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© Semiconductor Components Industries, LLC, 2009
July, 2009 − Rev. 1
1 Publication Order Number:
AND8206/D
AND8206/D
NCP30x Series Reference
Designs for Supply-Voltage
Sequencing Control
Applications
Prepared by: X.H. Meng
ON Semiconductor
Supply−Voltage Sequencing Requirements
As we know, highly integrated system chips that combine
multiple digital and analog functions into a single package
often require multiple power supplies. Usually the
microprocessor’s input/output (I/O) and core voltages are
two separate and independent power requirements. A
typical contemporary microprocessor’s I/O section usually
operates at 3.3 V or 2.5 V, but the core sections work at 1.8 V,
1.5 V, 1.3 V or lower. Improper supply sequencing can result
in device latch−up, incorrect device initiation, or
degradation of long−term reliability. And considering of
different outputs sequence results from different power
solutions, it’s important to add a part of circuit to control the
supply−voltage up and down sequencing to guarantee the
microprocessor operating normally. For example, DSPs and
some other multi−voltage needed processors require their
I/O voltage to be present before applying the core voltage.
On the contrary, some systems based on FPGAs needs the
core voltage to be fully created before the I/O supplying. So
it could be happen that different processors, FPGAs and
ASICs on the same board may have different outputs
sequencing requirements. For robust system operation it can
prove important to add a circuit block to control the
supply−voltage up and down sequencing to guarantee the
microprocessor operating normally.
Depending on the different power supply solutions we can
use particular method to realize the sequencing control. We
will describe the implementations of using discrete
components and devices of NCP30x families to control the
sequencing.
Use of Discrete Components
A simply approach for sequencing the supply voltage of
two power requirements is to add a delay between them. The
way is to monitor the primary supply and allow the second
supply coming up with a small delay after ensuring that
primary supply reaches a certain level. Figure 1 illustrates
this method for a system in which the power voltage supply
is provided by a remote power converter module without
individual output on/off control. One comparator, NCS2200
properly is used to drive the switch short or open. The
reference voltage on the negative input of the comparator
sets the level to be reached by the V
cc1
before V
cc2
turning
on.
An RC combination on the other input adds a delay to the
trigger. A P−channel MOSFET on the V
cc2
operating as a
high−side switch model controlled by the comparator
guarantees no power flows to the output before Vcc1 rising
above the preset V
ref
. A small N−channel MOSFET controls
the P−channel MOSFET switch.
-
+
V
ref
C1
R1
NCS2200
V
CC1
OUT2
V
CC2
R2
OUT1
Figure 1. RC Approach with Comparator
and MOSFET
This approach can guarantee that the V
cc2
will not be
present to out2 before V
cc1
reaching the preset V
ref
. The
timing delay after V
cc1
rises above the V
ref
depends upon
V
cc1
. It’s not a fixed and reliable constant and will be
affected by the slew rate of V
cc1
input. For example,
assuming the V
cc1
is applied below the V
ref
through the
duration of C1 charging. At this time if V
cc1
rises above the
V
ref
, the delay time will be shorter and hence it may cause
the error. Or, when the V
cc1
goes down below the V
ref
,
because of the RC combination the output of the comparator
will turn over after a little delay due to the C1 discharge. All
these above cannot be accepted by the system designers.
Another drawback is that at least six components are needed
to realize this approach.
APPLICATION NOTE
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