Datasheet



SLUS610 − JULY 2004
20
www.ti.com
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).
t
CC
+
C
L
ǒ
V
IN(max)
* V
LSS
Ǔ
ǒ
V
REF_K(min)
R
SENSE
Ǔ
where:
D t
CC
is the constant-current voltage ramp time, in seconds
D V
REF_K(min)
is the minimum clamp voltage, 33 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 and profiles
of load charging. The quantity t
SS2
is the duration of the normal rate current ramp period, and is given by
equation (6).
t
SS2
+ 0.35 C
RAMP
where:
D C
RAMP
is given in microfarads, and t
SS2
is in seconds
Since fault timing is generated by the constant-current charging of C
FLT
, the capacitor value is determined by
equation (7) or (8).
C
FLT(min)
+
55 t
SS2
3.75
C
FLT(min)
+
55
ǒ
t
SS2
) t
CC
Ǔ
3.75
where:
D C
FLT(min)
is the recommended capacitor value, in microfarads
D t
SS2
is the result of equation (6), in seconds
D t
CC
is the result of equation (5), in seconds
For the typical application example, with the 100-µF filter capacitor in front of the dc-to-dc converter, equations
(3) and (4) estimate the load voltage ramping to −46 V during the soft-start period. If the module should operate
down to −72-V input supply, approximately another 1.58 ms of constant-current charging may be required.
Therefore, equations (6) and (8) (because of the constant-current sourcing) are used to determine C
FLT(min)
.
The result of 0.043 µF suggests the 0.047-µF standard value.
(5)
(6)
(7)
(8)