Analog Devices DC1717B Demo Manual page 4

DEMO MANUAL DC1717B
DESIGN PROCEDURE FOR MODIFICATION OF DC1717B
The second summand, t
account if the associated LTC4417 input does not have
any bypass capacitor and the rail can be disconnected
from the input instantly.
The third one must be calculated as:
t = R • C
GATE _ THRES S
It is possible to determine the minimum capacitive load
required to hold the output up during switchover as a:
I
LOAD(MAX)
C ≥
LOAD(MIN)
V
OUT(DROOPMAX)
where:
• I is the maximum load current, A
LOAD(MAX)
• V is the maximum acceptable voltage
OUT(DROOPMAX)
droop, V
4
, should be taken into
pVALID(OFF)
 
 
V
THRES
–In 1–
   
S
V
 
 
SINK
• T
DISCON
R
As shown in the equation (3), the use of external slew rate
control will add additional delay to the total switchover
time. Unfortunately, the actual components cannot be
chosen until the load capacitance is known. This circular
issue can only be resolved through an iterative process.
The process starts by calculating the C
that t
(2)
GATE_THRES
C . Using the calculated C
LOAD(INIT)
from the expression of the T
calculated R
S
C with the calculated R
LOAD
If C
LOAD(INIT)
(3)
the newly calculated C
If the C
LOAD(INIT)
calculate R using the higher value and repeat this process.
S
⎡ ⎤
⎛ ⎞
V
THRES
• C –In 1– ⎥ = 10µs
⎢
⎜ ⎟
S S
V
⎝ ⎠
⎣ ⎦
SINK
Calculate R with C
S L(INIT)
Recalculate C with R
L S
Is recalculated C
No
L
lower than initial
C ?
L(INIT)
Done.
Use C and R
L(INIT) S
Figure 2
LOAD(MIN)
=10μs. For clarity this value will be labeled
LOAD(INIT)
. To ensure the newly
DISCON
based on C is sufficient, calculate
LOAD(INIT)
.
S
(the initial calculated C
LOAD
then the process is completed.
LOAD
is lower than the newly calculated C
, assuming
, calculate R
S
) is higher than
,
LOAD
Rev 0