lm5025cmtcx National Semiconductor Corporation, lm5025cmtcx Datasheet - Page 12

• Manufacturer Part Number: lm5025cmtcx
• Description: Active Clamp Voltage Mode Pwm Controller
• Manufacturer: National Semiconductor Corporation

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Feed-Forward Ramp
An external resistor (R
V
The slope of the signal at the RAMP pin will vary in proportion
to the input line voltage. This varying slope provides line feed-
forward information necessary to improve line transient re-
sponse with voltage mode control. The RAMP signal is
compared to the error signal at the COMP pin by the pulse
width modulator comparator to control the duty cycle of the
main switch output. The Volt Second Clamp comparator also
monitors the RAMP pin and if the ramp amplitude exceeds
2.5V the present cycle is terminated. The ramp signal is reset
to GND at the end of each cycle by either the internal clock
or the Volt Second comparator, which ever occurs first.
Soft-Start
The softstart feature allows the power converter to gradually
reach the initial steady state operating point, thus reducing
start-up stresses and surges. At power on, a 90 µA current is
sourced out of the softstart pin (SS) into an external capacitor.
The capacitor voltage will ramp up slowly and will limit the
COMP pin voltage and therefore the PWM duty cycle. In the
IN
and GND are required to create the PWM ramp signal.
FF
) and capacitor (C
FF
) connected to
12
event of a fault as determined by V
dervoltage (UVLO) or second level current limit, the output
gate drivers are disabled and the softstart capacitor is fully
discharged. When the fault condition is no longer present a
softstart sequence will be initiated. Following a second level
current limit detection (CS2), the softstart current source is
reduced to 1 µA until the first output pulse is generated by the
PWM comparator. The current source returns to the nominal
90 µA level after the first output pulse (~1V at the SS pin).
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the
integrated circuit in the event the maximum junction temper-
ature is exceeded. When activated, typically at 165°C, the
controller is forced into a low power standby state with the
output drivers and the bias regulator disabled. The device will
restart after the thermal hysteresis (typically 25°C). During a
restart after thermal shutdown, the softstart capacitor will be
fully discharged and then charged in the low current mode (1
µA) similar to a second level current limit event. The thermal
protection feature is provided to prevent catastrophic failures
from accidental device overheating.
CC
undervoltage, line un-