LTC3375
19
3375fc
For more information www.linear.com/3375
operaTion
of either an upstream high voltage buck regulator, or any
EN pin causing its associated buck switching regulator
to power-up, which can sequentially power-up the other
buck regulators. The RST pin gets pulled HIGH 230ms
after the last enabled buck is in its PGOOD state. An
application showing sequential regulator start-up is shown
in the T
ypical Applications section (Figure 8).
In Figure 2b, PB is held LOW at instant t
4
for 10 seconds.
This causes ON to return to a LOW state, which can se-
quence a power-down by either shutting down an upstream
high voltage buck, or by shutting down one of the internal
buck switching regulators.
In Figure 2c,
KILL
is pulled LOW while the pushbutton is in
the “on” state. This causes a hard reset to be generated at
t
4
, all regulators are powered down 50ms later at time t
5
.
An I
2
C signaled reset will have the same effect as pulling
KILL low momentarily.
In Figure 2d, PB is held LOW at instant t
4
for a time
greater than 50ms but less than 10 seconds. This causes
a transient IRQ signal. This unlatched interrupt can be
used to signal a user pushbutton request. In this case a
software reset may be initiated if so desired. In Figure 2d,
the microprocessor initiates the power-down sequenc
-
ing after the user pushbutton signal at time t
6
. At time
t
7
, once all the converters are powered down, the micro
brings KILL LOW. 50ms later at time t
8
ON goes LOW. In
this case, a hard reset is issued until 1 second after KILL
returns high at t
9
.
None of the pushbutton based IRQ signals are reported
in an I
2
C register. As such, any IRQ signals that are not
revealed by polling the I
2
C read back may be interpreted
as caused by the pushbutton.
Power-Up and Power-Down Via Enable Pin or I
2
C
All regulators can be enabled either via its enable pin or
I
2
C. If the use of the pushbutton interface is not desired PB
and KILL should be tied to V
CC
, and the user may simply
enable any of the buck switching regulators by asserting
a HIGH signal on any of the EN pins or by writing a buck
switching regulator EN command to the I
2
C. If no I
2
C en-
able has been written, the buck switching regulator may
be powered down by simply returning its EN pin to a LOW
state. If it is wished to power-down the converters via I
2
C,
an IGNORE_EN command may be written causing the
LTC3375 to treat the state of the EN pin as LOW regard
-
less of its input. Then, the buck switching converters can
be powered down via I
2
C regardless of their associated
EN pin. Alternatively a RESET_ALL command may be
written that will force all the buck switching regulators to
power-down and remain powered down for a minimum
of one second before they are allowed to be re-enabled.
PB
KILL
IRQ
HARD RESET
t
1
t
2
SEQUENCE UP
SEQUENCE
DOWN
t
3
t
4
t
5
t
6
t
7
t
8
t
9
ON
<10 SEC
PB_ON
t
PB_LO
t
KILLLO
t
HR
Figure 2d. Power-Up Using PB and Power-Down Using a “Soft” Reset
(15 pages)
(36 pages)
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