AN169: X80070, The Flexible -48V Hot Swap Solution

X80070, The Flexible -48V Hot Swap Solution
®
Application Note
May 4, 2005
AN169.0
Author: Carlos Martinez
Overview
zener diode controlling the base of a pass transistor to
provide a regulated 12V. With the transistor, a minimum
current through the zener provides regulation, while the
transistor supplies any current loads required by the
X80070. This circuit dissipates less power on average than a
zener diode/resistor only combination that must always
provide the maximum current needed by the X80070 plus
zener regulation current.
Systems that use -48V power supplies generally have a
requirement that boards may be inserted and removed from
the backplane without powering down the system. This “Hot
Swap” or “Hot Plug” application requires that minimum
current flows into the board when the board and the
backplane contacts meet. To meet this specification requires
an FET switch that provides power to the high power
components on the board only after the board has been
connected to the backplane; and turns off the power to the
board before it is removed from the backplane.
This power supply circuit can handle input voltages from -30V
to -170V. At -170V, the resistor dissipates a maximum of 2W
and the transistor dissipates a maximum of 800mW. The board
uses an MJD340 transistor that is rated at 300V and 1.5W.
The X80070 family of devices (X80070, X80071, X80072,
X80073) provide multiple control mechanisms to facilitate
the managed application and removal of power to a board.
These mechanisms include:
Overvoltage/Undervoltage Circuits
Inside the X80070 are comparators and programmable
voltage references to monitor for one overvoltage and two
undervoltage trip points. Intersil programs the overvoltage and
undervoltage trip points during manufacturing. There are two
optional overvoltage values: 74.3V for the X80070 and
X80072 devices and 67.4V for the X80071 and X80073
devices. The overvoltage and undervoltage options are
shown in Table 1 below.
• Delayed turn on of the FET after application of power.
• Manual or remote control of the FET to allow the device to
be turned on only under system control or turned off prior
to removal of the board.
• Control of the gate FET base current for more
manageable FET turn on characteristics.
The active undervoltage detector is selected by the BATTON pin. BATT-ON LOW (or unconnected) sets a higher
undervoltage level (42.4V). BATT-ON pulled HIGH sets a
lower undervoltage level (33.2V).
• Dual undervoltage thresholds for operation in a line or
battery operated system.
• Various retry options to handle different requirements for
FET turn-on operation.
A resistor divider connected between the plus and minus
input voltages and the VUV/OV pin (See Figure 1) determines
the overvoltage and undervoltage shutdown thresholds and
the operating voltage range. Using the thresholds in Table 1
and the equations of Figure the desired operating voltage
can be determined. Figure shows the resistance values
needed for various operating voltages.
This application note reviews these features and provides
some design guidelines for the operation of, and selection of
components for the X80070 device.
X80070 Power Supply Circuit
The X80070 requires a 12V power supply, provided on the
VDD pin. The circuit in Figure 3 shows a resistor and a 12V
TABLE 1. OVERVOLTAGE/UNDERVOLTAGE DEFAULT THRESHOLDS
THRESHOLD
SYMBOL
DESCRIPTION
FALLING
VOV
Overvoltage (X80070, 72)
3.87V
VOV
Overvoltage (X80071, 73)
3.51V
VUV1
Undervoltage 1
2.24V
VUV2
Undervoltage 2
1.76V
MAX/MIN VOLTAGE1
LOCKOUT VOLTAGE2
3.9V
74.3
74.9
3.54V
67.4
68
2.21V
43.0
42.4
1.73V
33.8
33.2
RISING
NOTES:
1. Max/Min Voltage is the maximum and mimimum operating voltage assuming the recommended VUV/OV resistor divider.
2. Lockout voltage is the voltage where the X8007x turns off the FET.
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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BATT-ON = VRGO
20
VUV2
222
214
0
150
VN
VUV1
206
R2 + R7
V S = V UV ⁄ OV  ----------------------
 R7 
R7
40
198
VUV/OV or:
Operating
Voltage
60
190
VS
VOV
182
R2
80
174
R7
V UV ⁄ OV = V S  ----------------------
 R2 + R7
BATT-ON = VEE
166
VP
100
158
Voltage divider:
Operating Voltage (Volts)
Application Note 169
R2 in kΩ (for R7=10K)
FIGURE 1. OVERVOLTAGE UNDERVOLTAGE DIVIDER
FIGURE 2. OPERATING VOLTAGE VS. RESISTOR RATIO
FIGURE 3. APPLICATION CIRCUIT
X80070 hotswap and support circuits
The primary components in the hotswap circuit are the FET
and the sense resistor. The FET can be any N-Channel type.
This application uses the IRF23N15 MOSFET from
International Rectifier, but any FET device operates similarly.
