SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
FEATURES
D Wide Input Supply Range: –36 V to –80 V
D Transient Rating to –100 V
D Programmable Current Limit
D Programmable Current Slew Rate
D Enable Input (EN)
D Fault Timer to Eliminate Nuisance Trips
D Open-Drain Fault Output (FAULT)
D Requires Few External Components
D 8-Pin MSOP Package
DESCRIPTION
The TPS2390 and TPS2391 integrated circuits
are hot swap power managers optimized for use
in nominal –48-V systems. They are designed for
supply voltage ranges up to –80 V, and are rated
to withstand spikes to –100 V. In conjunction with
an external N-channel FET and sense resistor,
they can be used to enable live insertion of plug-in
cards and modules in powered systems. Both
devices provide load current slew rate and peak
magnitude limiting, easily programmed by sense
resistor value and a single-external capacitor.
They also provide single-line fault reporting,
electrical isolation of faulty cards, and protection
against nuisance overcurrent trips. The TPS2390
latches off in response to current faults, while the
TPS2391 periodically retries the load in the event
of a fault.
⋅APPLICATIONS
D –48-V Distributed Power Systems
D Central Office Switching
D Wireless Base Station
APPLICATION DIAGRAM
VOUT+
R3
10 kΩ
1W
–48V_RTN
+
C3
100 µF
100 V
D2
DC/DC
CONVERTER
MODULE
+
COUT
VOUT–
TPS2390/1
R2
100 kΩ
1
FAULT
2 EN
3
C1
0.1 µF
–48V_IN
RTN
8
GATE
7
Q1
IRF530
FLTTIME ISENSE 6
4 IRAMP
C2
3900 pF
–VIN
5
R1
0.02 Ω
1/4 W 1%
DGK PACKAGE
(TOP VIEW)
FAULT
1
8
RTN
EN
2
7
GATE
FLTTIME
3
6
ISENS
IRAMP
4
5
–VIN
UDG–02085
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments
semiconductor products and disclaimers thereto appears at the end of this data sheet.
Copyright 2002, Texas Instruments Incorporated
!"# $ %& '# "$ (&)*%"# +"#',
+&%#$ % ! # $('%%"#$ (' #-' #' !$ '."$ $# &!'#$
$#"+" + /" "#0, +&%# ( %'$$1 +'$ # '%'$$" *0 %*&+'
#'$#1 "** (" "!'#' $,
www.ti.com
1
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
ABSOLUTE MAXIMUM RATINGS (See Note 1)
TPS2390/1
UNIT
–0.3 V to 15
V
–0.3 V to 100
V
–0.3 V to 100
V
–0.3 V to 100
V
Continuous output current, FAULT
10
mA
Continuous total power dissipation
see Dissipation Rating Table
Input voltage range, all pins except RTN, EN, FAULT(2)
Input voltage range, RTN(2)
Input voltage range, EN(2)(3)
Output voltage range, FAULT(2)(4)
Operating junction temperature range, TJ
–55_C to 125_C
_C
Storage temperature range, Tstg
–65_C to 150_C
_C
260_C
_C
Lead temperature soldering 1,6 mm (1/16 inch) from case for 10 seconds
NOTES 1: Stresses beyond those listed under ”absolute maximum ratings” may cause permanent damage to the device. These are stress ratings
only and functional operation of the device at these or any other conditions beyond those indicated under ”recommended operating
conditions” is not implied. Exposure to absolute–maximum–rated conditions for extended periods may affect device reliability.
2: All voltages are with respect to –VIN (unless otherwise noted).
3: With 100-kΩ minimum input series resistance, –0.3 V to 15 V with low impedance.
4: With 10-kΩ minimum series resistance, –0.3 V to 80 V with low impedance.
