DN384 - Accurate Power Supply Sequencing Prevents System Damage

Accurate Power Supply Sequencing Prevents System Damage
Design Note 384
Jeff Heath and Akin Kestelli
Introduction
Many complex systems—such as telecom equipment,
memory modules, optical systems, networking equipment, servers and base stations—use FPGAs and other
digital ICs that require multiple voltage rails that must
start up and shut down in a specific order, otherwise
the ICs can be damaged. The LTC®2924 is a simple and
compact solution to power supply sequencing in a 16pin SSOP package (see Figures 1 and 2).
How it Works
Four power supplies can be easily sequenced using
a single LTC2924 and multiple LTC2924s can be just
as easily cascaded to sequence any number of power
supplies. With slightly reduced functionality, six power
supplies can be sequenced with a single LTC2924.
The LTC2924 controls the start-up and shutdown sequence and ramp rates of four power supply channels
via output pins (OUT1 to OUT4). Each OUT pin uses a
10µA current source connected to an internal charge
pump and a low resistance switch to GND. This combination makes the outputs flexible enough to connect
them directly to many power supply shutdown pins or
to external N-channel MOSFET switches.
The LTC2924 monitors the output voltage of each
sequenced power supply via four input pins (IN1 to
IN4). These inputs use precision comparators and a
trimmed bandgap voltage reference to provide better
than 1% accuracy. The power ON and power OFF voltage thresholds are set using resistive dividers for each
of the four channels. The power ON threshold and the
power OFF threshold are individually selectable on a
channel by channel basis (see “Selecting the Hysteresis
Current and IN Pin Feedback Resistors” in the data
sheet for details).
The LTC2924 timer pin (TMR) is used to provide an
optional delay between the completion of start-up of one
supply and the start-up of the next power supply. The
delay time is selected by placing a capacitor between
the TMR pin and ground (delay = 200ms/μF), whereas
floating the TMR pin removes any delay. The start-up
delay can be different than the shutdown delay. Figure 3
shows a simple circuit where the shutdown delay is half
the start-up delay.
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All other trademarks are the property of their respective owners.
VON = 2.64V
VOFF = 1.98V
3.3V
SHDN
Q2
1V
VON = 0.93V
VOFF = 0.915V
0.1μF
5V TO 5V
SHDN
5V
0.1μF
VON = 3.97V
VOFF = 2.97V
0.1μF
Q1
3V
VON = 2.79V
VOFF = 2.73V
0.1μF
VCC
5V EARLY
10k
10k
OUT1
IN1
OUT2
IN2
OUT3
IN3
OUT4
IN4
ON LTC2924
SYSTEM
CONTROLLER
6.04k
100k
1.62k
66.5k
1.69k
18.2k
3.09k
20k
DONE
FAULT
TMR
PGT
150nF
150nF
HYS/CFG
GND
49.9k
Q1-Q2: IRL3714S
ALL RESISTORS 1%
DN384 F01
Figure 1. Typical Application with External N-Channel MOSFETs
03/06/384_conv
POWER SUPPLY 1
POWER SUPPLY 2
POWER SUPPLY 3
5V EARLY*
VON 3.01V
VOFF 2.68V
3.3V
SHDN
VON 4.49V
VOFF 3.99V
5V
SHDN
VON 1.43V
VOFF 1.27V
1.6V
SHDN
VON 2.25V
VOFF 2V
2.5V
SHDN
POWER SUPPLY 4
0.1μF
24.9k
15.8k
49.9k
33.2k
9.31k
11.81k
7.87k
8.45k
VCC
OUT1
10k
10k
OUT2
LTC2924
IN1
IN2
OUT3
IN3
OUT4
IN4
ON
SYSTEM
CONTROLLER
DONE
FAULT
TMR
*5V EARLY MUST BE ON BEFORE
SEQUENCING SUPPLIES
150nF
PGT
HYS/CFG
GND
49.9k
150nF
DN384 F02
Figure 2. Four Power Supply Sequencer Using Shutdown Pins
The LTC2924 also includes a power good timer (PGT).
The LTC2924 starts the PGT as each individual power
supply is enabled. If any power supply fails to reach
its nominal specified voltage within the allotted time
interval, a power ON fault is detected.
no software and designs can be fine tuned during system
integration simply by changing resistor and capacitor
values. Ease of design, low component cost, and a
small footprint make the LTC2924 an excellent choice
for power supply sequencing and monitoring.
VCC
Conclusion
The LTC2924 fits into a wide variety of power supply
sequencing and monitoring applications. With very
few external components and a 16-pin narrow SSOP,
an LTC2924-based sequencing solution requires very
little board space.
The power supply enable pins require no configuration
by the designer, yet are versatile enough to directly drive
shutdown pins or external N-channel MOSFETs. Softstart of power supplies can be achieved simply by adding
a capacitor. If the sequencing of more than four power
supplies is required, the LTC2924 can be cascaded to
sequence a virtually unlimited number of power supplies.
Tailoring the LTC2924 to a specific application requires
Data Sheet Download
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Linear Technology Corporation
0.1μF
VCC
IN1
OUT1
IN2
OUT2
IN3
IN4
LTC2924
VCC
FAULT
TMR
150nF
OUT4
PGT
ON
150nF
OUT3
GND
10k
DONE
2N7002
DN384 F03
POWER ON TIMER DELAY = 30ms
POWER OFF TIMER DELAY = 15ms
Figure 3. Programming Different ON/OFF Delays
For applications help,
call (408) 432-1900
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