reference signal and line current. But, if

signal gain at switching frequency is too high, the system

behaves more like a current mode system and can cause

subharmonic oscillation.

Multiplier

The multiplier has high noise immunity and superior

linearity over its full operating range. The current gain is

error amplifier output voltage required at the input to the

multiplier is: