lm5025asdx National Semiconductor Corporation, lm5025asdx Datasheet - Page 13

• Manufacturer Part Number: lm5025asdx
• Description: Active Clamp Voltage Mode Pwm Controller
• Manufacturer: National Semiconductor Corporation

Feed-Forward Ramp
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 20µA current is
sourced out of the softstart pin (SS) into an external capaci-
tor. The capacitor voltage will ramp up slowly and will limit
the COMP pin voltage and therefore the PWM duty cycle. In
the event of a fault as determined by V
undervoltage (UVLO) or second level current limit, the output
gate drivers are disabled and the softstart capacitor is fully
(Continued)
CC
undervoltage, line
13
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 1µA until the first output pulse is generated by the
PWM comparator. The current source returns to the nominal
20µA 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 tem-
perature 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 de-
vice will restart after the thermal hysteresis (typically 25˚C).
During a restart after thermal shutdown, the softstart capaci-
tor will be fully discharged and then charged in the low
current mode (1µA) similar to a second level current limit
event. The thermal protection feature is provided to prevent
catastrophic failures from accidental device overheating.
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