LTC2928
17
2928fa
For more information www.linear.com/LTC2928
applicaTions inForMaTion
Should multiple channels fault simultaneously, the re-
ported channel is given priority according to the channel
number (1,2,3,4).
In the event of an external fault
, the LTC2928 fault manager
reports overvoltage channels to the fault channel outputs
(CMP3, CMP4). If no channel is overvoltage, the default
report is channel 4 (High, High). As such, in certain ap
-
plications, a potential reporting ambiguity exists.
Operating without Pass Transistors
The LTC2928 enable outputs may directly drive the shut
-
down/enable, run/soft-start or control inputs on DC/DC
converters. However, since the LTC2928 enable outputs
may drive to a relatively high voltage with low current
(10µA), care must be taken not to exceed the maximum
voltage rating on the DC/DC converter enable input. The
gate voltage available from the LTC2928 enable output
ranges between V
CC
+ 4.5V to V
CC
+ 6V. Use a resistor
to limit an enable output to an external supply voltage. A
resistor between 4.7k and 27k is recommended.
Cascading Multiple LTC2928s
LTC2928s can be cascaded in two ways, simultaneously.
The first method, time extension, allows an unlimited
number of supplies to be sequenced in additional time
positions. The second method, supply extension, allows
additional supplies to be sequenced in the same time po
-
sition. The examples below demonstrate how to achieve
time and supply extension.
Central to configuring a cascade application is the assign
-
ment of LTC2928 properties such as master/slave and
first/not-first/last status. Master/Slave and first/not-first
designation is made with the MS1 and MS2 three-state
configuration inputs (see Table 2). An LTC2928 is con
-
figured as “LAST” by pulling DONE to V
CC
with a 2.4k to
5.1k resistor.
Next, the appropriate connection of one or both bidirectional
communication lines must be made. To achieve supply
extension, the CAS pins between master and slave devices
are tied together (Figure 1). To achieve time extension, the
DONE pin of the preceding LTC2928 is connected to the
ON pin of the subsequent LTC2928 (Figure 2). Both con
-
nections are allowed to exist within one system (Figure 4).
It is important not to corrupt these communication lines
with added passive or active loads.
CAS Connection: Supply Extension
When more than four supplies need to be synchronized
in time
, use the CAS connection. Consider the application
in Figure 1. This application allows for 8 supplies in 8
distinct time positions, or all at once depending upon the
choice of RT resistors. The upper device is designated as
master and the lower device is the slave. Both LTC2928s
are configured as “FIRST” and “LAST” and the CAS pins
are tied together.
RT1
V
CC
V
CC
RT2
RT3
RT4
DONE
STMR
CAS
LTC2928
(SLAVE/FIRST)
ON
RT1
V
CC
V
CC
RT2
RT3
RT4
DONE
CAS
LTC2928
ON
Figure1. Using the CAS pin to
synchronize additional power supplies
(slave STMR is not used).
The master controls the sequencing. The time delay be-
tween adjacent positions is configured with a capacitor
on the master’
s STMR pin and the slave STMR is ignored.
After application of the ON signal to start the sequence-up
phase, the CAS pin pulls low, enabling any supply config
-
ured for time position 1. After supplies in time position 1
cross their sequence-up threshold,
CAS is released and
pulled high. If no supplies are configured for a particular
time position, CAS pulls high after 25µs. CAS remains
high for one STMR period (100µs minimum) and then
