UCC1921
UCC2921
UCC3921
Latchable Negative Floating Hot Swap Power Manager
FEATURES
DESCRIPTION
• Precision Fault Threshold
The UCC3921 family of negative floating hot swap power managers provides complete power management, hot swap, and fault handling capability. The IC is referenced to the negative input voltage and is powered
through an external resistor connected to ground, which is essentially a
current drive as opposed to the traditional voltage drive. The onboard
10V shunt regulator protects the IC from excess voltage and serves as a
reference for programming the maximum allowable output sourcing current during a fault. All control and housekeeping functions are integrated
and externally programmable. These include the fault current level, maximum output sourcing current, maximum fault time, selection of Retry or
Latched mode, soft start time, and average power limiting. In the event of
a constant fault, the internal timer will limit the on time from less than
0.1% to a maximum of 3% duty cycle. The duty cycle modulation depends on the current into PL, which is a function of the voltage across
the FET, thus limiting average power dissipation in the FET. The fault
level is fixed at 50mV across the current sense amplifier to minimize total
• Programmable:
Average Power Limiting, Linear
Current Control, Overcurrent Limit
and Fault Time
• Fault Output Indication Signal
• Automatic Retry Mode or Latched
Operation Mode
• Shutdown Control
• Undervoltage Lockout
• 250µs Glitch Filter on the SDFLTCH
pin
• 8-Pin DIL and SOIC
(continued)
BLOCK DIAGRAM
UDG-99052
3/98
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UCC1921
UCC2921
UCC3921
DESCRIPTION (continued)
CT charges to 2.5V, the output device is turned off and
performs a retry some time later (provided that the selected mode of operation is Automatic Retry Mode).
When the output current reaches the maximum sourcing
current level, the output acts as a current source, limiting
the output current to the set value defined by IMAX.
dropout. The fault current level is set with an external
current sense resistor, while the maximum allowable
sourcing current is programmed with a voltage divider
from VDD to generate a fixed voltage on IMAX. The current level, when the output acts as a current source, is
equal to VIMAX/RSENSE. If desired, a controlled current
start up can be programmed with a capacitor on IMAX.
Other features of the UCC3921 include undervoltage
lockout, 8-pin Small Outline (SOIC) and Dual-In-Line
(DIL) packages, and a Latched Operation Mode option,
in which the output is latched off once CT charges to
2.5V and stays off until either SDFLTCH is toggled (for
greater than 1ms) or the IC is powered down and then
back up.
When the output current is below the fault level, the output device is switched on. When the output current exceeds the fault level, but is less than the maximum
sourcing level programmed by IMAX, the output remains
switched on, and the fault timer starts charging CT. Once
CONNECTION DIAGRAM
ABSOLUTE MAXIMUM RATINGS
IVDD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50mA
SDFLTCH Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10mA
PL Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10mA
IMAX Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VDD
Storage Temperature . . . . . . . . . . . . . . . . . . . −65°C to +150°C
Junction Temperature . . . . . . . . . . . . . . . . . . . –55°C to +150°C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300°C
DIL-8 , SOIC-8 (Top View)
N or J, D Packages
All voltages are with respect to VSS (the most negative voltage).
Currents are positive into, negative out of the specified terminal.
Consult Packaging Section of Databook for thermal limitations and
considerations of packages.