ELECTROSTATIC DISCHARGE (ESD) PROTECTION
MIN
UNIT
Human Body Model (HBM)
1.5
kV
Charged Device Model (CDM)
1.5
kV
RECOMMENDED OPERATING CONDITIONS†
MIN
Nominal input supply, –VIN to RTN
Operating junction temperature range
† All voltages are with respect to –VIN (unless otherwise noted)
MAX
UNIT
–80
NOM
–36
V
–40
85
_C
DISSIPATION RATING TABLE
PACKAGE
TA < 25_C
POWER RATING
DERATING FACTOR
ABOVE TA = 25_C
TA = 85_C
POWER RATING
MSOP-8
420 mW
4.3 mW/_C
160 mW
AVAILABLE OPTIONS
OPERATING
TA
–40_C
40_C to 85_C
2
FAULT
OPERATION
PACKAGED DEVICES
MSOP (DGK)
Latch off
TPS2390DGK
Periodically retry
TPS2391DGK
www.ti.com
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
ELECTRICAL CHARACTERISTICS
VI(–VIN) = –48 V with respect to RTN, VI(EN) = 2.8 V, VI(ISENS) = 0, all outputs unloaded, TA = –40_C to 85_C
(unless otherwise noted)(1)(2)
input supply
PARAMETER
TEST CONDITIONS
ICC1
ICC2
Supply current, RTN
Supply current, RTN
VI(RTN) = 48 V
VI(RTN) = 80 V
VUVLO_L
VHYS
UVLO threshold, input voltage rising
To GATE pull-up, referenced to RTN
UVLO hysteresis
MIN
TYP
MAX
UNIT
700
1000
µA
1000
1500
µA
–36
–30
–25
V
1.8
2.3
3.0
V
enable input (EN)
PARAMETER
TEST CONDITIONS
VTH
VHYS_EN
Threshold voltage, input voltage rising
IIH
High-level input current
To GATE pull-up
EN hysteresis
VI(EN) = 5 V
MIN
TYP
MAX
UNIT
1.3
1.4
1.5
V
30
60
90
mV
–2
1
2
µA
linear current amplifier (LCA)
PARAMETER
VOH
ISINK
High-level output, GATE
II
VREF_K
VIO
Input offset voltage
TEST CONDITIONS
MIN
TYP
11
14
Output sink current
VI(ISENS) = 0 V
VI(ISENS) = 80 mV, VO(GATE) = 5V, Fault mode
50
100
Input current, ISENS
0 V < VI(ISENS) < 0.2 V
–1
Reference clamp voltage
VO(IRAMP) = open
VO(IRAMP) = 2 V
33
40
–7
MAX
17
UNIT
V
mA
1
µA
46
mV
6
mV
ramp generator
PARAMETER
TEST CONDITIONS
ISRC1
ISRC2
IRAMP source current, slow turn-on rate
VOL
AV
Low-level output voltage
IRAMP source current, normal rate
VO(IRAMP) = 0.25 V
VO(IRAMP) = 1 V, 3 V
VI(EN) = 0 V
VO(IRAMP) = 1 V, 3 V
Voltage gain, relative to ISENS
MIN
TYP
MAX
UNIT
–850
–600
–400
nA
–11
–10
–9
µA
2
mV
9.5
10.0
10.5
mV/V
MAX
UNIT
overload comparator
PARAMETER
VTH_OL
tDLY
TEST CONDITIONS
Current overload threshold, ISENS
Glitch filter delay time
VI(ISENS) = 200 mV
MIN
TYP
80
100
120
mV
2
4
7
µs
fault timer
PARAMETER
TEST CONDITIONS
VOL
ICHG
Low-level output voltage
VFLT
IDSG
Fault threshold voltage
Discharge current, retry mode
TPS2391
D
Output duty cycle
TPS2391
Charging current, current limit mode
VI(EN) = 0 V
VI(ISENS) = 80 mV, VO(FLTTIME) = 2 V
VI(ISENS) = 80 mV, VO(FLTTIME) = 2 V
IRST
Discharge current, timer reset mode
VO(FLTTIME) = 2 V,VI(ISENS) = 0 V
NOTES 1: All voltages are with respect to the –VIN terminal unless otherwise stated.
2: Currents are positive into and negative out of the specified terminal.
www.ti.com
MIN
TYP
MAX
UNIT
5
mV
–55
–50
–45
µA
3.75
4.00
4.25
V
0.38
0.75
µA
1
1.5
%
1
mA
3
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
ELECTRICAL CHARACTERISTICS (continued)
VI(–VIN) = –48 V with respect to RTN, VI(EN) = 2.8 V, VI(ISENS) = 0, all outputs unloaded, TA = –40_C to 85_C
(unless otherwise noted)(1)(2)
FAULT output
PARAMETER
IOH
RDS(ON)
TEST CONDITIONS
High-level output (leakage) current
Driver ON resistance
MIN
VI(EN) = 0 V,
VO(FAULT) = 65 V
VI(ISENS) = 80 mV, VO(FLTTIME) = 5V,
IO(FAULT) = 1 mA
TYP
35
MAX
UNIT
10
µA
60
Ω
NOTES 1: All voltages are with respect to the –VIN terminal unless otherwise stated.
