8
LT1248
ripple fed back to the error amplifier is minimized. Note
that switching ripple on the high impedance lines could get
into the multiplier from the I
AC
pin and cause instability.
The LT1248 provides an internal 25k resistor in series with
the low impedance multiplier current input so that only a
capacitor from the I
AC
pin to GND is needed to filter out the
noise. The maximum multiplier output current, which
limits the system line current, is set by the R
SET
according
to the formula: I
M(MAX)
= 3.75V/R
SET
.
Oscillator Frequency and Maximum Line
Current Settling
Oscillator frequency is set by R
SET
and C
SET
. Ramp ampli-
tude is 5V and C
SET
charging current is set by V
REF
/R
SET
.
Typical discharging time for C
SET
= 1nF is 250ns. R
SET
should always be determined first to set the maximum
multiplier output current for system line current limit. For
a 300W preregulator, with R
SET
= 15k, I
M(MAX)
= 3.75V/15k
= 250µA. With a 4k resistor R
REF
from M
OUT
to the 0.2Ω
line current sense resistor R
S
, the line current limit is: (I
M
• 4k)/R
S
. As a general rule, R
S
is chosen according to:
R
S
= I
M(MAX)
• R
REF
• V
LINE(MIN)
K(1.414)P
OUT(MAX)
where P
OUT(MAX)
is the maximum power output and K is
usually between 1.1 and 1.3 depending on efficiency and
resistor tolerance. With R
SET
selected, C
SET
can then be
determined by: C
SET
= 1.5/(Frequency • R
SET
). For 100kHz,
C
SET
= 1.5/(100kHz • 15k) = 1nF. For optional double
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
from V
REF
to R
S
(Figure 2) senses the voltage across the
line current sense resistor and the current limit is set by:
I
LINE
= [(7.5V/R1) + 50µA](R2/R
S
), where 50µA is I
PKLIM
.
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
With I
LINE
and R
S
chosen, let R1 = 10k, then R2 =
(I
LINE
• R
S
)/0.8mA.
Always use R
SET
to set the primary line current limit. The
PK
LIM
comparator is only for secondary protection. The
secondary limit should be higher than the primary limit;
6.5A is good (5A for primary limit) for a 300W regulator.
When line current reaches the primary limit, V
OUT
drops to
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
+
–
I
LINE
R
S
0.2
Ω
I
PKLIM
C1
1nF
7.5V
V
REF
PK
LIM
C1 IS TO REJECT NOISE, CURRENT
LIMIT DELAY IS ABOUT 2µs.
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
regulator output DC level: V
OUT
= V
REF
[(R1 + R2)/R2], with
R1 = 1M, R2 = 20k, V
OUT
is 382V.
Figure 3
30k
V
REF
200k
V
CC
1N4148
1N4685
3.6V
EN/SYNC
VN2222
SYNC PULSE
AT LEAST 200ns
1248 F03
