LT1249CS8#PBF Linear Technology, LT1249CS8#PBF Datasheet - Page 8

• Manufacturer Part Number: LT1249CS8#PBF
• Description: IC PFC CTRLR AVERAGE CURR 8SOIC
• Manufacturer: Linear Technology

Specifications of LT1249CS8#PBF

• Mode: Average Current
• Frequency - Switching: 100kHz
• Current - Startup: 250µA
• Voltage - Supply: 15.5 V ~ 27 V
• Operating Temperature: 0°C ~ 100°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS
LT1249
The Figure 3 circuit therefore has 382V on V
overvoltage level = (V
hysteresis, V
feedback recovers and the switch turns back on.
M
loop, offset line current is determined by multiplier offset
current and input offset voltage of the current amplifier.
A negative 4mV current amplifier V
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
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
under this condition, the amplifier M1 (see Block Dia-
gram), becomes active in the current loop when VA
goes down to 1V. The M1 can put out up to 15 A to the 4k
resistor at the inverting input to cancel the current ampli-
fier negative V
Undervoltage Lockout
The LT1249 turns on when V
remains on until V
enters the lockout state. In the lockout state, the LT1249
only draws 250 A, the oscillator is off, the V
GTDR pins remain low to keep the power MOSFET off.
Start-Up and Supply Voltage
The LT1249 draws only 250 A before the chip starts at
16V on V
line to V
up while switching starts (see Figure 4). Then the auxiliary
winding takes over and supplies the operating current.
Note that D3 and the large value C3, in both Figures 4 and
5, are only necessary for systems that have sudden large
load variation down to minimum load and/or very light
load conditions. 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
variations, D3 is replaced with a short and C3 is omitted.
The turns ratio between the primary winding and the
8
OUT
is a high impedance current output. In the current
CC
CC
CC
up until switching resumes. For less severe load
supplies the trickle current and C4 holds the V
. To trickle start, a 90k resistor from the power
OUT
OS
and keep V
then has to drop 22V to 404V before
CC
U
falls below 10V, whereupon the chip
OUT
INFORMATION
U
+ 44V), or 426V. With a 22 A
OUT
CC
error to within 2V.
is higher than 16V and
W
OS
translates into
OUT
REF
U
OUT
, and an
and the
would
OUT
OUT
CC
auxiliary winding determines V
– 2V) = N
In Figure 5 a new technique for supply voltage eliminates
the need for an extra inductor winding. It uses capacitor
charge transfer to generate a constant current source
which feeds a Zener diode. Current to the Zener is equal to
(V
switching frequency. For V
1000pF and f = 100kHz, Zener current will be 36mA. This
is enough to operate the LT1249, including the FET gate
drive.
Output Capacitor
The peak-to-peak 120Hz output ripple is determined by:
For 180 F at 300W load, I
LINE
OUT
V
where I
P-P
– V
= (2)(I
ALL CAPACITORS ARE RATED 35V
D1
MAIN INDUCTOR
1000pF
P
450V
LOAD
Z
/N
D2
C2
D2
)(C)(f), where V
Figure 4. Power Supply for LT1249
S
Figure 5. Power Supply for LT1249
D1
. For 382V V
LOAD
+
DC: DC load current
Z: capacitor impedance at 120Hz
N
N
C3
390 F
35V
DC)(Z)
P
S
+
+
LOAD
OUT
18V
LINE
OUT
Z
D3
C1
2 F
C2
2 F
CC
is Zener voltage and f is
DC = 300W/385V = 0.78A,
and 18V V
90k
1W
R1
+
= 382V, V
according to: V
MAIN INDUCTOR
+
R1
90k
1W
C4
56 F
35V
C3
390 F
D3
CC
Z
, N
+
= 18V, C =
V
P
CC
/N
OUT
C4
56 F
1249 F05
S
/(V
V
CC
1249 F04
19.
CC