Datasheet

´
SS RAMP
t = 1260 C
( )
2
´
AVG
LSS SS
LOAD RAMP SENSE
i
V = t
2 × C × C × 100 × R
( )
´ -
LOAD IN(MAX) LSS
CC
REF_K(MIN)
SENSE
C V V
t =
V
R
SS2 RAMP
t = 0.35 C´
SS2
FLT(MIN)
54 t
C =
3.75
´
( )
SS2 CC
FLT(MIN)
54 t t
C =
3.75
´ +
TPS2394
SLVSAA9 –AUGUST 2010 www.ti.com
Due to the three-phase nature of the load current at turn-on, the load voltage ramp has potentially three distinct
phases. 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 TPS2394, the typical duration of the soft-start period, t
SS
, is given by Equation 3.
(3)
Where:
t
SS
is the soft-start period in ms
C
RAMP
is given in µF
During this current ramp period, the load voltage magnitude which is attained is estimated by Equation 4.
(4)
Where:
V
LSS
is the load voltage reached during soft-start
i
AVG
is 3.18 µA for the TPS2394
C
LOAD
is the load capacitance in Farads
t
SS
is the soft-start period in s
The quantity i
AVG
in Equation 4 is a weighted average of the two charge currents applied to C
RAMP
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, V
IN(MAX)
, the HSPM transitions to the constant-current charging of the load. The
remaining amount of time required at I
MAX
is determined from Equation 5.
(5)
Where:
t
CC
is the constant-current voltage ramp time, in seconds
V
REF_K(MIN)
is the minimum clamp voltage, 34 mV
With this information, the minimum recommended value timing capacitor C
FLT
can be determined. The delay time
needed will be either a time t
SS2
or the sum of t
SS2
and t
CC
, according to the estimated time to charge the load.
The quantity t
SS2
is the duration of the normal rate current ramp period, and is given by Equation 6.
(6)
Where:
C
RAMP
is given in µF
Since fault timing is generated by the constant-current charging of C
FLT
, the capacitor value is determined from
either Equation 7 or Equation 8, as appropriate.
(7)
(8)
Where:
C
FLT(MIN)
is the recommended capacitor value, in µ-Farads
t
SS2
is the result of Equation 6, in seconds
t
CC
is the result of Equation 5, in seconds
12 Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): TPS2394