DN319 - Low Voltage Hot Swap Controller Ignores Backplane Noise and Surges

Low Voltage Hot Swap Controller Ignores Backplane Noise
and Surges – Design Note 319
Mitchell Lee
First generation single supply Hot Swap™ controllers,
such as the LTC ®1422, combine the features of various
discrete inrush limiting circuits into a single IC, including
an electronic circuit breaker, adjustable power-up rate,
reset output, high side MOSFET gate drive, undervoltage
lockout and a wide operating voltage range. The LTC4211
advances these features by adding a dual level, dual
response time electronic circuit breaker, adjustable softstart with inrush current limiting, fault detection, faster
response time to severe overloads and operation from
2.5V to 16.5V—all in a small 10-lead MSOP package.
Control 25W with a 10-Lead MS Package
Figure 1 shows a 5V, 5A Hot Swap application designed
for inclusion on a removable circuit board. As is customary, the circuit board ground and VCC planes are wired
to long connector pins, eliminating reliability problems
associated with pin-to-receptacle arcing. Until the PC
board is firmly seated in the backplane, the MOSFET
switch remains safely off and isolates CLOAD (typically
100μF or more) from the backplane. Once the connector
is fully seated, the R1-R2 divider which serves both as a
connection sense and an undervoltage lockout, activates
the LTC4211. When activated, the LTC4211 waits one
TIMER pin period as set by CTIMER and then turns on
the MOSFET. RESET signals a successful start-up as
measured by divider R3-R4 and the FB pin.
Dual Level Current Control
Advanced current control features separate the LTC4211
from other Hot Swap controllers. The SENSE pin monitors load current and shuts off the MOSFET in the event
the current exceeds 50mV/RSENSE. Instead of using a
simple circuit breaker approach, the SENSE pin has
two thresholds: a slow 50mV trip point whose timing
is governed by CFILTER at the FILTER pin, and a fast
150mV (3×) trip point which triggers in just 300ns to
interrupt the flow of current in the event a catastrophic
fault occurs on the output. Thus, the LTC4211 ignores
temporary surges and overloads but responds quickly
when a genuine fault is detected.
Inrush Limiting
Conspicuous by its absence in Figure 1 is a gate capacitor which might otherwise define the ramp rate
and therefore the inrush current of the output during
start-up. Instead, the 50mV SENSE pin threshold is
used to servo the inrush current to a value of 50mV/
RSENSE. The 50mV threshold circuit breaker function
is suspended during this critical period, but the 150mV
SENSE threshold remains active to catch catastrophic
faults. Once the soft-start period is over, the 50mV
circuit breaker is armed.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
and Hot Swap is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.
BACKPLANE PCB EDGE
CONNECTOR CONNECTOR
(FEMALE)
(MALE)
VCC
5V
RSENSE
7mΩ
LONG
SMAJ10A*
FAULT
CONNECTION
SENSE
10
SHORT
2
R1
22.1k
1%
LONG
3
R2
10k
1%
*CLAMP AND SNUBBER, TYPICAL
8
9
SHORT
100nF
GND
RX*
10Ω
C X*
100nF
M1
Si4410DY
VCC
7
SENSE
GATE
FB
FAULT
6
FILTER
RESET
TIMER
4
R3
36k
R4
15k
LTC4211CMS
ON
CFILTER
100pF
VOUT
5V
5A
+
CLOAD
(≥100μF TYP)
1
LOGIC
RESET
GND
CTIMER
47nF
5
DN319 F01
GND
Figure 1. Single Channel 5V Hot Swap Controller in 10-Lead MS with Dual Level Current Control
09/03/319_conv
There are several advantages to dispensing with the
usual gate capacitor including the elimination of an
external component and the elimination of the turn-off
delay inherent in discharging that capacitor during a
fault. Even so, the LTC4211 is not restricted to using its
internal current control mode at start-up—an external
capacitor can still be employed in the ordinary way,
integrating the 10μA GATE pin current to produce a
well-controlled soft-start ramp. An 8-pin version, the
LTC4211CS8 is available as a back wards-compatible
upgrade to the LTC1422CS8 in applications demanding
current limited start-up.
Adaptive Response to Overloads
During start-up, the LTC4211 operates in the aforementioned 50mV/RSENSE current limit mode (regardless
of whether a gate capacitor is included in the design) at
any time the 50mV threshold is exceeded. For example,
if the LTC4211 attempts to start up into a short circuit,
the current is first limited to 50mV/RSENSE and then cut
off after the expiration of one TIMER pin cycle.
After successful start-up, the timed circuit breaker
function takes over. The 50mV threshold still applies,
but instead of instantly tripping on an overcurrent
condition, the FILTER circuit delays turn-off, thereby
rejecting temporary surges and spikes. This prevents
minor backplane disturbances from interrupting delivery
of power to critical subsystems and memory.
Figure 2 illustrates the action of the FILTER pin. An
overload of approximately 8A is drawn from the 5V
output which exceeds the 50mV/7mΩ SENSE pin
threshold. CFILTER delays shutdown 100μs. A shorter
duration overload would have been rejected. Extreme
overloads must be recognized and cleared immediately
to prevent collateral damage. The 150mV SENSE pin
threshold reacts to these overloads in just 300ns,
bypassing the FILTER pin delay.
Figure 2 also shows the importance of input clamping in
high current applications. Readily recognized is the di/dt
dip at the input, VCC, precipitated by the ≈1A/μs load current slew rate coupled with a backplane/wiring harness
inductance of nearly 3μH. There is no input bypassing.
Upon commutation of the 8A load current, a potentially
destructive consequence of the inductive feedpoint
impedance rears its ugly head: a voltage spike limited
only by the input clamp. The LTC4211’s high absolute
maximum VCC rating of 17V eases selection of an appropriate clamp, as these devices tend to have wide
tolerances.
Recovery from Faults
Once the circuit breaker has tripped, for whatever reason,
the LTC4211 safely latches off and helps guard against
any damage to the MOSFET or affected circuitry. The
FAULT pin then alerts the system controller for further
action. The circuit breaker can be reset by either cycling
the ON pin low under microprocessor control or by
allowing the chip to reset itself (by tying FAULT back
to the ON pin).
FAULT also serves as an input. If FAULT is pulled low
externally by another open drain logic signal, the
LTC4211 circuit breaker trips and turns off the output.
This feature allows multiple supplies to shut down
simultaneously when a short circuit occurs on any one
output. The FAULT pin of each LTC4211 is wired to a
common point, so that a fault on one is communicated
to the others.
VCC
5V/DIV
FILTER
1V/DIV
LOAD CURRENT
10A/DIV
20μs/DIV
DN319 F02
Figure 2. Temporary Surges and spikes are Rejected During
the Time Set By the FILTER Pin. A Persistent Overload of ≈8A
Trips the Circuit Breaker After 100μs FILTER Delay
Data Sheet Download
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call (408) 432-1900
dn319f_conv LT/TP 0903 351.5K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 2003
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