LT1248CS datasheet(9/12 Pages) LINER

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LT1248

APPLICATI

I FOR ATIO

Note that VSENSE is the summing node and it stays at 7.5V.

When overshoot occurs on VOUT, the overcurrent from R1

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

keep VSENSE locked at 7.5V. The equivalent AC resistance,

seen by the comparator input pin OVP, is R2 in parallel

with R3, which is 10k. Therefore, with the comparator trip

level of 1.05VREF and R3 of 20k, the comparator trips when

VOUT overshoot exceeds 10%. Overvoltage trip level:

OUT =









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 – 4mV current amplifier VOS translates into 20mA line

current and 5W input power for 250V line if 0.2

Ω 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 Diagram), becomes

active in the current loop when VAOUT goes down to 2.2V.

The M1 can put out up to 7

µA to the resistor at the ISENSE

pin to cancel any current amplifier negative VOS and keep

VOUT error to within 2V.

Figure 6

VCC

90k

18V

1248 F06

390

µF

LINE

MAIN INDUCTOR

1000pF

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 250

µA, the oscillator is off, and the VREF and

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

Start-Up and Supply Voltage

The LT1248 draws only 250

µA 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. 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 varia-

tion down to minimum load and/or very light load condi-

tions. 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 wind-

ing determines VCC according to:

Figure 5

VCC

90k, 1W

µF

1248 F05

µF

390

µF

LINE

MAIN INDUCTOR

0.47

µF

VREF = 7.5V

1.05VREF

OVERVOLTAGE

COMPARATOR

LT1248

20k

330k

0.047

µF

REGULATOR OUTPUT

VOUT = 382V

1248 F04

VSENSE

OVP

VAOUT

ERROR AMP

20k

–

Figure 4

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