Datasheet

( )
=
FLT(MIN)
54
C
3.75
+
´
t
SS2
t
CC
SS2
=
FLT(MIN)
54
C
3.75
´
t
RAMP
SS2
t ´
= 0.35
C
( )
LOAD
IN(MAX)
REF_K(MIN)
SENSE
C V -V
LSS
t =
CC
V
R
´
TPS2350
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SLUS574D JULY 2003REVISED OCTOBER 2013
APPLICATION INFORMATION
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 IMAX 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
Continuing this calculation example, using a 220-µF input capacitor (C
LOAD
), Equation 3 and Equation 4 estimate
the load voltage ramping to approximately –45 V during the soft-start period. If the module should operate down
to –72-V input supply, approximately another 1.4 ms of constant-current charging may be required. Therefore,
Equation 6 and Equation 8 are used to determine C
FLT(MIN)
, and the result is approximately 0.039-µF.
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