ELECTRICAL CHARACTERISTICS Unless otherwise specified, TA = 0°C to 70°C for the UCC3921 and –40°C to 85°C
for the UCC2921, and –55°C to 125°C for the UCC1921; IVDD = 2mA, CT = 1nF (the minimum allowable value), there is no
resistor connected between the SDFLTCH and VSS pins. TA = TJ.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
ISOURCE = 2mA
9
1
2
mA
9.5
10.0
V
ISOURCE = 10mA
9.15
9.6
10.15
V
6
7
8
V
47.5
50
53.5
mV
46
50
53.5
mV
50
500
nA
VDD Section
IDD
Regulator Voltage
UVLO Off Voltage
Fault Timing Section
Overcurrent Threshold
TJ = 25°C
Over Operating Temperature
Overcurrent Input Bias
CT Charge Current
VCT = 1V, IPL = 0
–50
–36
–22
µA
Overload Condition, VSENSE - VIMAX = 300mV
–1.7
–1.2
–0.7
mA
CT Discharge Current
VCT = 1V, IPL = 0
0.6
1
1.5
µA
CT Fault Threshold
2.2
2.45
2.6
V
CT Reset Threshold
0.41
0.49
0.57
V
1.7
2.7
3.7
%
Output Duty Cycle
Fault Condition, IPL = 0
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2
UCC1921
UCC2921
UCC3921
ELECTRICAL CHARACTERISTICS Unless otherwise specified, TA = 0°C to 70°C for the UCC3921 and –40°C to 85°C
for the UCC2921, and –55°C to 125°C for the UCC1921; IVDD = 2mA, CT = 1nF (the minimum allowable value), there is no
resistor connected between the SDFLTCH and VSS pins. TA = TJ.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
IOUT = 0mA
8.5
10
V
IOUT = –1mA
6
8
V
Output Section
Output High Voltage
Output Low Voltage
IOUT = 0mA, VSENSE – VIMAX = 100mV
0
10
mV
IOUT = 2mA, VSENSE – VIMAX = 100mV
200
600
mV
Linear Amplifier Section
Sense Control Voltage
VIMAX = 100mV
85
100
115
mV
VIMAX = 400mV
370
400
430
mV
50
500
nA
4.35
4.85
5.35
V
Input Bias
Power Limiting Section
VSENSE Regulator Voltage
IPL = 64µA
Duty Cycle Control
IPL = 64µA
0.6
1.2
1.7
%
IPL = 1mA
0.045
0.1
0.17
%
300
500
ns
mV
Overload Section
Delay to Output
Note 1
Output Sink Current
VSENSE – VIMAX = 300mV
40
100
Threshold
Relative to IMAX
140
200
260
mA
3
5
VDD+1
V
VSDFLTCH = 5V
50
110
250
µA
250
500
1000
µs
6
9.5
5
8.5
Shutdown/Fault/Latch Section
Shutdown Threshold
Input Current
Filter Delay Time (Delay to Output)
Fault Output High
ISDFLTCH = –100µA
Fault Output Low
Output Duty Cycle
Fault Condition, IPL = 0
1.7
V
V
0
10
2.7
3.7
%
0
%
ISDFLTCH = –100µA, Fault Condition, IPL = 0
mV
Note 1: Guaranteed by design. Not 100% tested in production.
PIN DESCRIPTIONS
voltage on IMAX over the current sense resistor. If
desired, a controlled current start up can be programmed
with a capacitor on IMAX, and a programmed start delay
can be achieved by driving the shutdown with an open
collector/drain device into an RC network.
CT: A capacitor is connected to this pin in order to set
the fault time. The fault time must be longer than the time
to charge external load capacitance. The fault time is
defined as:
TFAULT =
2 • CT
ICH
OUT: This pin provides gate output drive to the MOSFET
pass element.
where ICH = 36µA + IPL, and IPL is the current into the
power limit pin. Once the maximum fault time is reached
the output will shutdown for a time given by:
PL: This feature ensures that the average MOSFET
power dissipation is controlled. A resistor is connected
from this pin to the drain of the NMOS pass element.
When the voltage across the NMOS exceeds 5V, current
will flow into the PL pin which adds to the fault timer
charge current, reducing the duty cycle from the 3%
level. When IPL>>36µA, then the average MOSFET
power dissipation is given by:
TSD = 2 • 10 6 • CT
IMAX: This pin programs the maximum allowable
sourcing current. Since VDD is a regulated voltage, a
voltage divider can be derived from VDD to generate the
program level for IMAX. The current level at which the
output appears as a current source is equal to the
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PMOSFET avg = IMAX • 1 • 10 −6 • R PL
3
UCC1921
UCC2921
UCC3921
PIN DESCRIPTIONS (continued)
SENSE: Input voltage from the current sense resistor.
When there is greater than 50mV across this pin with
respect to VSS, then a fault is sensed, and CT starts to
charge.
If an 5k < RLATCH < 250kΩ resistor is placed from this pin
to VSS, then the latched operating mode will be invoked.
Upon the occurrence of a fault, under the latched mode
of operation, once the CT capacitor charges up to 2.5V
the NMOS pass element latches off. A retry will not
periodically occur. To reset the latched off device, either
SDFLTCH is toggled high for a duration greater than 1ms
or the IC is powered down and then up.
SDFLTCH: This pin provides fault output indication,
shutdown control, and operating mode selection.
