AN161: Power Sequencing in Distributed Power Applications: Sequenced Turn-on of Seven DC/DC Power Supplies with Latched Shutdown

Power Sequencing in Distributed Power Applications: Sequenced
Turn-on of Seven DC/DC Power Supplies with Latched Shutdown
®
Application Note
April 26, 2005
AN161.0
Author: Carlos Martinez
resequencing requires a separate reset signal or the main supply to be cycled off, then on.
It is becoming more common today to see many different
power supplies in a system. There can be from 3 to 7 or more
different DC/DC controllers. This is typically the result of the
proliferation of new microprocessors, ASICs and FPGAs, each
with their own variety of power supply voltages and power
sequencing requirements.
The X80140 provides a selectable delay for each supply. The
options are 100ms, 500ms, 1s, and 5s. Sequencing seven supplies requires a minimum of 700ms (plus the turn on time for
each supply.) The eighth voltage monitor in the circuit is used
as a watchdog timer (WDT). This timer starts when the master
(first) voltage is applied. For the circuit to work, the timer is set
to 1 second or 5 seconds. If the WDT (ViGDO) output goes
active before all of the supplies have reached their proper level,
then one or both of the RESET outputs will be active. In this
event, the latch is clocked and the MR pins are pulled HIGH,
turning off all supplies. With the WDT set to 5 seconds there is
some flexibility in the individual turn on delays. With the WDT
set to 1 second, all delay times will likely need to be set to
100ms.
In this application, two X80140 quad sequencers (See Figure
1) are used to sequence seven DC/DC converters (See Figure
2). The X80140 devices monitor the output of each DC/DC converter. After the supply reaches a minimum operating level, the
X80140 powers up the next supply after a fixed delay. When all
supplies have reached their proper level, the RESET outputs
go inactive (HIGH) (See Figure 3 for timing).
Because of the way the voltage monitors are configured, the
failure of any one supply causes either of the two reset outputs
to become active. This triggers the latch, which pulls the MR pin
on both devices LOW, thus turning off all supplies. To allow
RESET
VCC
Reset Logic
and Delay
POR
Control and
Fault Registers
MR
Bus Interface
SDA
VSS
SCL
WP
A0
A1
EEPROM
2kbits
VP
OSC
VMON Logic
V1MON
Divider
Reset
4
VREF1
V2MON
VREF2
VSS
4
V1GDO
Select
0.1s
0.5s
1s
5s
V2GDO
delay1
V3MON
V3GDO
delay2
VREF3
V4MON
VREF4
delay3
Delay circuit
repeated 4 times
VSS
V4GDO
delay4
VSS
FIGURE 1. X80140 BLOCK DIAGRAM
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 161
5V
Power
Supply
On/Off
3.3V
Power
Supply
On/Off
1.2V
Power
Supply
On/Off
1.8V
Power
Supply
On/Off
2.5V
Power
Supply
On/Off
1.2V
Power
Supply
On/Off
1.8V
Power
Supply
On/Off
2.5V
1
2
3
4
5
6
Note: Power supply #7
is not monitored if V4MON
is used as a WDT.
7
X80140/41/42/43
V4GDO V4MON
V3GDO V3MON
V2GDO V2MON
V1GDO V1MON
RESET
A0
SCL
SCA
VCC
MR
X80140/41/42/43
V4GDO V4MON
V3GDO V3MON
5V
74HC76
V2GDO V2MON
K
RESET
A0
SCL
SCA
VCC
Q
J 1
V1GDO V1MON
Q
C P
MR
5V
74HC4078
5V
K
Q
J 2
Q
Reset
C P
SCL
SDA
FIGURE 2. EXAMPLE OF RELAY POWER SUPPLY SEQUENCING
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AN161.0
April 26, 2005
Application Note 161
V1MON
tDELAY1
V1MON
threshold
Timing Not
To Scale
100ms
500ms
1sec
Example: 7 Independent
Power Supplies in relay timing
Programmable
Delay
Power Supply
#2 ON
V1GDO
Power Supply
#2 OUTPUT
tDELAY2
V2MON
100ms
threshold 500ms
1sec
Programmable
Delay
Power Supply
#3 ON
V2GDO
Power Supply
#3 OUTPUT
V3MON
threshold
tDELAY3
100ms
500ms
1sec
Programmable
Delay
Power Supply
#4 ON
V3GDO
Power Supply
#n-1 ON
Vn-1GDO
Power Supply
#n OUTPUT
VnMON
threshold
tDELAYn
100ms
500ms
1sec
Programmable
Delay
Power Supply
#n ON
VnGDO
RESET1
RESET2
RESET “Wire” AND (J-K FF #1 clock)
Q1
MR
Assume WDT times out before supplies are good
WDT (V4GDO) also J-K FF#2 clock
Q2
MR
FIGURE 3. RELAY SEQUENCING OF DC/DC SUPPLIES. (TIMING)
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verify that the Application Note or Technical Brief is current before proceeding.
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April 26, 2005
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