2: Currents are positive into and negative out of the specified terminal.
TERMINAL FUNCTIONS
TERMINAL
NAME
I/O
DESCRIPTION
NO.
EN
2
I
Enable input to turn on/off power to the load.
FAULT
FLTTIME
1
O
Open-drain, active-low indication of a load fault condition.
3
I/O
Connection for user-programming of the fault timeout period.
GATE
IRAMP
7
O
Gate drive for external N-channel FET.
4
I/O
Programming input for setting the inrush current slew rate.
ISENS
6
I
Current sense input.
RTN
8
I
Positive supply input for the TPS2390 and TPS2391.
–VIN
5
I
Negative supply input and reference pin for the TPS2390 and TPS2391.
DETAILED PIN DESCRIPTIONS
EN: Enable input to turn on/off power to the load. The EN pin is referenced to the –VIN potential of the circuit.
When this input is pulled high (above the nominal 1.4-V threshold) the device enables the GATE output, and
begins the ramp of current to the load. When this input is low, the linear current amplifier (LCA) is disabled, and
a large pull-down device is applied to the FET gate, disabling power to the load.
FAULT: Open-drain, active-low indication of a load fault condition. When the device EN is deasserted, or when
enabled and the load current is less than the programmed limit, this output is high impedance. If the device
remains in current regulation mode at the expiration of the fault timer, or if a fast-acting overload condition
causes greater than 100-mV drop across the sense resistor, the fault is latched, the load is turned off, and the
FAULT pin is pulled low (to –VIN). The TPS2390 remains latched off for a fault, and can be reset by cycling either
the EN pin or power to the device. The TPS2391 retries the load at approximately a 1% duty cycle.
FLTTIME: Connection for user-programming of the fault timeout period. An external capacitor connected from
FLTTIME to –VIN establishes the timeout period to declare a fault condition. This timeout protects against
indefinite current sourcing into a faulted load, and also provides a filter against nuisance trips from momentary
current spikes or surges. The TPS2390 and TPS2391 define a fault condition as voltage at the ISENS pin at
or greater than the 40-mV fault threshold. When a fault condition exists, the timer is active. The devices manage
fault timing by charging the external capacitor to the 4-V fault threshold, then subsequently discharging it to reset
the timer (TPS2390), or discharging it at approximately 1% the charge rate to establish the duty cycle for retrying
the load (TPS2391). Whenever the internal fault latch is set (timer expired), the pass FET is rapidly turned off,
and the FAULT output is asserted.
4
www.ti.com
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
DETAILED PIN DESCRIPTIONS (continued)
GATE: Gate drive for external N-channel FET. When enabled, and the input supply is above the UVLO
threshold, the gate drive is enabled and the device begins charging an external capacitor connected to the
IRAMP pin. This pin voltage is used to develop the reference voltage at the non-inverting input of the internal
LCA. The inverting input is connected to the current sense node, ISENS. The LCA acts to slew the pass FET
gate to force the ISENS voltage to track the reference. The reference is internally clamped at 40 mV, so the
maximum current that can be sourced to the load is determined by the sense resistor value as IMAX ≤ 40
mV/RSENSE. Once the load voltage has ramped up to the input dc potential, and current demand drops off, the
LCA drives the GATE output to about 14 V to fully enhance the pass FET, completing the low-impedance supply
return path for the load.
IRAMP: Programming input for setting the inrush current slew rate. An external capacitor connected between
this pin and –VIN establishes the load current slew rate whenever power to the load is enabled. The device
charges the external capacitor to establish the reference input to the LCA. The closed-loop control of the LCA
and pass FET acts to maintain the current sense voltage at ISENS at the reference potential. Since the sense
voltage is developed as the drop across a resistor, the charging current ramp rate is set by the voltage ramp
rate at the IRAMP pin. When the output is disabled via the EN input or due to a load fault, the capacitor is
discharged and held low to initialize for the next turn-on.
ISENS: Current sense input. An external low value resistor connected between this pin and –VIN is used to feed
back current magnitude information to the TPS2390/91. There are two internal device thresholds associated
with the voltage at the ISENS pin. During ramp-up of the load’s input capacitance, or during other periods of
excessive demand, the HSPM acts to limit this voltage to 40 mV. Whenever the LCA is in current regulation
mode, the capacitor at FLTTIME is charged to activate the timer. If, when the LCA is driving to its supply rail,
a fast-acting fault such as a short-circuit, causes the ISENS voltage to exceed 100 mV (the overload threshold),
the GATE pin is pulled low rapidly, bypassing the fault timer.
