Datasheet

LTC2928
12
2928f
The LTC2928 is a four channel, cascadable power supply
sequencer and supervisor for use with external N-channel
MOSFETs or power supplies with shutdown/enable inputs.
An unlimited number of power supplies may be fully
sequenced at configurable points in time using multiple
LTC2928s. A single LTC2928 controls up to four supplies
(four positives or three positives and one negative) and
one external resistor per supply configures each supply
enable (disable) time position. Device power is applied
through either V
CC
(2.9V to 6V) or HV
CC
(7.2V to 16.5V).
When applying power through HV
CC
, a regulated 3.3V is
output on V
CC
. An internal charge pump provides (V
CC
+ 5.5V) gate drive voltages at the enable outputs EN1 to
EN4 for driving external pass FETs.
The LTC2928 monitors four supply thresholds per supply
(sequence-up, sequence-down, undervoltage, overvoltage)
during a full system cycle. A full cycle comprises a se-
quence-up phase, monitor phase and sequence-down
phase. A sequence timer sets the time delay between
supply enables during both the sequence-up and sequence-
down phases. The power-good timer is a watchdog for
stalled supplies during the sequence-up and sequence-
down phases. A microprocessor reset signal is issued
once all supplies are above their undervoltage threshold
for the chosen reset time. During the monitor phase, all
enabled supply voltages are continuously compared with
their undervoltage threshold. If any supply falls below its
undervoltage threshold, the reset output pulls low. The
reset output pulls high once all enabled supplies have
been in compliance for the chosen reset time. During any
phase, all positive supplies are continuously monitored
for overvoltage conditions.
Implementing complex power-on and power-off schemes
is simple using the LTC2928. System errors are easily
diagnosed with the LTC2928 fault event reporting feature.
Basic LTC2928 operation is discussed below. Configura-
tion tables are given in the Applications Information sec-
tion. The Applications Information also includes many
of the unique and advantageous extensions to the basic
operation, such as Master/Slave configurations.
Sequence-Up Phase
While V
CC
is ramping up, the LTC2928 enters a power-on
mode and pulls down its CAS pin. When V
CC
is stabilized,
the LTC2928 is configured for operation and releases its
CAS pin. Multiple LTC2928s with common CAS connec-
tions will therefore synchronize with each other. The ON
input is used to start the sequence-up and sequence-
down process. The state of ON is ignored until CAS is
released. When the voltage at the ON input exceeds 1V, the
sequence-up phase is enabled after a small propagation
delay (20
µ s typical). If the ON signal prematurely pulls
below 0.97V during the sequence-up phase, a command
fault is generated, causing enable (EN) outputs to pull low.
For more details refer to the discussion on system faults
later in this document.
OPERATIO
U
t
STMR
t
MON(UV)
t
RTMR
t
CAS(MINIMUM)
15 CYCLES
7 CYCLES
CAS and STMR Relationship
Undervoltage Timing
ON
CAS
STMR
Vx
RST
RTMR
t
STMR
V
MON(TH)
t
MON(OV)
t
RTMR
7 CYCLES
Overvoltage Timing
Vx
OV
RTMR
V
OV(TH)
2928 F13
TIMING DIAGRAMS