Interface into and out of this pin is usually performed
through level shift transistors. When open, and under a
non-fault condition, this pin pulls to a low state with
respect to VSS. When a fault is detected by the fault
timer, or undervoltage lockout, this pin will drive to a high
state with respect fo VSS, indicating the NMOS pass
element is OFF. When > 250µA is sourced into this pin
for > 1ms, it drives high causing the output to disable the
NMOS pass device.
VDD: Current driven with a resistor to a voltage approximately 10V more positive than VSS. Typically a resistor is
connected to ground. The 10V shunt regulator clamps
VDD approximately 10V above VSS, and is also used as
an output reference to program the maximum allowable
sourcing current.
VSS: Ground reference for the IC and the most negative
voltage available.
APPLICATION INFORMATION
UDG-96275-1
Figure 1. Fault Timing Circuitry for the UCC3921, Including Power Limit Overload
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4
UCC1921
UCC2921
UCC3921
APPLICATION INFORMATION (continued)
Figure 1 shows the detailed circuitry for the fault timing
function of the UCC3921. For the time being, we will discuss a typical fault mode, therefore, the overload comparator, and current source I3 does not work into the
operation. Once the voltage across the current sense resistor, RS, exceeds 50mV, a fault has occurred. This
causes the timing capacitor to charge with a combination
of 36µA plus the current from the power limiting amplifier.
The PL amplifier is designed to only source current into
the CT pin and to begin sourcing current once the voltage across the output FET exceeds 5V. The current IPL
is related to the voltage across the FET with the following
expression:
V
− 5V
I PL = FET
RPL
where VFET is the voltage across the NMOS pass device.
Later it will be shown how this feature will limit average
power dissipation in the pass device. Note that under a
condition where the output current is more than the fault
level, but less than the max level, VOUT ≈ VSS (input
voltage), IPL = 0, the CT charging current is 36µA.
UDG-96276
t5 = t3: Illustrates 3% duty cycle.
t0: Safe condition. Output current is nominal, output
voltage is at the negative rail, VSS.
t6 = t4: Retry. CT has discharged to 0.5V, but fault is
still exceeded, CT begins charging again, FET is on,
VOUT pulled down towards VSS.
t1: Fault control reached. Output current rises above
the programmed fault value, CT begins to charge at
~36µA.
t7: Output short circuit. If VOUT is short circuited to
ground, CT charges at a higher rate depending upon
the values for VSS and RPL.
t2: Maximum current reached. Output current reaches
the programmed maximum level and becomes a constant current with value IMAX.
t8: Fault occurs. Output is still short circuited, but the
occurrence of a fault turns the FET off so no current is
conducted.
t3: Fault occurs. CT has charged to 2.5V, fault output
goes high, the FET turns off allowing no output current
to flow, VOUT floats up to ground.
t9 = t4: Output short circuit released, still in fault mode.
t4: Retry. CT has discharged to 0.5V, but fault current is
still exceeded, CT begins charging again, FET is on,
VOUT pulled down towards VSS.
t10 = t0: Fault released, safe condition. Return to normal operation of the hot swap power manager.
Figure 2. Retry Operation Mode
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5
UCC1921
UCC2921
UCC3921
APPLICATION INFORMATION (cont.)
UDG-96277
t0: Safe condition. Output current is nominal, output
voltage is at the negative rail, VSS.
is still exceeded, CT begins charging again, FET is on,
VOUT pulled down towards VSS.
t1: Fault control reached. Output current rises above
the programmed fault value, CT begins to charge at
~36µA.
t8 = t3: Fault occurs. CT has charged to 2.5V, fault output goes high as indicated by the SDFLTCH voltage,
the FET turns off allowing no output current to flow,
VOUT floats up to ground, and since there is an 82kΩ
resistor from SDFLTCH to VSS, the internal latchset
signal goes high.
t2: Maximum current reached. Output current reaches
the programmed maximum level and becomes a constant current with value IMAX.
t9: Output is latched off. Even though CT has discharged to 0.5V, there will not be a retry since the
latchset signal was allowed to remain high.
t3: Fault occurs. CT has charged to 2.5V, fault output
goes high as indicated by the SDFLTCH voltage. The
FET turns off allowing no output current to flow, VOUT
floats up to ground, and since there is an 82kΩ resistor
from the SDFLTCH pin to VSS, the internal latchset signal goes high.