RTN: Positive supply input for the TPS2390/91. For negative voltage systems, the supply pin connects directly
to the return node of the input power bus. Internal regulators step down the input voltage to generate the various
supply levels used by the TPS2390 and TPS2391.
–VIN: Negative supply input and reference pin for the TPS2390/91. This pin connects directly to the input supply
negative rail. The input and output pins and all internal circuitry are referenced to this pin, so it is essentially the
GND or VSS pin of the device.
www.ti.com
5
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
TYPICAL CHARACTERISTICS
LIVE INSERTION EVENT
VIN = –48 V
LIVE INSERTION EVENT
VIN = –80 V
EN (20 V/div.)
EN (20 V/div.)
VDRAIN (50 V/div.)
VDRAIN (20 V/div.)
Power Applied
Power Applied
CLOAD = 100 µF
CIRAMP = 3900 pF
CFLT = 0.1 µF
CLOAD = 50 µF
ILOAD
(500 mA/div.)
ILOAD (500 mA/div.)
t – TIme – 1 ms/div
t – TIme – 1 ms/div
Figure 1
Figure 2
LOAD CURRENT RAMP PROFILES
START-UP FROM ENABLE ASSERTION
IRAMP (2 V/div.)
EN (5 V/div.)
CIRAMP = .022 µF
VDRAIN 50 V/div.
CIRAMP =
3900 pF
CIRAMP = .047 µF
IRAMP (5 V/div.)
CLOAD = 100 µF
EN driven from logiclevel signal, ref to –VIN
ILOAD (1 A/div.)
ILOAD
t – TIme – 10 ms/div
t – TIme – 1 ms/div
Figure 4
Figure 3
6
CFLT = 0.33 µF
CLOAD = 600 µF
(500 mA/div.)
www.ti.com
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
TYPICAL CHARACTERISTICS
TURN-ON INTO SHORTED LOAD
(TPS2391)
TURN-ON INTO SHORTED LOAD
(TPS2390)
VDRAIN (50 V/div.)
VDRAIN (50 V/div.)
FLTTIME (2 V/div.)
FLTTIME (2 V/div.)
ILOAD (1 A/div.)
ILOAD (1 A/div.)
FAULT (20 V/div.)
CIRAMP = 3900 pF
CFLT = 0.047 µF
FAULT (20 V/div.)
CIRAMP = 3900 pF
CFLT = 0.047 µF
t – TIme – 1 ms/div
t – TIme – 1 ms/div
Figure 5
Figure 6
FAULT RETRY OPERATION
(TPS2391)
RECOVERY FROM A FAULT – LARGE SCALE VIEW
(TPS2391)
FAULT (50 V/div.)
FAULT (50 V/div.)
FLTTIME (2 V/div.)
VDRAIN
(20 V/div.)
FLTTIME (2 V/div.)
VDRAIN (20 V/div.)
ILOAD (1 A/div.)
CIRAMP = 3900 pF
CFLT = 0.047 µF
CLOAD = 100 µF
RLOAD = 12.5 Ω
ILOAD (1 A/div.)
CIRAMP = 3900 pF
CFLT = 0.047 µF
CLOAD = 100 µF
t – TIme – 50 ms/div
t – TIme – 50 ms/div
Figure 7
Figure 8
www.ti.com
7
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
TYPICAL CHARACTERISTICS
SUPPLY CURRENT
vs
AMBIENT TEMPERATURE
RECOVERY FROM A FAULT – EXPANDED VIEW
(TPS2391)
1200
1000
ICC – Supply Current – µA
FAULT (50 V/div)
FLTTIME (2 V/div.)
