TPS2392
TPS2393
SLUS536C AUGUST 2002 REVISED AUGUST 2004
www.ti.com
21
APPLICATION INFORMATION
setting the fault timing capacitor
The fault timeout period is established by the value of the capacitor connected to the FLTTIME pin, CFLT. The
timeout period permits riding out spurious current glitches and surges that may occur during operation of the
system, and prevents indefinite sourcing into faulted loads swapped into a live system. However, to ensure
smooth voltage ramping under all conditions of load capacitance and input supply potential, the minimum
timeout should be set to accommodate these system variables. To do this, a rough estimate of the maximum
voltage ramp time for a completely discharged plug-in card provides a good basis for setting the minimum timer
delay.
Due to the three-phase nature of the load current at turn-on, the load voltage ramp has potentially three distinct
phases and is seen by comparing Figure 1 and Figure 2. This profile depends on the relative values of load
capacitance, input dc potential, maximum current limit and other factors. The first two phases are characterized
by the two different slopes of the current ramp; the third phase, if required to complete load charging, is the
constant-current charging at IMAX. Considering the two current ramp phases to be one period at an average
di/dt simplifies calculation of the required timing capacitor.
For the TPS2392 and TPS2393, the typical duration of the soft-start ramp period, tSS, is given by equation (3).
t
SS + 1183
C
IRAMP
where:
D tSS is the soft-start period in milliseconds, and
D CIRAMP is given in F
During this current ramp period, the load voltage magnitude which is attained is estimated by equation (4).
V
LSS +
i
AVG
2
C
L
C
IRAMP
100
R
SENSE
t
SS
2
where:
D VLSS is the load voltage reached during soft-start
D iAVG is 3.38 A for the TPS2392 and TPS2393
D CL is the amount of the load capacitance
D tSS is the softstart period, in seconds
The quantity iAVG in equation (4) is a weighted average of the two charge currents applied to CIRAMP during
turn-on, considering the typical output values.
If the result of equation (4) is larger than the maximum input supply value, then the load can be expected to
charge completely during the inrush slewing portion of the insertion event. However, if this voltage is less than
the maximum supply input, VIN(max), the HSPM transitions to the constant-current charging of the load. The
remaining amount of time required at IMAX is determined from equation (5).
t
CC +
C
L
V
IN(max) * VLSS
V
REF_K(min)
R
SENSE
where:
D tCC is the constant-current voltage ramp time, in seconds
D VREF_K(min) is the minimum clamp voltage, 33 mV.
(3)
(4)
(5)