Datasheet

V
ILIM
+ 2.7 I
PH(max)
R
CS
I
PH(max)
+ I
OUT
)
ǒ
V
IN
* V
OUT
Ǔ
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
the voltage on the ILIM pin set by a divider from the controller's reference. In case a threshold of V
(ILIM)
/2.7 is
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:
• I
PH(max)
is a maximum value of the phase current allowed
• R
CS
is a value of the current sense resistor used (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.
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