VDRAIN (20 V/div)
CIRAMP = 3900 pF
CFLT = 0.047 µF
CLOAD = 100 µF
VRTN = 80 V
800
600
VRTN = 40 V
400
VRTN = 36 V
200
0
ILOAD (1 A/div)
–40
t – TIme – 1 ms/div
–15
10
35
60
85
TA – Ambient Temperature – °C
Figure 9
Figure 10
IRAMP OUTPUT CURRENT
vs
AMBIENT TEMPERATURE, SLOW TURN-ON
GATE HIGH-LEVEL OUTPUT
vs
AMBIENT TEMPERATURE
–0.50
17.0
ISRC1 – IRAMP Output Current – µA
VI(ISENS) = 0 V
VOH – Output Voltage – V
16.5
VRTN = 80 V
16.0
VRTN = 48 V
15.5
15.0
–0.54
VRTN = 80 V
–0.58
VRTN = 48 V
VRTN = 36 V
–0.62
14.5
VO(IRAMP) = 0.25 V
VRTN = 36 V
–066
14.0
–40
–15
10
35
60
85
–15
10
35
60
TA – Ambient Temperature – °C
TA – Ambient Temperature – °C
Figure 12
Figure 11
8
–40
www.ti.com
85
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
TYPICAL CHARACTERISTICS
IRAMP OUTPUT CURRENT
vs
AMBIENT TEMPERATURE, NORMAL RATE
TIMER CHARGING CURRENT
vs
AMBIENT TEMPERATURE
–9.5
–45
VI(ISENS) = 80 mV
VO(FLTTIME) =2V
–9.7
–47
ICHG – Charging Current – µA
ISRC2 – IRAMP Output Current – µA
Average for VO(IRAMP) = 1 V, 3 V
VRTN = 36 V to 80 V
–9.9
–10.1
–10.3
VRTN = 80 V
–49
–51
–53
VRTN = 36 V
VRTN = 48 V
–10.5
–55
–40
–15
10
35
60
85
–40
TA – Ambient Temperature – °C
–15
Figure 13
60
85
60
85
FAULT LATCH THRESHOLD
vs
AMBIENT TEMPERATURE
4.25
0.50
VRTN = 48 V
VI(ISENS) = 80 mV
VO(FLTTIME) = 2 V
VRTN = 36 V to 80 V
VFLT – Threshold Voltage – V
IDSG – Discharge Current – µA
35
Figure 14
FLTTIME DISCHARGE CURRENT
vs
AMBIENT TEMPERATURE (TPS2391)
0.47
10
TA – Ambient Temperature – °C
0.44
0.41
0.38
0.35
0.32
4.13
4.00
3.88
0.29
0.26
–40
–15
10
35
60
85
3.75
–40
–15
10
35
TA – Ambient Temperature – °C
TA – Ambient Temperature – °C
Figure 15
Figure 16
www.ti.com
9
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
APPLICATION INFORMATION
When a plug-in module or printed circuit card is inserted into a live chassis slot, discharged supply bulk
capacitance on the board can draw huge transient currents from the system supplies. Without some form of
inrush limiting, these currents can reach peak magnitudes ranging up to several hundred amps, particularly in
high-voltage systems. Such large transients can damage connector pins, PCB etch, and plug-in and supply
components. In addition, current spikes can cause voltage droops on the power distribution bus, causing other
boards in the system to reset.
The TPS2390 and TPS2391 are hot swap power managers designed to limit these peaks to preset levels, as
well as control the slew rate (di/dt) at which charging current ramps to the programmed limit. These devices use
an external N-Channel pass FET and sense element to provide closed-loop control of current sourced to the
load. Input supply undervoltage lockout (UVLO) protection allows hot swap circuits to turn on automatically with
the application of power, or to be controlled with a system command via the EN input. External capacitors control
both the current ramp rate, and the time–out period for load voltage ramping. In addition, an internal overload
comparator provides circuit breaker protection against shorts occurring during steady-state (post-turn-on)
operation of the card.
The TPS2390 and TPS2391 operate directly from the input supply (nominal –48 VDC rail). The –VIN pin
connects to the negative voltage rail, and the RTN pin connects to the supply return. Internal regulators convert
input power to the supply levels required by the device circuitry. An input UVLO circuit holds the GATE output
low until the supply voltage reaches a nominal 30-V level. A second comparator monitors the EN input; this pin
must be pulled above the 1.4-V enable threshold to turn on power to the load.
Once enabled, and when the input supply is above the UVLO threshold, the GATE pull-down is removed, the
linear control amplifier (LCA) is enabled, and a large discharge device in the RAMP CONTROL block is turned
off. Subsequently, a small current source is now able to charge an external capacitor connected to the IRAMP
pin. This results in a linear voltage ramp at IRAMP. The voltage ramp on the capacitor actually has two discrete
slopes. As shown in Figure 17, charging current is supplied from either of two sources. Initially at turn-on, the
600-nA source is selected, to provide a slow turn-on rate. This slow turn-on helps ensure that the LCA is pulled
out of saturation, and is slewing to the voltage at its non-inverting input before normal rate load charging is
allowed. This mechanism helps reduce current steps at turn-on. Once the voltage at the IRAMP pin reaches
approximately 0.5 V, an internal comparator deasserts the SLOW signal, and the 10-µA source is selected for
the remainder of the ramp period.