t10: Output remains latched off. CT has discharged all
the way to 0V.
t4: Since the user does not want the chip to LATCH off
during this cycle, he toggles SDFLTCH high for greater
than 1ms {t6 - t4 > 1ms}.
t11: The output has been latched off for quite some
time. The user now wishes to reset the latched off output, thus toggling SDFLTCH high for greater than 1ms
{t13 - t11}.
t5: The latchset signal is reset.
t12 = t5: The latchset signal is reset.
t6: Forcing of SDFLTCH is released after having been
applied for > 1ms.
t13: Forcing of SDFLTCH is released after having been
applied for > 1ms. The fault had also been released
during the time the output was latched off, safe condition, return to normal operation of the hot swap power
manager.
t7: Retry (since the latchset signal has been reset to its’
low state) - CT has discharged to 0.5V, but fault current
Figure 3. Latched Operation Mode: RLATCH = 82k
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6
UCC1921
UCC2921
UCC3921
APPLICATION INFORMATION (continued)
output FET failure or to build redundancy into the system.
During a fault, CT will charge at a rate determined by the
internal charging current and the external timing capacitor. Once CT charges to 2.5V, the fault comparator
switches and sets the fault latch. Setting of the fault latch
causes both the output to switch off and the charging
switch to open. CT must now discharge with the 1µA current source, I2, until 0.5V is reached. Once the voltage at
CT reaches 0.5V, the fault latch resets, which re-enables
the output and allows the fault circuitry to regain control
of the charging switch. If a fault is still present, the fault
comparator will close the charging switch causing the cycle to repeat. Under a constant fault, the duty cycle is
given by:
Duty Cycle =
Determining External Component Values
To set RVDD (see Fig. 4) the following must be achieved:
VIN min
10V
>
+ 2mA
RVDD
R1 + R 2
1µA
I PL + 36 µA
Average power dissipation in the pass element is given
by:
PFETAVG = VFET • I MAX •
1µA
I PL + 36 µA
UDG-96278
Figure 4.
Where VFET>>5V IPL can be approximated as:
VFET
RPL
In order to estimate the minimum timing capacitor, CT,
several things must be taken into account. For example,
given the schematic in Figure 4 as a possible (and at this
point, a standard) application, certain external component values must be known in order to estimate CTMIN.
Now, given the values of COUT, Load, RSENSE, VSS, and
the resistors determining the voltage on the IMAX pin,
the user can calculate the approximate startup time of
the node VOUT. This startup time must be faster than the
time it takes for CT to charge to 2.5V (relative to VSS),
and is the basis for estimating the minimum value of CT.
In order to determine the value of the sense resistor,
RSENSE, assuming the user has determined the fault current, RSENSE can be calculated by:
and where IPL>>36µA, the duty cycle can be approximated as :
1µA • RPL
VFET
Therefore, the maximum average power dissipation in
the MOSFET can be approximated by:
PFET AVG = VFET • I MAX •
= IMAX • 1µA • RPL
1µA • RPL
VFET
Notice that in the approximation, VFET cancels, thereby
limiting the average power dissipation in the NMOS pass
element.
RSENSE =
Next, the variable IMAX must be calculated. IMAX is the
maximum current that the UCC3921 will allow through
the transistor, M1, and it can be shown that during
startup with an output capacitor the power MOSFET, M1,
can be modeled as a constant current source of value
IMAX where
Overload Comparator
The linear amplifier in the UCC3921 ensures that the
output NMOS does not pass more than IMAX (which is
VIMAX/RSENSE). In the event the output current exceeds
the programmed IMAX by 0.2V/RSENSE, which can only
occur if the output FET is not responding to a command
from the IC, CT will begin charging with I3, 1mA, and
continue to charge to approximately 8V. This allows a
constant fault to show up on the SDFLTCH pin, and also
since the voltage on CT will continue charging past 2.5V
in an overload fault mode, it can be used for detection of
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50mV
I FAULT
I MAX =
VIMAX
where VIMAX = voltage on pin IMAX.
