LT1249IS8#PBF Linear Technology, LT1249IS8#PBF Datasheet - Page 7

• Manufacturer Part Number: LT1249IS8#PBF
• Description: IC PFC CTRLR AVERAGE CURR 8SOIC
• Manufacturer: Linear Technology

Specifications of LT1249IS8#PBF

• Mode: Average Current
• Frequency - Switching: 100kHz
• Current - Startup: 250µA
• Voltage - Supply: 15.5 V ~ 27 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS
Line Current Limiting
Maximum voltage across R
1.1V. Therefore, line current limit is 1.1V divided by the
sense resistor R
current limit is 5.5A. As a general rule, R
according
where P
usually between 1.1 and 1.3 depending on efficiency and
resistor tolerance. When the output is overloaded and line
current reaches limit, output voltage V
line current constant. System stability is still maintained
by the current loop which is controlled by the current
amplifier. Further load current increase results in further
V
power factor.
Synchronization
The LT1249 can be externally synchronized in a frequency
range of 127kHz to 160kHz. Figure 2 shows the synchro-
nizing circuit. Synchronizing occurs when CA
pulled below 0.5V with an external transistor and a Schottky
diode. The Schottky diode and the 10k pull-up resistor are
necessary for the required fast slewing back up to the
normal operating voltage on CA
turned off. Positive slewing on CA
than the oscillator ramp rate of 0.5V/ s.
The width of the synchronizing pulse should be under
60ns. The synchronizing pulses introduce an offset volt-
age on the current amplifier inputs, according to:
ts = pulse width
fs = pulse frequency
I
V
R2 = resistor for the midfrequency “zero” in the current loop
g
C
m
OUT
C
= CA
R
= CA
= current amplifier transconductance ( 320 mho)
S
V
drop and clipping of the line current, which degrades
OS
OUT
OUT
OUT(MAX)
(
I
M MAX
source current ( 150 A)
(
operating voltage (1.8V to 6.8V)
ts fs I
K
( .
1 414
)
S
is the maximum power output and K is
)(
. With a 0.2
U
R
C
MOUT
g
)
m
P
OUT MAX
INFORMATION
V
U
C
)(
R
MOUT
(
V
2
0 5
LINE MIN
OUT
)
sense resistor R
(
is internally limited to
W
OUT
after the transistor is
OUT
)
)
should be faster
will drop to keep
S
OUT
is chosen
U
S
pin is
line
With ts = 30ns, fs = 130kHz, V
voltage shift is 5mV. Note that this offset voltage will add
slight distortion to line current at light load.
Overvoltage Protection
In Figure 3, R1 and R2 set the regulator output DC level:
V
V
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 LT1249 voltage error amplifier
senses the output voltage and quickly shuts off the current
switch when overvoltage occurs. When overshoot occurs
on V
because amplifier feedback keeps V
When this overcurrent reaches 44 A amplifier sinking
limit, the amplifier loses feedback and its output snaps low
to turn the multiplier off.
Overvoltage trip level: V
OUT
OUT
OUT
= V
is 382V.
V
OUT
, the overcurrent from R1 will go through VA
R1
1M
R2
20k
REF
[(R1 + R2)/R2]. With R1 = 1M and R2 = 20k,
Figure 2. Synchronizing the LT1249
CA
V
7.5V
Figure 3. Overvoltage Protection
REF
OUT
V
R2
10k
SENSE
1nF
1N5712
0.047 F
0.47 F
2N2369
C1
–
+
EA
V
OUT
CC
MULTIPLIER
R1
10k
LT1249
= (44 A)(R1)
C
330k
R3
= 3V and R2 = 10k, offset
2k
80pF
SENSE
VA
OUT
1249 F02
locked at 7.5V.
44 A
22 A
5V
0V
LT1249
1249 F03
OUT
7