LT1508ISW 데이터 시트보기 (PDF) - Linear Technology
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LT1508ISW Datasheet PDF : 16 Pages
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LT1508
APPLICATIONS INFORMATION
VIN
L
+
VIN
IIN
–
RS
0.15Ω
D1 VOUT
COUT
470µF
RREF R5
4k
4k
C4
300pF
C3
R6
0.001µF
20k
VIN
RIAC
1M
IM
IAC
MOUT
ISENSE –
CA
+
Figure 2
CAOUT
LT1508
1508 • F02
80
60
40
20
0
–20
–40
0.1
60
45
30
15
0
–15
–30
1
10
100
1000
FREQUENCY (Hz)
1508 • F03
Figure 3
Current Amplifier (PFC Section)
The current amplifier has a 110dB DC gain, 3MHz unity-
gain frequency and a 2V/µs slew rate. It is internally
clamped at 8.5V. Note that in the current averaging opera-
tion, high gain at twice the line frequency is necessary to
minimize line current distortion. Because CAOUT may need
to swing 5V over one line cycle at high line condition,
20mV AC will be needed at the inputs of the current
amplifier for a gain of 260 at 120Hz. Especially at light load
when the current loop reference signal is small, lower gain
will distort the reference signal and line current. But, if
signal gain at switching frequency is too high, the system
behaves more like a current mode system and can cause
subharmonic oscillation.
To avoid subharmonic oscillations, the amplified downslope
of the inductor current must be less than the slope of the
oscillator ramp.
VCA(OUT)
VRS
≤
(VOSC)(L)(fSW)
(VOUT)(RS)
= (5V)(500µH)(100k) = 4.4
(382V)(0.15Ω)
If the current amplifier gain at 100kHz is less than 4.4,
there will be no subharmonic oscillation. The open-loop
gain of the current loop is given by:
VRS = (VOUT)(RS)
VCA(OUT) (j)(2πf)(L)(VOSC)
=
(382V)(0.15Ω)
(j)(2πf)(500µH)(5V)
=
3648
(j)(f)
The current error amp, with R5 = 4k, R6 = 20k, C3 =
0.001µF and C4 = 300pF, provides zero pole compensa-
tion resulting in 16kHz loop crossover frequency. The
current amp gain at 100kHz is 1.7. The resulting current
loop gain and phase margin is shown in Figure 4.
80
60
60
45
40
30
20
15
0
0
–20
–15
–40
0.1
–30
1
10
100
1000
FREQUENCY (kHz)
1508 • F04
Multiplier
Figure 4
The multiplier has high noise immunity and superior
linearity over its full operating range. The current gain is
IM = (IACIEA2)/(200µA2) with IEA = (VAOUT – 2V)/ 25k. The
error amplifier output voltage required at the input to the
multiplier is:
8
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