The voltage at IRAMP is divided down by a factor of 100, and applied to the non-inverting input of the LCA. Load
current magnitude information at the ISENS pin is applied to the inverting input. This voltage is developed by
connecting the current sense resistor between ISENS and –VIN. The LCA slews the gate of the external pass
FET to force the ISENS voltage to track the divided down IRAMP voltage. Consequently, the load current slew
rate tracks the linear voltage ramp at the IRAMP pin, producing a linear di/dt of the load current. The IRAMP
capacitor is charged to about 6.5 V; however, the LCA input is clamped at 40 mV. Therefore, the current sourced
to the load during turn-on is limited to a value given by IMAX ≤ 40 mV/RSENSE, where RSENSE is the value of
the sense resistor.
The resultant load current, regulated by the controller, charges the module’s input bulk capacitance in a safe
fashion. Under normal conditions, this capacitance eventually charges up to the dc input potential. At this point,
the load demand drops off, and the voltage at ISENS decreases. The LCA now drives the GATE output to its
supply rail. The 14-V typical output level ensures sufficient overdrive to fully enhance the external FET, while
not exceeding the typical 20-V VGS rating of common N-Channel power FETs.
10
www.ti.com
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
APPLICATION INFORMATION
10 µ A
600 nA
VDD
SLOW
+
LCA
IRAMP
7
GATE
6
ISENS
1
FAULT
4
99 R
RAMP
CONTROL
R
40mV
OC
EN
2
+
ON
EN_A
1.4V
+
OL
50 µ A
100 mV
4V
FLTTIME
3
OVERLOAD
COMP
+
0.4 µ A
RTN
8
0.5V
+
30 V
S
Q
R
Q
DCHG
RETRY
VDD
14V
–VIN
+
TIMER
BLOCK
FAULT
LOGIC
’91 ONLY
ON
5
UDG–02091
Figure 17. Block Diagram
Fault timing is accomplished by connecting a capacitor between the FLTTIME and –VIN pins, allowing
user-programming of the timeout period. Whenever the hot swap controller is in current control mode as
described above, the LCA asserts an overcurrent indication (OC in the Figure 17 diagram). Overcurrent fault
timing is inhibited during the slow turn-on portion of the IRAMP waveform. However, once the device transitions
to the normal rate current ramp (VO(IRAMP) ≥ 0.5 V), the external capacitor is charged by a 50-µA source,
generating a voltage ramp at the FLTTIME pin. If this voltage reaches the 4-V fault threshold, the fault is latched,
and the open-drain driver is turned on to assert the external FAULT output. Fault capacitor charging ceases,
and the capacitor is now discharged. In addition, latching of a fault condition causes rapid discharge of the
IRAMP capacitor. In this manner, the soft-start function is now reset and ready for the next output enable, if and
when conditions permit.
The TPS2390 latches off in response to faults; once a fault timeout occurs, the discharge signal (DCHG) turns
on a large NMOS device to rapidly discharge the external capacitor, resetting the timer for any subsequent
device reset. The TPS2390 can only be reset by cycling power to the device, or by cycling the EN input.
www.ti.com
11
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
APPLICATION INFORMATION
In response to a latched fault condition, the TPS2391 enters a fault retry mode, wherein it periodically retries
the load to test for continued existence of the fault. In this mode, the FLTTIME capacitor is discharged slowly
by a about a 0.4-µA constant-current sink. When the voltage at the FLTTIME pin decays below 0.5 V, the LCA
and RAMP CONTROL circuits are re-enabled, and a normal turn-on current ramp ensues. Again, during the
load charging, the OC signal causes charging of the FLTTIME capacitor until the next delay period elapses. The
sequential charging and discharging of the FLTTIME capacitor results in a typical 1% retry duty cycle. If the fault
subsides (GATE pin drives to high-level output), the timing capacitor is rapidly discharged, duty-cycle operation
stops, and the fault latch is reset.
