LT1509ISW#TR Linear Technology, LT1509ISW#TR Datasheet - Page 10

• Manufacturer Part Number: LT1509ISW#TR
• Description: IC PFC CTRLR AVERAGE CURR 20SOIC
• Manufacturer: Linear Technology

Specifications of LT1509ISW#TR

• Mode: Average Current
• Frequency - Switching: 100kHz
• Voltage - Supply: 27V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Startup: -
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIONS
10
LT1509
line current reaches the secondary limit, the comparator
takes over control and hysteresis may occur causing
audible noise.
Overvoltage Protection (PFC Section)
Because of the slow loop response necessary for power
factor correction, output overshoot can occur following a
sudden load reduction or removal. To protect downstream
components, the LT1509 provides an overvoltage com-
parator which senses the output voltage and quickly
reduces the line current demand. Referring back to Figure
1, V
current flows in R3, 7.5V appears at both the V
OVP pins. When V
overcurrent from R1 will flow through R2 as well as R3.
The voltage amplifier feedback will keep V
Therefore, the equivalent AC resistance seen by the OVP
pin is R2 in parallel with R3 or 10k. With these values and
the overvoltage comparator trip level internally set at
1.05V
10%. Overvoltage trip level is given by:
For additional protection, the OVP pin can be connected
to V
Figure 7). This ensures overvoltage protection during
safety agency abnormal testing conditions, such as
opening R1 or shorting R2.
The output of the multiplier looks like a high impedance
current source. In the current loop, offset line current is
determined by multiplier offset current and input offset
voltage of the current error amplifier. A – 4mV current
amplifier V
input power for 250VAC line if a 0.15 sense resistor is
used. Under a no-load condition or when the load power
is less than the offset output power, the offset line current
could slowly charge the output to an overvoltage level.
This is because the best the overvoltage comparator can
do is to reduce the multiplier output current to zero.
Unfortunately, this does not guarantee zero output current
if the current amplifier has offset. To regulate V
(%)V
OUT
OUT
REF
OUT
is 382V and during normal operation, since no
, the comparator trips when V
through an independent resistor divider (see
OS
= 5%
translates to 27mA line current and 6.7W
U
OUT
R2 + R3
R3
overshoots its preset value, the
INFORMATION
U
W
OUT
SENSE
overshoots
U
OUT
SENSE
at 7.5V.
under
and
this condition, the amplifier M1 (see Block Diagram)
becomes active. When VA
plies up to 7 A of current to the resistor at the I
in order to cancel a negative V
within 2V.
Undervoltage Lockouts and Soft Start
The LT1509 turns on when V
on until V
lockout state. In the lockout state, the oscillator is off and
the V
SS1 to GND1 determines the ramp-up time of the PFC
section. SS1 is released from a zero when V
the lockout threshold. Once released, an internal 14 A
current source ramps the voltage error amplifier’s refer-
ence voltage to 7.5V. SS1 voltage then continues beyond
7.5V. A second capacitor from SS2 to GND1 determines
the start-up time from the PWM section. A PWMOK
comparator (see Block Diagram) holds SS2 low until the
OVP pin reaches 7V. This corresponds to the PFC output
voltage reaching approximately 93% of its preset voltage.
SS2 is diode coupled to the PMW comparator which is
connected to the V
low at any time will disable PWM output. Once released,
the 14 A current source ramps the PWM comparator
R5
20k
R4
1.05M
REF
and gate driver pins remain low. A capacitor from
CC
V
OUT
falls below 10V, whereupon the chip enters the
R1
1M
R2
20k
C
0.47 F
V
OVERVOLTAGE = 420V
pin by a second diode. Holding SS2
OUT
V
OVP
SENSE
1.05V
= 382V
Figure 7
OUT
0.047 F
REF
CC
–
+
–
+
OS
reduces to 2.2V, M1 sup-
reaches 16V and remains
and keep V
ERROR
AMP
OVERVOLTAGE
COMPARATOR
330k
VA
OUT
LT1509
CC
1509 • F07
OUT
rises above
SENSE
error to
pin