Datasheet

LTC2928
17
2928f
process repeats until CAS clocks through time position 8
and DONE pulls low.
To sequence down, the ON input is pulled low. Supplies in
time position 8 are disabled. After supplies in time posi-
tion 8 fall below their sequence-down threshold, CAS is
released and pulled high. CAS remains high for one STMR
period and then pulls low again to disable supplies in time
position 7. The process repeats until CAS clocks through
time position 1 and DONE pulls high.
DONE-ON Connection: Time Extension
When additional time positions and/or additional supplies
require control, use the DONE—ON connection. Consider
the application in Figure 2. This application allows for 8
supplies in 16 distinct time positions (4 supplies within
the first 8 time positions, and 4 more within the second 8
time positions). Both LTC2928s are designated as master.
Each has its own STMR capacitor allowing for different
sequence timing. The leftmost device is designated as
“FIRST”, and the rightmost device is “NOT FIRST” and
“LAST”.
It is critical to note here that the DONE pin of the first device
is connected to the ON pin of the second device, and that
this connection forms a bi-directional communication line
for the purposes of sequence control. DONE and ON do not
function as typical DONE and ON pins. The handshaking
that occurs between these pins is described below.
FLT
DONE
STMR
LTC2928
(MASTER/FIRST)
8 SUPPLIES, 16 TIME POSITIONS
ON
V
CC
DONE
STMR
LTC2928
(MASTER/NOT FIRST)
ON
V
CC
FLT
2928 F02
Figure 2. Using the DONE—ON interface to extend number of
supplies and time positions (STMR capacitors may be different).
To start the sequence-up process, the first ON input is
pulled high. The first device sequences the first 4 sup-
plies as usual. The first DONE pin has recognized that the
first device is not the last because it has not been pulled
up to V
CC
with a resistor. Knowing this information, the
first DONE pin pulls up the second ON input. The second
device now sequences its 4 supplies normally. When the
second device is finished, the second DONE pin pulls low
as expected.
To start the sequence-down process, the first ON input is
pulled low. The first device has recognized that the first
DONE pin was high and already knows that it is not the
last device. The first DONE pin therefore pulls the second
ON pin low. The second DONE pin was low and is the last
device. Therefore, the second device starts its sequence-
down procedure. When finished, the second DONE stays
low, and the second ON pin, knowing that it is not first,
pulls up the first DONE pin. The first DONE pin senses the
pulled up condition and triggers the sequence-down pro-
cess for the first device. When the first device is finished,
the DONE pin pulls down, overriding the pull-up from the
second ON pin. The second device then releases its DONE
pin, which pulls up to V
CC
, and the process is complete.
Time extension can cascade to more than two devices as
shown in Figure 3. It is a simple matter of adding more
LTC2928s in the middle of the cascade, with a master/not-
first designation.
DONE
STMR
LTC2928
(MASTER/NOT FIRST)
12 SUPPLIES, 24 TIME POSITIONS
ON
FLT
V
CC
DONE
STMR
LTC2928
(MASTER/NOT FIRST)
ON
DONE
STMR
LTC2928
(MASTER/FIRST)
ON
FLT
V
CC
FLT
2928 F03
Figure 3. Additional supply and time extension.
APPLICATIO S I FOR ATIO
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