Note that because of the timing inhibit during the initial slow ramp period, the duty cycle in practice is slightly
greater than the nominal 1% value. However, sourced current during this period peaks at only about one-eighth
the maximum limit. The duty cycle of the normal ramp and constant-current periods is approximately 1%.
The FAULT LOGIC within the TIMER BLOCK automatically manages capacitor charge and discharge rates
(DCHG signal), and the enabling of the GATE output (ON signal). For the TPS2391, the FAULT output remains
asserted continuously during retry mode; it is only released if the fault condition clears.
These hot swap controllers contain an OVERLOAD COMPARATOR which also monitors the ISENS voltage.
If sense voltage excursions above 100 mV are detected, the fault is latched, LCA disabled, and the FET gate
is rapidly pulled down, bypassing the fault timer. The timer block does apply a 4-µs deglitch filter to the OL signal
to help reduce nuisance trips. As with overcurrent faults, the TPS2390 latches the output off. For the TPS2391,
an overload fault causes charging of the timer capacitor, to initiate fault retry timing.
setting the sense resistor value
Due to the current-limiting action of the internal LCA, the maximum allowable load current for an implementation
is easily programmed by selecting the appropriate sense resistor value. The LCA acts to limit the sense voltage
VI(ISENS) to its internal reference. Once the voltage at the IRAMP pin exceeds approximately 4 V, this limit is
the clamp voltage, VREF_K. Therefore, a maximum sense resistor value can be determined from equation (1).
R SENSE v 33 mV
IMAX
(1)
where:
•
•
12
RSENSE is the resistor value, and
IMAX is the desired current limit.
www.ti.com
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
APPLICATION INFORMATION
When setting the sense resistor value, it is important to consider two factors, the minimum current that may be
imposed by the TPS2390 or TPS2391, and the maximum load under normal operation of the module. For the
first factor, the specification minimum clamp value is used, as seen in equation (1). This method accounts for
the tolerance in the sourced current limit below the typical level expected (40 mV/RSENSE). (The clamp
measurement includes LCA input offset voltage; therefore, this offset does not have to be factored into the
current limit again.) Second, if the load current varies over a range of values under normal operating conditions,
then the maximum load level must be allowed for by the value of RSENSE. One example of this is when the load
is a switching converter, or brick, which draws higher input current, for a given power output, when the
distribution bus is at the low end of its operating range, with decreasing draw at higher supply voltages. To avoid
current-limit operation under normal loading, some margin should be designed in between this maximum
anticipated load and the minimum current limit level, or IMAX > ILOAD(max), for equation (1).
For example, using a 20-mΩ sense resistor for a nominal 1-A load application provides a minimum of 650 mA
of overhead for load variance/margin. Typical bulk capacitor charging current during turn-on is 2 A
(40 mV/20 mΩ).
setting the inrush slew rate
The TPS2390 and TPS2391 devices enable user-programming of the maximum current slew rate during load
start-up events. A capacitor tied to the IRAMP pin (C2 in the typical application diagram) controls the di/dt rate.
Once the sense resistor value has been established, a value for ramp capacitor CIRAMP, in microfarads, can be
determined from equation (2).
C IRAMP +
11
100
R SENSE
ǒdtdiǓ
(2)
MAX
where:
•
•
RSENSE is in ohms, and
(di/dt)MAX is the desired maximum slew rate, in amps/second.
For example, if the desired slew rate for the typical application shown is 1500 mA/ms, the calculated value for
CIRAMP is about 3700 pF. Selecting the next larger standard value of 3900 pF (as shown in the diagram) provides
some margin for capacitor and sense resistor tolerances.
As described earlier in this section, the TPS2390 and TPS2391 initiate ramp capacitor charging, and
consequently, load current di/dt at a reduced rate. This reduced rate applies until the voltage on the IRAMP pin
is about 0.5 V. The maximum di/dt rate, as set by equation (2), is effective once the device has switched to the
10-µA charging source.
www.ti.com
13
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
APPLICATION INFORMATION
setting the fault timing capacitor
The fault timeout period is established by the value of the capacitor connected to the FLTTIME pin, CFLT. The
timeout period permits riding out spurious current glitches and surges that may occur during operation of the
system, and prevents indefinite sourcing into faulted loads swapped into a live system. However, to ensure
smooth voltage ramping under all conditions of load capacitance and input supply potential, the minimum
timeout should be set to accommodate these system variables. To do this, a rough estimate of the maximum
voltage ramp time for a completely discharged plug-in card provides a good basis for setting the minimum timer
delay.
