Dec 2003 Hot Swap Controller Enforces Tracking in Split Supply Systems

DESIGN FEATURES
Hot Swap Controller Enforces
Tracking in Split Supply Systems
by Ted Henderson
Introduction
Split power supplies are widely used in
audio, video and data communication
systems. These systems typically use
±5V, ±12V or ±15V supply voltages
and require a wide range of operating currents. The LT4220 Hot Swap
controller—which operates over any
combination of split supplies ranging from ±2.7V to ±16.5V—allows
a circuit board to be safely inserted
or removed from a live backplane
without glitching the power supplies
while controlling load currents from
milliamps to amps.
Glitches can cause anything from
objectionable “pops” in audio systems,
data loss in digital systems or even
connector damage. Pops can also
originate from shifting bias points or
complementary stages wherein half of
the circuit is correctly powered and the
other half has not yet been powered.
The LT4220 offers the usual Hot
Swap features, such as limiting inrush
current to the local supply bypass capacitors and isolating faults from the
system supply should they occur, but
it also coordinates voltage tracking of
the split supplies. Tracking ensures
RS +
VIN+
that both the positive and negative
supplies power-up either coincidently
or ratiometrically, thereby eliminating
glitch and pop problems. This complete
split supply Hot Swap control system
is packaged in a small 16-lead SSOP
plastic package.
The LT4220
The LT4220 contains two independent,
yet coupled, Hot Swap controllers, one
for the negative supply and one for the
positive supply. The control action is
carefully coordinated such that the
supplies turn on together, turn off
together and in the case of an over-current fault, both outputs are tripped off
simultaneously. Best of all, the LT4220
enforces active tracking between the
two supplies during power-up to ease
the design requirements of the split
supply circuitry and eliminate abnormal circuit behavior arising from
asymmetrical supply ramps.
The LT4220 provides other important Hot Swap features, including
input voltage monitors, output voltage monitors, a circuit breaker with
selectable automatic retry, timed cur-
Q1
VOUT+
C1
ON+
FB+
C6
PWRGOOD
FAULTB
FB–
ON–
LT4220
TRACK
C2
VIN–
RS –
Q2
Figure 1. LT4220 Simplified block diagram
28
VOUT–
rent limiting with foldback, and gate
drives for N-channel MOSFET devices
to be used on both the negative and
positive supplies.
Basic Operation
Figure 1 shows a simplified block diagram of the LT4220. The inputs are
monitored and power-up is not started
until both are good. The outputs are
monitored and PWRGD signals when
both are good. Tracking monitors the
outputs via the FB pins and controls
the gate drives to assure correct
power-up. N-channel MOSFETs are
used on both supplies, eliminating
the need for complementary devices.
FAULT indicates when a current limit
condition has caused the timer to time
out. Connecting FAULT back to ON+
enables automatic retry. Ramp rates
are adjusted by gate capacitors and
associated gate charging currents.
Nevertheless, when track is enabled
the actual rate is no faster than the
slowest ramp.
Typical Hot Swap Application
Figure 2 shows a complete circuit
design for a ±12V, 10A Hot Swap
circuit using the LT4220. Q1 and Q2
N-channel MOSFET devices control
the ±12V output power-up profiles
after insertion. Resistors RS+ and
RS– sense the load current, enabling
the LT4220 to protect against temporary overloads and short circuits.
R5 and R7 prevent high frequency
parasitic oscillations sometimes associated with power MOSFET devices
operating in their linear regions. The
amount of inrush current is set by the
appropriate choice of C1 and C2. In
this case the inrush current is limited
to approximately 100mA for a 100µF
load capacitance. In case of an output
short circuit, both Hot Swap channels incorporate timed current limiting
with foldback to protect the MOSFET
devices against over-dissipation, and
Linear Technology Magazine • December 2003
DESIGN FEATURES
RS +
0.005Ω
VIN+
12V
VCC
R13
10Ω
C4
100nF
Z1*
R5
10Ω
CONNECTOR 1
CONNECTOR 2
7
R15
20k
R1
36.5k
R2
4.99k
C7
10nF
R4
4.99k
C6
1µF
C8
10nF
TRACK
12
9
6
Z2*
15
SENSE +
LT4220
R6
1k
12V
VOUT+
C1
10nF
CL1
14
GATE+
10
PWRGD
13 R16, 20k
FB+
R10
4.99k
FB
ON –
VEE
1
R14
10Ω
SENSEK SENSE –
2
3
RS –
0.005Ω
– 5
C2
4 R8 10nF
1k
R7
10Ω
CL2
D1
IN4001
D2
IN4001
+
–12V
VOUT –
Q2
SUB85N03-04
* 1SMA13AT3 TRANSIENT
VOLTAGE SUPPRESSOR
R9
36.5k
R11
36.5k
GATE –
C3
100nF
+
R12
4.99k
GND
C5
1µF
VIN–
16
VCC
ON+
11 FAULT
8
TIMER
R3
36.5k
–12V
VEE
Q1
SUB85N03-04
4220 TA01
Figure 2. LT4220 ±12V, 10A Hot Swap controller
were chosen to enable the GATE drive
outputs when both input supplies are
within 15% of their final value. Resistive divider ratios for R9/R10 and
R11/R12 were chosen to indicate that
the output power was good when both
outputs are within 15% of their final
value. The 15% value was chosen assuming that the system power supply
tolerances were ±10%.
