9
LT1248
S
A
PPLICATI
U
U
I FOR ATIO
Note that V
SENSE
is the summing node and it stays at 7.5V.
When overshoot occurs on V
OUT
, the overcurrent from R1
will go through R2 as well as R3. Amplifier feedback will
keep V
SENSE
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 1.05V
REF
and R3 of 20k, the comparator trips when
V
OUT
overshoot exceeds 10%. Overvoltage trip level:
%
%
V
R R
R
OUT
=
+
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
5
2 3
3
M
OUT
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 V
OS
translates into 20mA line
current and 5W input power for 250V line if 0.2& sense
resistor is used. Under no load or when the load power is
less than this offset input power, V
OUT
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 V
OUT
under this
condition, the amplifier M1 (see Block Diagram), becomes
active in the current loop when VA
OUT
goes down to 2.2V.
The M1 can put out up to 7礎 to the resistor at the I
SENSE
pin to cancel any current amplifier negative V
OS
and keep
V
OUT
error to within 2V.
Figure 6
V
CC
R1
90k
1W
18V
1248 F06
+
C3
390礔
C4
56礔
+
LINE
MAIN INDUCTOR
C2
1000pF
D3
D1
D2
Undervoltage Lockout
The LT1248 turns on when V
CC
is higher than 16V and
remains on until V
CC
falls below 10V, whereupon the chip
enters the lockout state. In the lockout state, the LT1248
only draws 250礎, the oscillator is off, and the V
REF
and
the GTDR pins remain low to keep the power MOSFET off.
Start-Up and Supply Voltage
The LT1248 draws only 250礎 before the chip starts at
16V on V
CC
. To trickle start, a 90k resistor from the power
line to V
CC
supplies the trickle current and C4 holds the V
CC
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 V
CC
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 V
CC
according to:
Figure 5
V
CC
N
P
N
S
R1
90k, 1W
C1
2礔
1248 F05
+
+
C2
2礔
C3
390礔
+
C4
56礔
+
LINE
MAIN INDUCTOR
D2
D3
D1
C1
0.47礔
V
REF
= 7.5V
1.05V
REF
OVERVOLTAGE
COMPARATOR
LT1248
R3
20k
330k
0.047礔
REGULATOR OUTPUT
V
OUT
= 382V
1248 F04
V
SENSE
OVP
VA
OUT
ERROR AMP
R1
1M
R2
20k
+
+
Figure 4
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