UC17131/2/3
UC27131/2/3
UC37131/2/3
APPLICATION INFORMATION
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:
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
INT
≈
500µ
s
(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
=
I
DISCHARGE
•
t
D
(
OFF
)
V
CDEL
_
MAX
−
V
FAULT
_
L
t
D
(
ON
)
min
=
CDEL
•
V
FAULT
_
L
−
V
FAULT
_
H
I
CHARGE
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
=
94
µ
A
•
500
µ
s
58
V
−
13
V
.
.
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-
VREF
11
6V
PRE-REGULATOR
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.
VCC
I
CC
7
+L
R
CSH
POWER
SUPPLY
C
REF
6
CSH
LS
5
INPUT
COMPARATOR
IN
VREF
GND
13
72V
C
CC
Q
OUT
1/2
VREF
4
TURN-ON/OFF
AND SHORT
CIRCUIT
PROTECTION
HS
OUT
CSL
3
GND
LOAD
2
–L
I
GND
12
HYST
R
9
LED
HYST
10
CDEL
C
CDEL
UDG-98027
Figure 2a. High side application.
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