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參數(shù)資料
| 型號(hào)： | LT1249C |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Octal D-Type Edge Triggered Flip-Flops with 3-State Outputs 20-SOIC 0 to 70 |
| 中文描述： | 功率因數(shù)控制器 |
| 文件頁(yè)數(shù)： | 7/12頁(yè) |
| 文件大?。?/td> | 143K |
| 代理商： | LT1249C |
LT1249
7
APPLICATIO
S I
N
FOR
ATIO
U
Line Current Limiting
Maximum voltage across R
MOUT
is internally limited to
1.1V. Therefore, line current limit is 1.1V divided by the
sense resistor R
S
. With a 0.2
sense resistor R
S
line
current limit is 5.5A. As a general rule, R
S
is chosen
according
W
U
R
I
R
V
K
P
S
M MAX
(
MOUT
)
LINE MIN
(
OUT MAX
(
=
(
)(
)(
)
1 414
)
)
)
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. When the output is overloaded and line
current reaches limit, output voltage V
OUT
will drop to keep
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
OUT
drop and clipping of the line current, which degrades
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
OUT
pin is
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
OUT
after the transistor is
turned off. Positive slewing on CA
OUT
should be faster
than the oscillator ramp rate of 0.5V/
μ
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:
V
( )( )
V
R
g
OS
C
C
m
=
+
.
0 5
2
ts = pulse width
fs = pulse frequency
I
C
= CA
OUT
source current (
≈
150
μ
A)
V
C
= CA
OUT
operating voltage (1.8V to 6.8V)
R2 = resistor for the midfrequency “zero” in the current loop
g
m
= current amplifier transconductance (
≈
320
μ
mho)
With ts = 30ns, fs = 130kHz, V
C
= 3V and R2 = 10k, offset
voltage shift is
≈
5mV. Note that this offset voltage will add
slight distortion to line current at light load.
Figure 2. Synchronizing the LT1249
R1
10k
R2
10k
2k
5V
0V
V
CC
80pF
1N5712
CA
OUT
1nF
2N2369
1249 F02
Overvoltage Protection
In Figure 3, R1 and R2 set the regulator output DC level:
V
OUT
= V
REF
[(R1 + R2)/R2]. With R1 = 1M and R2 = 20k,
V
OUT
is 382V.
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
OUT
, the overcurrent from R1 will go through VA
OUT
because amplifier feedback keeps V
SENSE
locked at 7.5V.
When this overcurrent reaches 44
μ
A amplifier sinking
limit, the amplifier loses feedback and its output snaps low
to turn the multiplier off.
Overvoltage trip level:
V
OUT
= (44
μ
A)(R1)
–
+
V
OUT
VA
OUT
C1
0.47
μ
F
R3
330k
0.047
μ
F
LT1249
MULTIPLIER
44
μ
A
22
μ
A
V
SENSE
R1
1M
R2
20k
V
7.5V
EA
1249 F03
Figure 3. Overvoltage Protection
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