The X80070 overcurrent threshold is 50mV. With a sense
resistor value of 25mohm, the overcurrent condition during
normal operation is 2 Amps. On an initial turn-on of the FET
(insertion,) the X80070 has an overcurrent threshold of
150mV. So, for a system that limits the current to 2A in
normal operation, the X80070 allows up to 6A of surge
current during insertion. When the PWRGD signal goes
active, the current limit drops back down to the normal level.
2
FET feedback components (C3 and R10) provide
compensation for the FET during turn on. The value of the
resistance is not critical, but should be greater than
1KOhms. The capacitor value depends on the load and the
desired maximum surge current.
A rough calculation for the value of C3 is:
I GATE × C LOAD
C3 = ------------------------------------------I INRUSHmax
Typical values for the feedback capacitor typically range
from 2.2 to 10nF.
The X80070 has a programmable gate current, ranging from
10µA to 150µA. The capacitor chosen for this application is
AN169.0
May 4, 2005
Application Note 169
4.7nF. This results in surge currents below the maximim 6A
limit into a 200µF load for all but the highest gate current
setting (150µA.)
PWRGD
GND-VEE
With larger loads, the gate current can be set to lower values
using the jumpers on the IGQ pins. Or, C3 can be increased
to maintain the desired surge current. The capacitor size is
limited by the turn on time of the FET. If the capacitor is too
large, the FET remains partially turned on for a longer
period, possibly heating the FET. If the capacitor is too small,
the internal capacitance in the FET act to turn the FET on
too fast and very large surge currents result.
OCI
Limit
Vgate
Isurge
When power is applied to the system, the FET tries to turn
on due to its internal gate to drain capacitance (Cgd) and the
feedback capacitor C3. The X80070 device, when powered,
pulls the gate output low to prevent the gate voltage from
rising and keep the FET from turning on. However, unless
VDD powers up very quickly, there will be a brief period of
time during initial power application when the X80070
circuits cannot hold the gate low. The use of an external
capacitor (C2) prevents this. Capacitors C2 and C3 form a
voltage divider to prevent the gate voltage from rising above
the FET turn on threshold before the X80070 can hold the
gate low. Use the following formula for choosing C2.
FIGURE 4. FET SHUTDOWN (INTERNAL FILTER)
PWRGD
GND-VEE
OCI
Limit
Vgate
V1 – V2
C2 = --------------------- C3
V2
Where:
ext.
delay
Isurge
V1 = Maximum input voltage,
V2 = FET threshold Voltage,
C2 = Gate capacitor,
C3 = Feedback capacitor.
In a system where VDD rises very fast, a smaller value of C2
may suffice, because the X80070 will control the voltage at
the gate before the voltage can rise to the FET turn on
threshold. This application board assumes that the input
voltage rises only to 12.5V before the X80070 reaches its
operational voltage on VDD. So C2 is chosen to be 10nF. For
a system that turns on very slowly, or one with a capacitor on
the VDD line, a larger C2 capacitor is recommended.
X80070 Current Control and Shutdown
In addition to an overcurrent level, there is an overcurrent
time delay. For the X80070, this delay is 5µs. That is, in an
overcurrent condition the X80070 waits for 5µs before
turning the FET off (See Figure 4). For additional filtering,
contact Intersil for X80070 devices that are factory
programmed with overcurrent filter values of 0µs, 10µs, or
20µs. If 20µs of filtering is not enough (or for a longer time
constant with the standard X80070,) an external filter
provides any required delay. See Figure 5 for the response
to an overcurrent condition during insertion with an external
200ohm resistor and 1µF capacitor. This is an RC time
constant of about 200µs.
3
FIGURE 5. FET SHUTDOWN (EXTERNAL FILTER)
Programmable Gate Current
One of the benefits of the X80070 is its ability to program the
turn-on surge current with simple jumpers on the IGQ0 and
IGQ1 pins. This can reduce the surge current without
changing the hardware. For example, if it is necessary to
increase the bulk capacitance on the board, a simple jumper
can bring the surge current back under control.
The table below shows various measured surge currents
with C3=4.7nF; IGate=10µA, 50µA, 70µA or 150µA; and
CLOAD=100µF, 200µF or 300µF. The flexibility of the X80070
is shown by the wide range of surge current settings
possible with a only a jumper change on the IGQ0 and IGQ1
pins. Actual waveforms for performance of the circuit using a
200µF load are shown in Figure 6 through Figure 10.
