LT1508IN Linear Technology, LT1508IN Datasheet - Page 9

• Manufacturer Part Number: LT1508IN
• Description: IC PWM/POWER FACTOR CNTRLR 20DIP
• Manufacturer: Linear Technology

Specifications of LT1508IN

• Mode: Average Current
• Frequency - Switching: 100kHz
• Voltage - Supply: 27V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Through Hole
• Package / Case: 20-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Startup: -

APPLICATIONS
See Figure 2 for R
VA
performance over a wide range of output power and input
voltage without the addition of feedforward line frequency
ripple. Care must be taken to avoid feeding switching
frequency noise into the multiplier from the I
internal 25k is provided in series with the low impedance
multiplier input so that only a capacitor from the I
GND1 is required to filter noise. The maximum multiplier
output current, which ultimately limits the input line cur-
rent, is set by a resistor from the R
according to the formula: I
shows I
Figure 5 data was taken with R
Oscillator Frequency and Maximum
Line Current Setting
The oscillator frequency is set by R
the resistor from the R
capacitor from the C
determined first. The oscillator frequency, which is equal
to the switching frequency for both the PFC and PWM
section, is determined by:
VA
OUT
f
OSC
OUT
is squared in the multiplier, resulting in excellent
M
Figure 5. Multiplier Current I
=
versus I
(R
= 2 +
300
150
0
SET
0
1.5
)(C
AC
VA
REF
VA
(P
OUT
U
SET
OUT
for various values of VA
IN
.
VA
SET
= 6V
= 6.5V
)(R
OUT
)
SET
INFORMATION
pin to GND1. R
U
(V
= 7V
I
S
M(MAX)
AC
pin to GND1 and C
250
)(25)(R
IN 2
( A)
SET
)(R
M
= 3.75V/R
= 15k.
REF
vs I
W
SET
IAC
AC
)
SET
and C
+ 25k)
1508 • F05
and VA
SET
500
OUT.
pin to GND1
SET
VA
VA
VA
VA
VA
VA
VA
SET
OUT
OUT
OUT
OUT
OUT
OUT
OUT
AC
OUT
should be
U
SET
. Figure 5
Note that
. R
AC
= 5.5V
= 5V
= 4.5V
= 4V
= 3.5V
= 3V
= 2.5V
pin. An
is the
pin to
SET
is
The multiplier output acts as the command signal to the
current loop error amplifier. During steady-state operation
the voltage across R
this the value for R
with R
converter with an efficiency (eff) of 0.8 at low line (90V
and R
A 0.15 resistor will yield a maximum peak input current
of (I
a 100kHz switching frequency with R
(100kHz)(15k) = 1nF. For added protection the LT1508
provides a second independent current limit comparator.
When the input voltage to the comparator (PK
below 0V, GTDR1 pin quickly goes low turning off the PFC
power switch. A resistor divider from V
senses the voltage across the line current sense resistor
(R
50 A] (R2/R
current which flows out of the PK
and R2 = 1.8k, I
average plus the input inductor peak ripple current.
Always use R
PK
the line current reaches the primary limit, V
longer be supported with the given input current and
begins to fall. System stability is maintained by the current
loop which is controlled by the current amplifier. When the
C1 IS TO REJECT NOISE, CURRENT
LIMIT DELAY IS ABOUT 2 s
S
R
(250 A)(4k)(90VAC)(0.8)
LIM
) and limits the peak input line current to [(7.5V/R1) +
M(MAX)
S
REF
SET
comparator is only for secondary protection. When
–
+
(I
set to 4k, R
= 15k, I
M(MAX)
)(R
300W 2
S
SET
REF
). The 50 A represents the PK
0.15
1.8k
R2
R
to set the primary line current limit. The
M(MAX)
/R
IN
)(R
P
S
I
LINE
S
OUT
S
REF
= 9.6A peak above the 6.67A peak
is determined by:
) = (250 A)(4k)/0.15 = 6.67A. For
REF
S
should be less than:
= (I
)(V
= 3.75/15k = 250 A. For a 300W
Figure 6
2
10k
R1
M
I
PKLIM
IN
)(R
)(eff)
= 0.169
REF
C1
1nF
LIM
7.5V
) = (I
SET
pin. With R1 = 10k
REF
PK
V
IN
= 15k, C
REF
LIM
)(R
to R
LT1508
S
LIM
OUT
). Based on
S
(Figure 6)
LIM
SET
–
+
pin) dips
LT1508
can no
= 1.5/
1508 • F06
input
RMS
9
)