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Application Note
14 September 2006
PDF Name: seq_multi_mod.pdf
Application Guidelines for Non-Isolated Converters
AN04-008: Guidelines for Se
q
uencin
g
of Multi
p
le Modules
Introduction
Today’s electronics systems require multiple power
supply voltages to power the various ICs. Board
designers need to not just provide multiple voltages, but
also to ensure proper voltage sequencing for each multi-
voltage IC during power-up and power-down. The most
common power supply architecture involves dual
voltages; usually 1.8V or lower for the core power supply
and 3.3V or 2.5V for supporting I/O.
Dual-voltage-supply architectures often need coordinated
management of both core and I/O voltages during power-
up and power-down, a requirement called sequencing.
Generally, there are three types of power sequencing
solutions that can be applied to provide a synchronized
start-up and shutdown sequence of core and I/O
voltages. The first method is called voltage cascading or
sequential start-up where one of the voltages, often the
core voltage, comes up first and reaches its final
regulated value. After a certain specified delay, the other
voltage is required to come up and reach its regulation
value (see Figure 1 for example waveforms). During
power-down, the sequencing order of the supply voltages
is reversed.
A second sequencing solution is called the ratiometric
approach, where both supply voltages reach the
regulation point at the same time. In this scheme, the
core and I/O voltages have different slew-rates, (see
Figure 2) to enable both voltages to reach their
regulation point at approximately the same time instant.
During power-down using the ratiometric sequencing
method, both output voltages start ramping down at the
same time instant but with different slew-rates so that
they reach zero at approximately the same time instant.
The third sequencing method is simultaneous start-up or
voltage tracking where both voltages start ramping up
with identical slew rates. After the core or lower voltage
reaches its regulationlevel, the I/O voltage continues with
the same slew-rate until it reaches its regulation voltage
(see Figure 3 for example wavwforms). The pattern is
reversed during the power-down sequence.
Simultaneous start-up of multiple voltages on an IC is the
most common sequencing approach used in
applications.
Using EZ-SEQUENCE
TM
All Austin Lynx
TM
II series modules (Austin MicroLynx
TM
II, Austin Lynx
TM
II, Austin SuperLynx
TM
II, 12V Austin
MicroLynx
TM
II, 12V Austin Lynx
TM
II, 12V Austin
SuperLynx
TM
II) include a sequencing feature, EZ-
SEQUENCE
TM
that helps users implement any of the
three types of the sequencing just discussed. This is
enabled through an additional sequencing (SEQ) pin.
When a voltage is applied to the SEQ pin, the output
voltage tracks this voltage on a one-to-one basis until the
output reaches the set-point voltage. This allows the
output voltage to be brought up in a controlled manner as
long as the final value of the SEQ voltage is made higher
V
COR
E
V
I/
O
Fig. 3. Voltage waveforms in simultaneous start-up.
than the set-point voltage of the module. By connecting
the SEQ pins of multiple modules together, all modules
can track their output voltages to the voltage applied on
the SEQ pin. When sequencing is not required in the
application, the SEQ pin can either be tied to VIN or left
V
COR
E
V
I/O
Fig. 1. Voltage waveforms in sequential start-up
V
COR
E
V
I/O
Fig. 2. Voltage waveforms in ratiometric start-up.