CLOAD
Igate
(µA)
100µF
200µF
300µF
10
1A
2.3A
3.2A
50
1.2A
2.9A
4A
70
1.6A
3.9A
>6A
150
3.4A
>6A
>6A
AN169.0
May 4, 2005
Application Note 169
VGATE
VGATE
VEE
VEE
PWRGD
PWRGD
ILOAD
ILOAD
FIGURE 6. FET TURN-ON IGATE = 10µA
FIGURE 8. FET TURN-ON IGATE = 70µA
VGATE
VGATE
VEE
VEE
PWRGD
PWRGD
FIGURE 9. F
ILOAD
ILOAD
FIGURE 7. FET TURN-ON IGATE = 50µA
FIGURE 9. FET TURN-ON IGATE = 150µA (RETRIES)
VGATE
VEE
PWRGD
ILOAD
FIGURE 10. FET TURN-ON IGATE = 150µA (FINAL TURN-ON)
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Application Note 169
X80070 Reset (FET control)
The X80070 device has a MR input that signals a “Hot side”
reset. Pulling this input LOW results in the FET gate going
LOW. This turns off the FET. There are three mechanisms
for controlling this input pin.
First, if the internal 50ms delay on power on is not sufficient,
the MR pin can connect through a resistor to VDD and
through a capacitor to VEE. This provides a delay to the FET
turn-on until the contact bounce on insertion has decayed.
Second, a manual reset switch can be provided. Pressing
the button and holding it turns off the FET. Releasing the
button restarts the FET turn-on process (assuming that the
input voltage is still within limits.) This switch can be part of a
mechanical board insertion/removal mechanism.
Third, a host PC or workstation control of this pin remotely
through the backplane and an isolated interface. The user
can specify a “Soft-extract” to bring the MR pin LOW and
turn off the FET or the user can specify a “Soft-Insert” to set
the MR pin HIGH and turn on the FET.
X80070 Options
The schematic in Figure 3 shows three jumper settings. The
first two of these are the control of gate current. The third
controls the undervoltage setting. The X80070 has two
under voltage thresholds. The first is set for 42.3V. The
second is for 33.2V. Typically, undervoltage 2 (UV2) is for
operation when there is a battery backup. This allows the
system more voltage margin when operating on batteries
where the voltage may not be as high.
There are four standard versions of the X80070. The main
differences are in the overvoltage and undervoltage settings
and the retry response. If the undervoltage or overvoltage
settings are not correct for a particular application, they can
be factory set to custom levels, on request.
Besides the voltage thresholds, there are a number of
parameters in the X80070 family that are factory
programmed. If needed, different versions of the X80070
can be provided. See Table 2 for these options, with the
standard X80070 settings.
Summary
The X80070 provides a flexible solution for -48V hotswap
applications. The ability to set the gate current, change the
response to overcurrent conditions, provide dual
undervoltage shutsdown thresholds and delay the turnon of
the power FET gives the designer many options for
configuring and changing the operation of the power supply.
This allows the power supply to handle evolving demands in
a system or meet the needs of many systems without major
design changes.
TABLE 2. X80070 FACTORY PROGRAMMABLE PARAMETERS (BOLD INDICATES DIFFERENCES IN STANDARD DEVICES)
OPTION
FACTORY PROGRAMMABLE OPTIONS
Over current Filter
0µs, 5µs, 10µs, 20µs
Retry Options
Always retry (don’t set FAR)
Always retry (set FAR after 1st fail)
Never Retry (set FAR after fail)
Retry 1 time (set FAR after fail)
Retry 2 times (set FAR after fail)
Retry 3 times (set FAR after fail)
Retry 4 times (set FAR after fail)
Retry 5 times (set FAR after fail)
Insertion Over current
Retry Delay
Gate Current IGQ1:0=0,0
RESET delay
X80070
X80071
X80072
5µs
5µs
5µs
5µs
Retry 5
times
(set FAR
after fail)
Retry 5
times
(set FAR
after fail)
Always
Always
retry (don’t retry (don’t
set FAR)
set FAR)
X80073
1X, 2X, 3X, 4X
3X
3X
3X
3X
100ms, 500ms, 1s, 5s
100ms
100ms
100ms
100ms
10µA, 20µA, 30µA, 40µA, 50µA, 60µA, 70µA, 80µA,
90µA, 100µA, 110µA, 120µA, 130µA, 140µA, 150µA,
160µA
50µA
50µA
50µA
50µA
100ms, 500ms, 1s, 5s
100ms
100ms
100ms
100ms
Over voltage threshold
Any voltage between 1.5V and 4.25V (+/-50mV)
3.9V
3.54V
3.9V
3.54V
Under voltage threshold 1
Any voltage between 1.5V and 4.25V (+/-50mV)
2.24V
2.24V
2.24V
2.24V
Under voltage threshold 2
Any voltage between 1.5V and 4.25V (+/-50mV)
1.76V
1.76V
1.76V
1.76V
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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AN169.0
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