12V
VIN +
12V
VOUT +
–12V
VIN –
–12V
VOUT –
TIME (10ms/DIV)
Power-Up Sequence
Figure 3. Power-up sequence
disconnect a faulted circuit from the
backplane. Foldback is especially valuable in difficult circumstances such
as start up into a 0Ω short circuit,
where simple protection schemes
may not be sufficient to protect the
output MOSFET devices. Resistive
divider ratios for R1/R2 and R3/R4
Bouncing contacts and voltage glitches
during board insertion wreak havoc
with sensitive analog circuitry powered
by split supplies. The LT4220 eliminates all of these issues (the results
shown in Figure 3). After the ON+ and
ON– pins exceed their undervoltage
lockout thresholds, the gates of Q1
and Q2 (GATE+, GATE–) are pulled
up by the internal current sources.
For large capacitive loads the inrush
current is limited by the gate slew rate
or by the foldback current limit. For
a desired inrush current that is less
than the foldback current limit, the
feedback capacitors C1 and C2 can
be used to control the output voltage
slew rates by integrating the gate
pullup currents. Once both output
supply voltages exceed their power
good thresholds and the MOSFETs
Q1/Q2 are fully enhanced, the PWRGD
signal is released and pulled high by
R16 (Figure 2).
Supply Tracking
When the TRACK pin is connected
to VIN+, track mode is enabled. The
function of this mode is to control the
GATE+ and GATE– pullup currents
A
VOUT +
12V
VOUT+
A+B
5V/DIV
VOUT +
12V
2V/DIV
2V/DIV
VOUT –
–5V
(INVERTED)
VOUT–
VOUT –
–5V
(INVERTED)
B
2ms/DIV
Figure 4. ±12V coincident supply tracking
Linear Technology Magazine • December 2003
2ms/DIV
Figure 5. +12V, –5V coincident
supply tracking (–5V signal inverted)
2ms/DIV
Figure 6. +12V, –5V ratiometric
supply tracking (–5V signal inverted)
29
DESIGN FEATURES
such that the desired output voltages
ramp characteristic is achieved. The
gate pullup currents are controlled via
the FB+ and FB– pins.
Figure 4 shows coincident tracking for a system operating with +12V
and –12V supplies as per the circuit
in Figure 2. The circuit in Figure 2
is easily converted to work with –5V
and +12V supplies by simply changing
R3, R9 and R11 to 12.4kΩ. The new
coincident tracking behavior is shown
in Figure 5. Ratiometric tracking is
sometimes preferable, especially in
signal processing applications. Figure 6 shows this mode of operation,
obtained by changing only R3 and R11
to 12.4kΩ. Note that in this case the
supply ramps are made to start and
finish at the same time.
Short-Circuit Protection
Current limiting provides protection
for the output MOSFET devices. The
current limit for either supply is set by
sense resistors RS+ and RS– (Figure 2).
The voltage across the sense resistor is
regulated by the current limit circuitry
to 50mV for conditions where foldback
current limiting is not enabled. The
LTC6903/LTC6904, continued from page 9
Conclusion
Though crystal based oscillators have
dominated the timing and clocking
market for many years, the LTC6903
(I2C) and LTC6904 (SPI) offer solutions
that are smaller, more flexible, more
LTC3205, continued from page 23
Both of these features are required to
keep the LTC3205 in direct-connect
mode as long as possible.
Conclusion
The LTC3205, designed specifically
for portable backlighting applications,
provides all of the necessary current
TIMER pin provides a means for setting the maximum time the LT4220
is allowed to operate in current limit.
Whenever the current limit circuitry
becomes active, by either the positive
or negative sense amplifier operating in
current limit, a pull-up current source
of 60uA is connected to the TIMER pin
and the voltage rises with a slope of
dV/dt = 60µA/CTIMER. If the overload
is removed, a small 3µA pulldown
current slowly discharges the timer
pin. If the timer succeeds in charging to a 1.24V threshold, an internal
fault latch is set and the FAULT pin is
pulled low. Both MOSFETs are quickly
turned off while the TIMER pin is slowly
discharged to ground.
The power dissipation will be high
in the output MOSFET devices when
the output is shorted with zero ohms.
To prevent excessive power dissipation
in these pass transistors the current
limit on each supply is reduced as the
output voltage falls. This characteristic, commonly referred to as “current
foldback”, reduces the fault current as
the output voltage drops and reaches
the lowest level into the short. The
foldback current limiting reduces
short circuit MOSFET dissipation by
a factor of 2.5. The FB± pins effectively
measure the MOSFET VDS voltage and
control the appropriate current limit
sense amplifier input offset to provide
the foldback current limit.
Automatic Restart
Normally the LT4220 latches off in
the presence of a fault. Nevertheless,
by removing R15 in Figure 2, you can
connect the FAULT and ON+ together to
enable automatic restart. FAULT pulls
the ON+ pin low allowing an automatic
restart to be initiated once the TIMER
pin ramps below 0.5V.
Conclusion
The LT4220 combines all of the functions necessary for split supply Hot
Swap control in one small 16-lead
SSOP plastic package. This device is
adaptable to applications covering a
wide range of positive and negative
supply voltages, ramping profiles,
capacitance and load currents, including optical/laser, audio and ECL
systems.
robust and lower power. Selecting
a frequency from the 1kHz–68MHz
frequency range is simple through
the serial ports, and both devices
operate over a wide range of supply
voltages.
regulation, power circuitry and control
logic to deliver efficient and accurate
power to a large number of LEDs in
a portable product. To further reduce
board level complexity, it uses only four
0603 sized ceramic capacitors keeping
the total solution height under 1mm.
A straightforward serial interface re-
for
the latest information
on LTC products,
visit
www.linear.com
duces the number of wires needed to
control all of the LEDs. Given its feature
set, the LTC3205 packs an amazing
amount of backlighting horsepower,
flexibility and performance into a very
small 4mm × 4mm footprint.
For more information on parts featured in this issue, see
http://www.linear.com/go/ltmag
30
Linear Technology Magazine • December 2003
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