Datasheet

7
UC17131/2/3
UC27131/2/3
UC37131/2/3
Choosing The CDEL Capacitor
The maximum amount of time that VCC power can be in-
terrupted and not require the outputs to go through a
turn-on delay cycle is user programmable by the CDEL
capacitor value. While VCC is interrupted, the outputs
will be in an indeterminate state and they may turn off
during this interval, t
INT
. However, as long as the pro-
grammed interruption time is not exceeded, the outputs
will immediately turn back on with the return of VCC.
For example:
ts
INT
≈ 500µ
(User specified)
CDEL is selected such that the time it takes for this ca-
pacitor to discharge from V
CDEL_MAX
(5.8V) to V
FAULT_L
(1.3V) with a discharge current of 94µA is just greater
than this t
INT
. This time is referred to as t
D(OFF)
in Fig.
3b.
()
CDEL
It
VV
DISCHARGE D
OFF
CDEL MAX FAULT L
=
•
−
__
If t
D(OFF)
is set equal to t
INT
, which the user has selected
to be 500µs, the minimum CDEL capacitor is calculated:
CDEL
As
VV
=
•
−
94 500
58 13
µµ
..
For this application, the CDEL capacitor value calculates
to 10.4nF. By using a 10nF capacitor on CDEL, VCC can
be interrupted for up to 478µs and the outputs will experi-
ence an indeterminate state during this interruption, but
resume normal operation when VCC power returns to
normal.
If the VCC power is interrupted for a time equal to or lon-
ger than t
D(OFF)
then the following relationships apply.
As the CDEL capacitor discharges past the V
FAULT_L
threshold, the output is fully disabled and must cycle
through a power up delay equal to t
D(ON)
. The charge
current for the CDEL capacitor is equal to 4µA. The out-
puts will turn on when the CDEL capacitor charges up to
the V
FAULT_H
threshold of 4.9V. The minimum turn-on
delay the outputs will experience will occur if t
INT
is ex-
actly equal to the t
D(OFF)
time and the CDEL capacitor
has only discharged to V
FAULT_L
. This would be the mini-
mum turn-on delay time and is calculated with the follow-
ing equation:
()
t
CDEL V V
I
DON
FAULT L FAULT H
CHARGE
min
__
=
•−
Using the 10nF CDEL capacitor, for example, the mini-
mum turn-on delay calculates to 9ms. If the CDEL ca-
pacitor discharges completely to zero, then the 10nF
CDEL capacitor would cause a turn-on delay of 12.25ms.
The outputs would be off for this amount of time after
VCC power is restored. The total amount of time the out-
puts could be disabled is equal to the t
INT
time, which
may include the indeterminate time of t
D(OFF)
, and the
t
D(ON)
time, as shown in Fig. 3b.
APPLICATION INFORMATION
C
CDEL
13
12
11
VREF
IN
6
10
3
7
VCC
LS
CSH
2
GND
6V
PRE-REGULATOR
INPUT
COMPARATOR
1/2
VREF
HYST
9
LED
TURN-ON/OFF
AND SHORT
CIRCUIT
PROTECTION
CDEL
4
HS
5
Q
OUT
VREF
GND
72V
R
CSH
–L
CSL
+L
OUT
C
CC
LOAD
I
GND
R
HYST
POWER
SUPPLY
C
REF
I
CC
Figure 2a. High side application.
UDG-98027