V

I

PH(max)

R

CS

I

V

V

OUT

2

L

f

SW

V

IN

TPS40090

TPS40091

www.ti.com

SLUS578B

– OCTOBER 2003 – REVISED MAY 2006

OVERCURRENT PROTECTION

The overcurrent function monitors the voltage level separately on each current sense input and compares it to

exceeded the PWM cycle on the associated phase is terminated. The voltage level on the ILIM pin is determined

by the following expression:

(5)

where:

•

•

(6)

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 now serves 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. 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 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.

Copyright

© 2003–2006, Texas Instruments Incorporated

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