TPS40090

TPS40091

SLUS578A − OCTOBER 2003 − REVISED JUNE 2004

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APPLICATION INFORMATION

If the overcurrent condition continues, each phase’s PWM cycle is terminated by the overcurrent signals. This

puts a converter in a constant current mode with the output current programmed by the ILIM voltage. Eventually

the supply and demand equilibrium on the converter output fails and the output voltage declines. When the

undervoltage threshold is reached, the converter enters a hiccup mode. The controller is stopped and the output

is not regulated any more, the softstart pin function changes. It serves now as a timing capacitor for a fault control

circuit. The soft-start pin is periodically charged and discharged by the fault control circuit. After seven hiccup

cycles expire, the controller attempts to restore normal operation. If the overload condition is not cleared, the

controller stays in the hiccup mode indefinitely long. In such conditions the average current delivered to the load

is roughly 1/8 of the set overcurrent value.

UNDERVOLTAGE PROTECTION

If the FB pin voltage falls lower than the undervoltage protection threshold (84.5%), the controller enters the

hiccup mode as it is described in the Overcurrent Protection section.

FAULT-FREE OPERATION

If the SS pin voltage is externally prevented from rising above the 1-V threshold, the controller does not execute

nor report most faults and the PGOOD output remains low. Only the overcurrent function and current-sense fault

remain active. The overcurrent protection continues to terminate PWM cycle every time when the threshold is

exceeded but the hiccup mode is not entered.