LT1248IS Linear Technology, LT1248IS Datasheet - Page 9

• Manufacturer Part Number: LT1248IS
• Description: IC PFC CTRLR AVERAGE CURR 16SOIC
• Manufacturer: Linear Technology

Specifications of LT1248IS

• Mode: Average Current
• Frequency - Switching: 300kHz
• Current - Startup: 250µA
• Voltage - Supply: 15.5 V ~ 27 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

A
Note that V
When overshoot occurs on V
will go through R2 as well as R3. Amplifier feedback will
keep V
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
V
M
loop, offset line current is determined by multiplier offset
current and input offset voltage of the current amplifier.
A – 4mV current amplifier V
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
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
condition, the amplifier M1 (see Block Diagram), becomes
active in the current loop when VA
The M1 can put out up to 7 A to the resistor at the I
pin to cancel any current amplifier negative V
V
OUT
OUT
REGULATOR OUTPUT
OUT
PPLICATI
V
V
OUT
overshoot exceeds 10%. Overvoltage trip level:
error to within 2V.
is a high impedance current output. In the current
OUT
SENSE
= 382V
R1
1M
R2
20k
SENSE
locked at 7.5V. The equivalent AC resistance,
5
REF
20k
R3
O
and R3 of 20k, the comparator trips when
is the summing node and it stays at 7.5V.
R
2
U
V
OVP
R
SENSE
3
S
R
V
3
Figure 4
REF
1.05V
I FOR ATIO
U
= 7.5V
OS
OUT
REF
translates into 20mA line
–
+
+
–
, the overcurrent from R1
0.047 F
0.47 F
OUT
C1
ERROR AMP
OVERVOLTAGE
COMPARATOR
W
goes down to 2.2V.
OUT
330k
OUT
would slowly
VA
LT1248
OS
OUT
under this
1248 F04
U
and keep
SENSE
Undervoltage Lockout
The LT1248 turns on when V
remains on until V
enters the lockout state. In the lockout state, the LT1248
only draws 250 A, the oscillator is off, and the V
the GTDR pins remain low to keep the power MOSFET off.
Start-Up and Supply Voltage
The LT1248 draws only 250 A before the chip starts at
16V on V
line to V
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
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
LINE
CC
CC
D1
1000pF
supplies the trickle current and C4 holds the V
MAIN INDUCTOR
C2
D2
. To trickle start, a 90k resistor from the power
D2
+
D1
CC
CC
C3
390 F
N
N
according to:
P
S
falls below 10V, whereupon the chip
Figure 5
Figure 6
+
+
18V
LINE
D3
CC
C1
2 F
C2
2 F
90k, 1W
+
R1
is higher than 16V and
MAIN INDUCTOR
R1
90k
1W
+
C4
56 F
C3
390 F
D3
LT1248
+
V
CC
1248 F05
1248 F06
REF
C4
56 F
CC
and
V
9
CC
up
CC