8

LT1248

ripple fed back to the error amplifier is minimized. Note

that switching ripple on the high impedance lines could get

The LT1248 provides an internal 25k resistor in series with

the low impedance multiplier current input so that only a

noise. The maximum multiplier output current, which

Oscillator Frequency and Maximum Line

Current Settling

should always be determined first to set the maximum

multiplier output current for system line current limit. For

= 250

usually between 1.1 and 1.3 depending on efficiency and

protection, the LT1248 provides a current limit compara-

tor. When the comparator trips at 0V, the GTDR pin quickly

goes low to shut off the MOS switch. A resistor divider

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

S

APPLICATI

I FOR ATIO

secondary limit should be higher than the primary limit;

6.5A is good (5A for primary limit) for a 300W regulator.

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 second-

ary 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.

Figure 2

+

–

0.2

Ω

C1

1nF

7.5V

C1 IS TO REJECT NOISE, CURRENT

LIMIT DELAY IS ABOUT 2

µs.

R2

1.6k

R1

10k

–

+

1248 F02

Overvoltage Protection

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 LT1248 provides an overvolt-

age comparator which senses the output voltage and

quickly shuts off the current switch. In Figure 4, because

there is no DC current going through R3, R1 and R2 set the

Figure 3

30k

200k

1N4148

1N4685

3.6V

EN/SYNC

VN2222

SYNC PULSE

AT LEAST 200ns

1248 F03