RSENSE
Given this information, calculation of the startup time is
now possible via the following:
7
UCC1921
UCC2921
UCC3921
APPLICATION INFORMATION (continued)
Resistive Load:
Current Source Load:
TSTART =
CT min =
3 • TSTART • ( 36 µA • R PL + VSS − 5V − I MAX • ROUT )
5 • RPL
COUT • VSS
I MAX − I LOAD
Resistive Load:
TSTART =
+
I MAX • ROUT
COUT • ROUT • n
I MAX • ROUT − VSS
l
Level Shift Circuitry to Interface with SDFLTCH
Some type of circuit is needed to interface with the
UCC3921 via SDFLTCH, such as opto-couplers or level
shift circuitry. Figure 6 depicts one implementation of
level shift circuitry that could be used, showing component values selected for a typical –48V telecommunications application. There are three communication
conditions which could occur; two of which are Hot Swap
Power Manager (HSPM) state output indications, and the
third being an External Shutdown.
Once TSTART is calculated, the power limit feature of the
UCC3921 must be addressed and component values derived. Assuming the user chooses to limit the maximum
25
R PL===∞
∞∞
22.5
IMAX = 4A
20
1) When open, and under a non-fault condition,
SDFLTCH is pulled to a low state. In Figure 6, the Nchannel level shift transistor is off, and the
FAULT OUT signal is pulled to LOCAL VDD through
R3. This indicates that the HSPM is not faulted.
17.5
15
R PL = 10M
12.5
R PL = 5M
10
2) When a fault is detected by the fault timer or undervoltage lockout, this pin will drive to a high state, indicating that the external power FET is off. In Figure 6,
the N-channel level shift transistor will conduct, and
the FAULT OUT signal will be pulled to a Schottky Diode voltage drop below LOCAL GND. This indicates
that the HSPM is faulted. The Schottky Diode is necessary to ensure that the FAULT OUT signal does
not traverse too far below LOCAL GND, making fault
detection difficult.
7.5
R PL = 2M
5
R PL = 1M
2.5
0
R PL = 500k
R PL = 200k
0
25
50
3 • ROUT • VSS • COUT
5 • RPL
75 100 125 150 175 200
VFET
Figure 5. Plot Average Power vs FET Voltage for
Increasing Values of RPL
allowable average power that will be associated with the
hot swap power manager, the power limiting resistor,
RPL, can be easily determined by the following:
RPL =
PFET avg
1µA • I MAX
where a minimum RPL exists
defined by RPL min =
VSS
(Refer to Figure 5).
5 mA
Finally, after computing the aforementioned variables, the
minimum timing capacitor can be derived as such:
Current Source Load:
UDG-96279
CT min =
3 • TSTART • ( 72 µA • R PL + VSS − 10V )
10 • RPL
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Figure 6. Possible Level Shift Circuitry to
Interface to the UCC3921, showing component
values selected for a typical telecom application.
8
UCC1921
UCC2921
UCC3921
APPLICATION INFORMATION (continued)
limited to 10mA or less: ISDFLTCHMAX < 10mA.
If a 5k < RLATCH < 250kΩ resistor is tied between
SDFLTCH & VSS, as optionally shown in Figure 6,
then the latched operating mode (described earlier)
will be invoked upon the occurrence of a fault.
SAFETY RECOMMENDATIONS
Although the UCC3921 is designed to provide system
protection for all fault conditions, all integrated circuits
can ultimately fail short. For this reason, if the UCC3921
is intended for use in safety critical applications where
UL or some other safety rating is required, a redundant
safety device such as a fuse should be placed in series
with the external power FET. The UCC3921 will prevent
the fuse from blowing for virtually all fault conditions, increasing system reliability and reducing maintenance
cost, in addition to providing the hot swap benefits of the
device.
3) To externally shutdown the HSPM, the SHUTDOWN
signal (typically held at LOCAL VDD) must be pulled
to LOCAL GND. Assuming SHUTDOWN is tied to
LOCAL GND, the P-channel level shift transistor will
conduct, driving SDFLTCH high (to roughly VDD plus
a diode). By sourcing > 250µA into SDFLTCH for >
1ms the output to the external power FET will be disabled. The current sourced into SDFLTCH must be
Ω
UDG-98053
Figure 7. Typical Telecommuications Application
(The “Negative Magnitude-Side” of the Supply is Switched in)
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9
UCC1921
UCC2921
UCC3921
APPLICATION INFORMATION (continued)
Ω
UDG-98054
Figure 8. Floating Positive Application
The “Ground-side” of the Supply is Switched In
UNITRODE CORPORATION
7 CONTINENTAL BLVD. • MERRIMACK, NH 03054
TEL. (603) 424-2410 • FAX (603) 424-3460
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