Due to the three-phase nature of the load current at turn-on, the load voltage ramp potentially has three distinct
phases ( compare Figures 1 and 2). This profile depends on the relative values of load capacitance, input dc
potential, maximum current limit and other factors. The first two phases are characterized by the two different
slopes of the current ramp; the third phase, if required for bulk capacitance charging, is the constant-current
charging at IMAX. Considering the two current ramp phases to be one period at an average di/dt simplifies
calculation of the required timing capacitor.
For the TPS2390 and TPS2391, the typical duration of the soft-start ramp period, tSS, is given by equation (3).
t SS + 1183
C IRAMP
(3)
where:
•
•
tSS is the soft-start period in ms, and
CIRAMP is given in µF
During this current ramp period, the load voltage magnitude which is attained is estimated by equation (4).
V LSS +
2
CL
i AVG
C IRAMP 100
R SENSE
ǒt SSǓ
2
(4)
where:
•
•
•
•
VLSS is the load voltage reached during soft-start,
iAVG is 3.38 µA for the TPS2390 and TPS2391,
CL is the amount of the load capacitance, and
tSS is the soft-start period, in seconds
The quantity iAVG in equation (4) is a weighted average of the two charge currents applied to CIRAMP during
turn-on, considering the typical output values.
14
www.ti.com
SLUS471C – JUNE 2002 – REVISED OCTOBER 29, 2002
APPLICATION INFORMATION
If the result of equation (4) is larger than the maximum input supply value, then the load can be expected to
charge completely during the inrush slewing portion of the insertion event. However, if this voltage is less than
the maximum supply input, VIN(max), the HSPM transitions to the constant-current charging of the load. The
remaining amount of time required at IMAX is determined from equation (5).
t CC +
CL
ǒVIN (max) * VLSSǓ
ǒ
V
REF_K (min)
R
SENSE
Ǔ
(5)
where:
•
•
tCC is the constant-current voltage ramp time, in seconds, and
VREF_K(min) is the minimum clamp voltage, 33 mV.
With this information, the minimum recommended value timing capacitor CFLT can be determined. The delay
time needed is either tSS or the sum of tSS and tCC, according to the estimated time to charge the load. Since
fault timing is generated by the constant-current charging of CFLT, the capacitor value is determined by equation
(6) or (7).
C
C
FLT
(MIN) +
FLT
(MIN) +
55
55
t
SS
3.75
(6)
ǒtSS ) tCCǓ
3.75
(7)
where:
•
•
•
CFLT(min) is the recommended capacitor value, in microfarads,
tSS is the result of equation (3), in seconds, and
tCC is the result of equation (5), in seconds.
For the typical application example, with the 100-µF filter capacitor in front of the dc-to-dc converter, equations
(3) and (4) estimate the load voltage ramping to –46 V during the soft-start period. If the module should operate
down to –72-V input supply, approximately another 1.58 ms of constant-current charging may be required.
Therefore, equation 7 is used to determine CFLT(min), and the result is approximately 0.1 µF.
www.ti.com
15
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications,
enhancements, improvements, and other changes to its products and services at any time and to discontinue
any product or service without notice. Customers should obtain the latest relevant information before placing
orders and should verify that such information is current and complete. All products are sold subject to TI’s terms
and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI
deems necessary to support this warranty. Except where mandated by government requirements, testing of all
parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for
their products and applications using TI components. To minimize the risks associated with customer products
and applications, customers should provide adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right,
copyright, mask work right, or other TI intellectual property right relating to any combination, machine, or process
in which TI products or services are used. Information published by TI regarding third–party products or services
does not constitute a license from TI to use such products or services or a warranty or endorsement thereof.
Use of such information may require a license from a third party under the patents or other intellectual property
of the third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of information in TI data books or data sheets is permissible only if reproduction is without
alteration and is accompanied by all associated warranties, conditions, limitations, and notices. Reproduction
of this information with alteration is an unfair and deceptive business practice. TI is not responsible or liable for
such altered documentation.
Resale of TI products or services with statements different from or beyond the parameters stated by TI for that
product or service voids all express and any implied warranties for the associated TI product or service and
is an unfair and deceptive business practice. TI is not responsible or liable for any such statements.
Mailing Address:
Texas Instruments
Post Office Box 655303
Dallas, Texas 75265
Copyright 2002, Texas Instruments Incorporated