Datasheet

t
SS
+
0.7 C
SS
5 10
*6
t
PG
+ 1.43 T
SS
V
OUT
+ 0.7 V
ǒ
R1
R2
) 1
Ǔ
TPS40090
TPS40091
SLUS578B –OCTOBER 2003– REVISED MAY 2006 www.ti.com
When the BP5 pin voltage crosses its lower undervoltage threshold and the power-on reset function is cleared,
the calibrated current source starts charging the soft start capacitor. The PGOOD pin is held low during the start
up. The rising voltage across the capacitor serves as a reference for the error amplifier assuring start-up in a
closed loop manner. When the soft start pin voltage reaches the level of the reference voltage V
REF
= 0.7 V, the
converter's output reaches the regulation point and further rise of the soft start voltage has no effect on the
output.
(2)
When the soft-start voltage reaches level of 1 V, the power good (PGOOD) function is cleared and reported on
the PGOOD pin. Normally, the PGOOD pin goes high at this moment. The time from when SS begins to rise to
when PGOOD is reported is:
(3)
OUTPUT VOLTAGE PROGRAMMING
The converter output voltage is programmed by the R1/R2 divider from the output of the differential amplifier. The
center point of the divider is connected to the inverting output of the error amplifier (FB), as shown in Figure 5.
(4)
CURRENT SENSE FAULT PROTECTION
Multiphase controllers with forced current sharing are inherently sensitive to failure of a current sense
component. In the event of such failure, the whole load current can be steered with catastrophic consequences
into a single channel where the fault has happened. The dedicated circuit in the TPS4009x controller prevents it
from starting up if any current sense pin is open or shorted to ground. The current-sense fault detection circuit is
active only during device initialization, and it does not provide protection should a current-sense failure happen
during normal operation.
OVERVOLTAGE PROTECTION
If the voltage at the FB pin (V
FB
) exceeds V
REF
by more than 16%, the TPS4009x enters into an overvoltage
state. In this condition, the output signals from the controller to the external drivers is pulled low, causing the
drivers to force all of the upper MOSFETs to the OFF position and all the lower MOSFETs to the ON position. As
soon as V
FB
returns to regulation, the normal operating state resumes.
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