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

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.

www.national.com

13